Body Surface Area (BSA): The 2026 Guide to Du Bois, Mosteller, and Why Medicine Uses It

Ask someone their weight and they answer instantly; ask their body surface area and most people have never considered having one. Yet BSA — roughly 1.7 to 1.9 square meters for a typical adult — is one of the most consequential numbers in medicine, the scaling factor behind chemotherapy doses, cardiac output measurements, and a century of physiology. This guide explains what body surface area actually is, walks through the two formulas you will meet everywhere — Du Bois from 1916 and Mosteller from 1987 — and shows where the number is used and where it is not. As always, the stance is honest: a BSA calculator produces an educational estimate, not medical advice, and dosing decisions belong to clinicians.

SECTION 01What Body Surface Area Actually Is

Body surface area is exactly what the name says: the total area of skin covering a body, measured in square meters. It is not observable with a tape measure — nobody wraps you in graph paper anymore — so it is estimated from height and weight through regression formulas fitted on direct measurements, most famously a small 1916 study that produced the Du Bois formula. A typical adult lands between 1.6 and 2.0 square meters; a newborn is around 0.25; a large adult can exceed 2.3.

Why medicine cares is a matter of scaling. Heat loss, metabolic burn, and drug distribution all track surface area better than raw weight, because area governs exchange with the environment while weight lumps together tissue the body treats very differently. BSA is therefore the body's most common index for scaling physiology — the denominator that turns a raw number like cardiac output into a comparable, person-sized number.

The key mental model: BSA answers 'how big is this body's interface with the world,' not 'how heavy is it.' That reframing explains both its power and its quirks. Two people of very different shapes can share a BSA while sharing nothing else, which is exactly why the number thrives in some contexts and would fail in others.

SECTION 02The Du Bois Formula

The classic formula, published in 1916 by Du Bois and Du Bois after direct measurement of nine subjects, reads: BSA equals 0.007184, times weight in kilograms raised to the power 0.425, times height in centimeters raised to the power 0.725. The fractional exponents are why the formula feels exotic — weight and height each contribute, but neither dominates, and the constant folds the units into a clean square-meter answer.

Work a quick case to see it live. For a 70-kilogram, 175-centimeter adult: 70 to the power 0.425 is about 6.08, and 175 to the power 0.725 is about 42.3. Multiply: 6.08 times 42.3 is roughly 257. Multiply by 0.007184 and you get about 1.85 square meters. That is a textbook adult value, and the arithmetic is exactly what a BSA calculator performs instantly.

Du Bois remains the default in oncology protocols a century later, which is remarkable for a formula fitted on nine people. It has known biases — it runs slightly high for very small children and somewhat off at body-size extremes — and later formulas were fitted to fix exactly that. But convention is a force in medicine, and where a protocol says Du Bois, the calculator's job is to compute Du Bois precisely.

SECTION 03The Mosteller Formula and Why Clinicians Like It

The Mosteller formula, published in 1987, wins the simplicity prize: BSA equals the square root of height in centimeters times weight in kilograms, divided by 3600. Take the same 70-kilogram, 175-centimeter adult: 175 times 70 is 12,250; 12,250 divided by 3600 is 3.403; and the square root of 3.403 is about 1.84 square meters. One square root, no fractional exponents, computable on any phone.

The two formulas agree closely across the ordinary adult range — within a percent or two for most people — which is why clinicians reach for Mosteller when they need a fast estimate and Du Bois when a protocol names it. The 3600 constant is chosen so that a person 180 centimeters tall weighing 100 kilograms — the so-called reference that makes the numbers tidy — computes to a clean value, but the practical meaning is simpler: divide by 3600, take the root, done.

Other formulas exist — Haycock, Gehan and George, Boyd — each fitted on different populations and each preferred in particular niches, with Haycock common in pediatrics. A good calculator lets you see more than one. If Du Bois and Mosteller agree within about two percent, your inputs are fine and the choice of formula barely matters; if they diverge wildly, the inputs — almost always the units — are wrong.

SECTION 04Where BSA Is Used

The flagship use is chemotherapy dosing. Most classic IV chemo protocols dose in milligrams per square meter, multiplying a protocol rate by the patient's BSA to get a dose — and sometimes capping the BSA at an arbitrary value like 2.0 or 2.2 to limit doses in very large patients. A rate of 50 milligrams per square meter with a BSA of 1.84 gives 92 milligrams. That multiplication is performed by pharmacists with protocol-specified rules, never improvised.

Cardiology uses BSA as the normalizer for flow: cardiac output in liters per minute divided by BSA gives cardiac index, typically 2.5 to 4.0 liters per minute per square meter, which lets a small and large person be compared fairly. Nephrology reports GFR per 1.73 square meters for the same reason. Burn care estimates affected area as a percentage of total BSA; nutrition and some ventilator settings touch it too. The pattern is consistent: BSA appears wherever a per-person scaling denominator is needed.

Where it is not used matters equally. Obesity classification uses BMI, not BSA; most oral drug dosing uses weight or fixed dosing; pediatric growth tracks weight-for-height on charts rather than area. BSA is a specialist's scaling tool, not a general health metric — powerful inside its contexts, meaningless outside them.

SECTION 05BSA Versus BMI: Sibling Formulas, Different Questions

Both BSA and BMI combine height and weight, and people constantly confuse them. BMI divides weight in kilograms by the square of height in meters, producing a number around 20 to 35 that classifies weight status against population bands. BSA multiplies height and weight through area formulas, producing square meters that describe physical size. One is a ratio built for classification; the other is an area built for scaling.

The divergence shows up in real cases. A tall, lean adult and a short, heavy adult can have similar BSA — both present a similar 'surface' to the world — while their BMIs sit in different categories entirely. Conversely, two people with identical BMI can differ in BSA if height differs. Neither formula sees composition: muscle, fat, and frame all blur together in both.

The practical rule: if the question is 'is my weight in a healthy range,' that is BMI territory, imperfect but relevant. If the question is 'how is this physiological quantity scaled for a body this size,' that is BSA territory. Asking one to do the other's job produces confident nonsense, which is the standard failure mode of health arithmetic everywhere.

SECTION 06Using a BSA Calculator Well

The workflow is short. Weigh yourself on a real scale, measure height without shoes, and enter both with the units the calculator expects — kilograms and centimeters for both classic formulas. The tool at /bsa-calculator.html computes Du Bois and Mosteller together and shows the agreement between them, which doubles as an input sanity check. Seconds later you have a number that previously required a nomogram and a steady hand.

For children, use pediatric-appropriate expectations: values scale from roughly 0.25 square meters at birth upward, and Haycock is the common pediatric formula. Height measurement matters more than people expect — a slouching, shoe-wearing height adds error that flows straight into the result. Round nothing before computing; round only the display.

Then interpret within the stated limits. The output is an educational estimate of an area nobody can measure exactly; formulas differ slightly by design; and any medical use — dosing above all — happens under clinical rules that may cap, adjust, or override the raw number. A calculator tells you what the formula computes. Only a clinician tells you what it means for a treatment.

SECTION 07Limits and Safety Notes

Every BSA limitation descends from one fact: the formulas were fitted on finite populations and extrapolate imperfectly at extremes. Very tall, very short, very heavy, and very lean bodies sit where the regression is weakest, and formulas disagree most there. Body composition is invisible by construction. Amputation, swelling, and pregnancy all distort the height-weight relationship the formulas assume.

The safety note is the one this site repeats deliberately: nothing here is medical advice. If you arrived at a BSA to check a chemotherapy dose, a cardiac index, or a pediatric regimen — stop, and take the question to the clinician who prescribed it; dose calculations include rules that no generic calculator knows. For learning, for checking a lab report's context, or for satisfying curiosity about a 1916 formula that still runs oncology, /bsa-calculator.html is exactly the right tool. Numeracy and medicine are allies precisely when each stays in its lane.

SECTION 08The Two Formulas in One Place

Mosteller: BSA equals the square root of height in centimeters times weight in kilograms, divided by 3600. Du Bois: BSA equals 0.007184 times weight to the power 0.425 times height to the power 0.725. Both take height in centimeters and weight in kilograms and return square meters. Everything below uses those exact forms with intermediate values rounded only for display.

Cardiac index, the one derived quantity used here, is cardiac output in liters per minute divided by BSA. With the two formulas and one division, all six examples are checkable by hand — which is the point. Formulas you can verify are formulas you can trust. The bsa calculator at /bsa-calculator.html runs both formulas side by side, which makes it the natural place to check every example below.

SECTION 09Example 1: Standard Adult by Mosteller — 70 kg, 175 cm

Multiply height by weight: 175 times 70 is 12,250. Divide by 3600: 12,250 over 3600 is 3.403. Take the square root: the square root of 3.403 is about 1.845.

So BSA is approximately 1.84 square meters — squarely in the typical adult band of 1.6 to 2.0.

Interpretation: this is the textbook adult case, and it is worth memorizing as an anchor. When any BSA output for a mid-sized adult lands far from 1.8, suspect the inputs before trusting the result.

SECTION 10Example 2: The Same Adult by Du Bois

Weight factor: 70 to the power 0.425 — read it as 70 to the 0.4 (about 6.29) times 70 to the 0.025 (about 1.092), which composes to roughly 6.08. Height factor: 175 to the power 0.725 is approximately 42.3.

Multiply: 6.08 times 42.3 is about 257.2. Multiply by 0.007184: 257.2 times 0.007184 is about 1.848.

Interpretation: Du Bois gives about 1.85 square meters against Mosteller's 1.84 — agreement within roughly half a percent. That is the normal relationship between the two formulas for an ordinary adult, and it doubles as your sanity check: if they disagree by more than a couple of percent, recheck the units.

SECTION 11Example 3: A Child — 15 kg, 95 cm

Mosteller: 95 times 15 is 1,425. Divided by 3600: 1,425 over 3600 is 0.3958. Square root: about 0.629.

So BSA is approximately 0.63 square meters — about a third of an adult's, for a child about a fifth of an adult's weight. Area does not scale with weight; it scales closer to height times weight, which is why children are not simply small adults in dosing arithmetic.

Interpretation: this is also where the Du Bois formula's known small-child bias shows up, and where pediatric practice leans on formulas like Haycock and on clinical judgment. The number is a fine educational illustration; pediatric dosing is another universe entirely.

SECTION 12Example 4: Multiplying a Dose — 50 mg per Square Meter

Take Example 1's BSA of 1.84 square meters and a hypothetical protocol rate of 50 milligrams per square meter. The dose is 50 times 1.84, which is 92 milligrams.

That is the entire multiplication — and that simplicity is exactly why the units matter so much. Enter the BSA as 18.4 (a square-meter slip) and the dose becomes 920 milligrams, a tenfold error that no downstream check will necessarily catch.

Interpretation: the example exists to show the shape of real dose arithmetic, not to suggest you perform it. Actual chemotherapy dosing adds protocol caps, rounding rules, cycle adjustments, and verification steps — pharmacist work, prescribed work, checked work. Understand the multiplication; never perform it.

SECTION 13Example 5: A Larger Adult — 85 kg, 160 cm

Mosteller: 160 times 85 is 13,600. Divided by 3600: 13,600 over 3600 is 3.778. Square root: about 1.944.

So BSA is approximately 1.94 square meters.

Interpretation: a heavier but shorter person lands near the same area as Example 1's taller, lighter adult — 1.94 versus 1.84. This is BSA's character in one comparison: it measures the interface, not the mass, and it compresses differences that weight-based thinking would exaggerate.

SECTION 14Example 6: Cardiac Index — 5.0 L/min over 1.84 m²

Cardiac output of 5.0 liters per minute, divided by BSA of 1.84 square meters: 5.0 over 1.84 is about 2.72 liters per minute per square meter.

The typical adult range for cardiac index runs roughly 2.5 to 4.0, so 2.72 sits comfortably inside it — a normal resting heart, scaled to body size.

Interpretation: this is BSA's normalizing job in one line — raw output becomes per-size output, and suddenly a small person and a large person can be compared on equal terms. The same pattern underlies GFR reporting per 1.73 square meters. Run all six cases yourself at /bsa-calculator.html; watching the tool reproduce your hand math is the best possible check on both.

SECTION 15Cross-Checks and Cautions

Three habits catch nearly every BSA error. Cross-formula agreement: Du Bois and Mosteller should land within about two percent for typical adults — compute both and compare. Anchor sense: a mid-sized adult should land near 1.8, a ten-year-old near 1.1 to 1.3, a newborn near 0.25; wild departures mean unit slips. And unit discipline: centimeters and kilograms, always — pounds and feet must be converted before entering, not after.

The caution is permanent: these are educational estimates of areas nobody measures exactly, and the two most consequential uses — dosing and cardiac index — are clinical acts surrounded by protocol. Use the worked examples to understand what the number is and where it comes from; leave the application to the people with the chart, the protocol, and the license.

A closing note on where these numbers come from: every BSA formula is a regression fitted on measured groups, which means each carries population-level error that is invisible in its decimals. Du Bois was fitted on nine adults in 1916; Mosteller on larger but still finite samples. The practical consequence is the rounding discipline used throughout this post — two decimals in square meters, no more. Numbers that pretend to more precision than their data support are not more accurate; they are merely better dressed, and in dose arithmetic, better-dressed numbers get trusted exactly as if they were right.

SECTION 16Mistake 1: Mixing Units — the Classic Tenfold Error

The dominant BSA mistake, responsible for most wild results, is unit mixing: pounds entered where kilograms are expected, feet and inches where centimeters belong. The formulas assume kilograms and centimeters, and every factor inside them — 3600, 0.007184 — is tuned to those units. A person who is 1.84 square meters becomes a nonsense figure when 160 pounds masquerades as 160 kilograms; the output looks like a number and means nothing.

The fix is mechanical, not mathematical. Convert before entering, with fixed factors: pounds to kilograms by dividing by 2.205, inches to centimeters by multiplying by 2.54. Better, use the tool's structured inputs and unit selectors instead of hand conversion. Then run the anchor test: a mid-sized adult should land near 1.8 square meters, so any result far from that band sends you back to the inputs, not to a second calculator.

SECTION 17Mistake 2: Treating All BSA Formulas as Interchangeable — or as Enemies

Two opposite errors live here. Some people average whatever formulas they find, or switch formulas until one matches an expectation, treating them as interchangeable decorations. Others see a half-percent difference between Du Bois and Mosteller and conclude one must be broken. Both miss the design: the formulas are different regressions, fitted on different populations, expected to agree closely for typical adults and to diverge slightly at extremes.

The fix is to use the relationship deliberately. Compute both — the calculator at /bsa-calculator.html shows them together — and read the gap as a diagnostic. Agreement within about two percent says your inputs are sound and either answer serves. Divergence beyond that says the inputs are wrong, almost always units. The formulas are not rivals to adjudicate; they are witnesses that corroborate each other.

SECTION 18Mistake 3: Rounding Before Multiplying

In dose arithmetic, BSA is a multiplier, and rounding it early contaminates everything downstream. Round 1.844 to 1.8 before multiplying a 50 milligram per square meter rate and you get 90 instead of 92.2 milligrams — a small shift in a toy example, but the habit scales badly: looser rounding, larger BSAs, or compounded adjustments turn it into real error. Precision lost at the start cannot be recovered at the end.

The fix is to keep full precision through the computation and round once, at the display, following whatever rule the context specifies — clinical protocols dictate their own rounding, and pharmacists apply it. In your own educational use, carry four decimal places in the intermediate steps and round only the final number. The arithmetic is cheap; the discipline is what separates a calculation from a guess.

SECTION 19Mistake 4: Confusing BSA with BMI

The two formulas share inputs, so people assume they share meaning, then draw conclusions in the wrong currency. Using BSA to ask whether weight is healthy is the common version — it cannot answer that; a person at 2.0 square meters is big, not bad. The reverse error appears too: dismissing BSA as 'basically BMI' in contexts like cardiac index where the square-meter denominator is the entire point.

The fix is a one-line distinction, worth memorizing. BMI is a ratio (weight over height squared) built to classify weight status against population bands. BSA is an area built to scale physiological quantities per body size. If the question involves a healthy range, it is BMI's — imperfectly. If the question involves scaling a measurement or a dose, it is BSA's. Cross-hiring either formula produces confident nonsense.

SECTION 20Mistake 5: Applying Adult Formulas to Children Without the Context

Children are not small adults, and BSA is where that cliché becomes arithmetic: a 15-kilogram child computes to about 0.63 square meters, and the pediatric world layers its own formulas — Haycock most commonly — plus weight-based rules and age-specific judgment over that number. Running a child through an adult-default tool and treating the result as clinically complete is a category error with real consequences downstream.

The fix is scope honesty. For learning — seeing how area scales, checking a number a clinician mentioned — adult tools are fine for any age. For anything approaching application, pediatric values belong to pediatric workflows. A calculator that asks for age and flags pediatric inputs is doing its part; the rest is the clinician's, and the parent's job is to bring questions, not computations.

SECTION 21Mistake 6: Phantom Precision

The final mistake is trusting the sixth decimal. Du Bois was fitted on nine people in 1916; Mosteller on larger but still finite samples; every formula carries population-level error larger than the decimals it prints. Reporting a BSA of 1.8446 and acting on the 0.0006 is precision theater — the formula's own uncertainty swamps it. The same humility applies to self-measured inputs: bathroom-scale drift and shoe-wearing heights inject errors larger than the formula differences people agonize over.

The fix is rounding to the task: two decimals in square meters is plenty for understanding, and honest input hygiene — a real scale, a wall-measured height without shoes — improves results more than any formula choice. Precision is a budget; spend it where the error actually lives, which is almost never in the calculator. Honest inputs and honest rounding are the whole discipline; the formulas were never the weak link.

SECTION 22Pro Tips for Trustworthy Numbers

First, adopt the anchor set: roughly 0.25 square meters for a newborn, around 1.1 to 1.3 for a ten-year-old, 1.6 to 2.0 for typical adults — one-second reality checks that catch unit slips instantly. Second, always compute both Du Bois and Mosteller and treat agreement as your input verifier. Third, measure once, properly: a wall-measured height and a consistent scale remove the two biggest input errors you control.

Fourth, keep a note of your own BSA alongside your height and weight; it recurs in enough contexts — reports, formulas, curiosity — that having a stable, correctly computed value beats recomputing casually. And fifth, hold the standing stance: a BSA calculator is an educational instrument, transparent arithmetic on your inputs. It cannot diagnose, cannot dose, and cannot replace the protocols that govern its most serious uses. Inside those limits it is excellent — the moment you feel it answering a medical question, that is the moment to take the question to a clinician instead.

🔑 Key takeaways

• BSA is the body's total surface in square meters — roughly 1.7 to 1.9 for a typical adult — estimated from height and weight, never measured directly.
• Du Bois (0.007184 times weight^0.425 times height^0.725) is the oncology default; Mosteller (square root of height times weight over 3600) is the quick check.
• The two formulas agree within a percent or two for most adults; wide disagreement almost always means a units error.
• Chemotherapy doses in milligrams per square meter and cardiac index (output divided by BSA) are the flagship uses; BMI classification is not a BSA use.
• BSA and BMI combine the same inputs to answer different questions — scaling versus classification — and neither sees body composition.
• Treat every output as an educational estimate, not medical advice; real dosing follows clinical protocols that can cap or adjust the raw number.
• Mosteller in one line: square root of (height times weight over 3600) — Example 1's 70 kg, 175 cm adult computes to about 1.84 m².
• Du Bois lands within about half a percent of Mosteller for the same adult (1.85 versus 1.84) — agreement is your input sanity check.
• Example 3's 15 kg child computes to about 0.63 m²: area scales closer to height times weight than to weight alone, which is why children are not small adults in dosing.
• Example 4 shows the whole dose arithmetic — 50 mg/m² times 1.84 m² equals 92 mg — and why a units slip becomes a tenfold error.
• Example 5's larger-but-shorter adult (1.94 m²) nearly matches Example 1's taller adult: BSA measures interface, not mass.
• Cardiac index is output divided by BSA — 5.0 over 1.84 gives 2.72 L/min/m², inside the typical 2.5 to 4.0 band.
• Verify with cross-formula agreement and size anchors, keep centimeters and kilograms straight, and leave all clinical application to protocols and clinicians.
• Unit mixing is the dominant BSA error — convert to kilograms and centimeters before entering, and run the 1.8-square-meter adult anchor test.
• Du Bois and Mosteller should agree within about two percent; compute both and read the gap as an input diagnostic, not a formula contest.
• Keep full precision through the math and round once at the end — rounding a 1.844 BSA to 1.8 before multiplying doses shifts real results.
• BMI classifies weight; BSA scales physiology. The two share inputs, not meaning — cross-hiring them produces confident nonsense.
• For children, adult tools are fine for learning, but pediatric formulas and pediatric judgment govern anything applied.
• Two decimals are honest; the sixth decimal is theater. Fix input hygiene first — a real scale and a wall-measured height.

❓ Frequently asked questions

What is a normal BSA for an adult?

Most adults fall between about 1.6 and 2.0 square meters, with roughly 1.7 to 1.9 common for mid-range height and weight. There is no 'abnormal' in the clinical sense — BSA describes size; it is not a health grade.

Which BSA formula should I use?

For a quick estimate, Mosteller — it is simple and accurate. If a medical protocol names a formula, that one governs. For most adults, Du Bois and Mosteller agree within a percent or two, so the choice rarely changes the story.

Why does chemotherapy use BSA instead of weight?

Historically, BSA correlated better than raw weight with how many drugs distribute and clear, taming overdoses in large patients and underdoses in small ones. Modern practice adds caps, adjustments, and newer scaling research, all under clinical protocol.

Is BSA the same as BMI?

No. BMI is weight divided by height squared and classifies weight status; BSA is an area estimate in square meters used for scaling physiological quantities. They share inputs and nothing else.

Can I calculate BSA for a baby or child?

The arithmetic runs, but children are better served by pediatric formulas such as Haycock and by age-appropriate interpretation. A generic adult tool will compute a small number without telling you whether it is being used correctly.

Why do my BSA and a hospital's BSA differ slightly?

Different formula, rounding, or measured-versus-stated height and weight. Differences of a few hundredths of a square meter are ordinary; larger gaps usually mean different inputs rather than different math.

Why do Mosteller and Du Bois give slightly different answers?

They were fitted on different populations with different mathematics, so small differences — a percent or two for typical adults — are by design. Larger gaps indicate input problems, usually units, not a formula failure.

Which formula do hospitals use?

It depends on the protocol: oncology traditionally names Du Bois, quick bedside estimates use Mosteller, and pediatrics often uses Haycock. The protocol's named formula governs; the rest are estimates for context.

Can I use pounds and feet with a conversion in my head?

Convert carefully with fixed factors — 2.205 pounds per kilogram and 2.54 centimeters per inch — before entering anything. Mental approximations shift BSA by amounts that matter in any downstream arithmetic.

Is a BSA of 2.2 too high?

BSA is a size description, not a health grade, so there is no 'too high' in itself. In dosing contexts, protocols sometimes cap BSA around 2.0 to 2.2 for certain drugs — a treatment rule, not a judgment of the body.

Why does my child's BSA look so small compared with mine?

Because area scales with height times weight rather than weight alone, a child at a fifth of adult weight has roughly a third of adult area. That asymmetry is exactly why pediatric dosing is its own discipline.

Can this calculator tell me if my chemo dose is right?

No. Doses come from protocols that specify the rate, formula, caps, and rounding, verified by pharmacists. A calculator can show the arithmetic transparently; only the care team can tell you what applies to you.

Why is my BSA wildly different from my friend's despite similar weights?

Height enters the formula with substantial power, so tall and short bodies diverge even at equal weight. If the gap looks extreme, check units first — pounds-for-kilograms is the classic culprit — then compare heights honestly measured.

Which is more accurate, Du Bois or Mosteller?

For typical adults they are effectively equivalent; differences run a percent or two. Each has known bias at extremes of size, and named protocols govern which applies clinically. For everyday estimates, Mosteller's simplicity makes it the practical choice.

How accurate is BSA as a concept?

It is a regression estimate with real population-level error — formulas were fitted on measured groups, not individuals. Two decimals in square meters overstate the certainty slightly, which is why two-decimal honesty is the recommended stance.

Do I need BSA at all if I know my BMI?

For different purposes, yes. BMI answers a classification question; BSA scales quantities like cardiac index or dose rates. If no one is asking for your BSA, nothing is wrong — it is a specialist number, not a general health metric.

Is it safe to compute my own chemotherapy dose now that I understand the math?

No — and the understanding is still valuable. Real dosing adds protocol caps, rounding rules, cycle adjustments, and multi-step verification. Use your numeracy to follow the care team's work and ask sharper questions, not to compute.

Why does the same calculator give me slightly different numbers on different days?

Because you do. Weight fluctuates by kilogram across days and height by centimeter between morning and evening. Small input changes move BSA a little; that is honest variation, not instability in the tool.

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