Body Surface Area Calculator

Body Surface Area

{{ format(selectedEffectiveBsa) }} m²

{{ consensusBadgeText }} {{ referenceBadgeText }} Dose basis {{ weightBasisAppliedBadgeLabel }} BMI {{ ST.formatNumber(bmiValue, 1) }} ({{ bmiBand }}) Dose {{ ST.formatNumber(doseTotalMgRounded, 0) }} mg Capped {{ format(resolvedCapM2) }} m²

Height:

Weight:

Formula:

Patient profile:

Input check

{{ message }}

Sex for IBW/AdjBW:

Dose weight basis:

Obesity gate BMI:

BMI

Reference lane:

Reference BSA:

m²

Consensus tolerance:

{{ ST.formatNumber(consensus_tolerance_pct, 1) }}%

Spread alert threshold:

{{ ST.formatNumber(spread_alert_pct, 1) }}%

Cap profile:

Cap BSA:

m²

Dose per m²:

mg/m²

Dose rounding increment:

Round BSA to:

{{ round_dp }} dp

Formula	Raw BSA	Δ vs mean	Protocol BSA	Protocol dose	Note	Copy
{{ row.label }}	{{ row.rawBsaDisplay }}	{{ row.deltaDisplay }}	{{ row.protocolBsaDisplay }}	{{ row.protocolDoseDisplay }}	{{ row.note }}

Active reference

{{ format(resolvedReferenceBsa) }} m²

Closest anchor

—

Surface footprint

{{ format(bsaFt2, 2) }} ft²

Cohort	Reference BSA	Difference	Index	Lane note	Copy
{{ row.label }}	{{ row.referenceDisplay }}	{{ row.deltaDisplay }}	{{ row.indexDisplay }}	{{ row.note }}

BMI is below the current obesity gate, so actual body weight remains the active basis and adjusted or ideal rows are comparison-only.

Add a non-zero dose per m² to turn the protocol table into dose totals. BSA rows are still available for weight-basis review.

Weight basis	Weight	Selected BSA	Protocol BSA	Protocol dose	Δ vs actual	Status	Copy
{{ row.label }}	{{ row.weightDisplay }}	{{ row.rawBsaDisplay }}	{{ row.protocolBsaDisplay }}	{{ row.protocolDoseDisplay }}	{{ row.deltaDisplay }}	{{ row.status }}

Formula Surface Map

Consensus Drift Plot

Reference Lane Map

Tags: Body Metrics , Diagnostics

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Body surface area, usually shortened to BSA, is an estimate of total body area expressed in square metres. In medicine it is often used when weight alone is not enough to compare bodies for dose normalization, burn-size assessment, or other values that are indexed to body size. The number is meant to give scale, not a diagnosis.

BSA matters because body size does not rise in a simple straight line. Two people can share the same weight but have different heights, body proportions, and clinical context. A surface-area estimate can sometimes fit the job better than a plain kilogram value, but it is still an estimate built from population-derived equations rather than a direct bedside measurement.

Diagram showing height and weight combining into an estimated body surface area that can then be used for dose normalization, burn-area context, or other body-size-indexed measurements

Different equations can produce slightly different BSA values because they were derived from different cohorts and mathematical assumptions. In many routine adult measurements the gap is small. In children, people with obesity, and bodies at the low or high ends of size, the gap can widen enough to change how confident you feel about a single number.

This estimate is informational and should not replace medication orders, prescribing guidance, or clinician review. Any dose basis, cap, or pediatric or oncology decision still needs the governing protocol and professional judgment.

Technical Details:

All BSA equations try to model the same thing: external body area derived from height and weight. They do not weight those inputs equally. Height and weight are raised, scaled, or combined in different ways because body size does not increase linearly, and because the original studies behind the formulas used different samples and measurement methods.

That is why formula choice matters most when the person being measured is far from the population used to build an equation. The Du Bois equation is the historic reference. Mosteller keeps the same idea but simplifies the arithmetic. Haycock is often checked in smaller bodies because it was validated in infants, children, and adults. Gehan and George and Boyd remain useful comparison formulas when you want to see whether the estimate stays stable as body size changes.

A practical reading of the calculation is that one formula creates the selected BSA, then optional weight-basis, cap, and dose rules can modify the value used in the final protocol estimate.

\begin{array}{lcl} BSA {Mosteller}_{} & = & \sqrt{\frac{h_{cm} w_{kg}}{3600}} \\ AdjBW & = & IBW + 0.4 (Actual - IBW) \\ Dose {mg}_{} & = & BSA {effective}_{} regimen (mg / m^{2}) \end{array}

Formula symbols and meanings
Term	Meaning
`h_cm`	Height in centimetres after any unit conversion.
`w_kg`	Weight in kilograms, using either actual weight or the active dosing basis.
`IBW`	Ideal body weight derived from height and the selected sex entry for dosing comparisons.
`AdjBW`	Adjusted body weight used for obesity-related scenario checks.
`BSA effective`	The selected BSA after any weight-basis change and any active cap.

Body surface area formulas compared by the calculator
Formula	Exact expression used	Why it helps to compare
Mosteller	√((height in cm × weight in kg) / 3600)	Simple adult baseline and the default starting point for many quick checks.
Du Bois	0.20247 × height in m^0.725 × weight in kg^0.425	Historic comparison equation that still appears in medical literature.
Haycock	0.024265 × height in cm^0.3964 × weight in kg^0.5378	Useful pediatric cross-check because it was validated across infants, children, and adults.
Gehan and George	0.0235 × height in cm^0.42246 × weight in kg^0.51456	Alternative comparison path when you want another direct-measurement-derived estimate.
Boyd	0.0003207 × height in cm^0.3 × weight in g^{0.7285 − 0.0188 log10(weight in g)}	Often separates more at larger body size, which makes it a useful stress test for formula spread.

Interpretation becomes more reliable when the rules around agreement, reference comparison, and dose adjustments are explicit instead of implied.

Interpretation rules used by the calculator
Check	Rule used here	What it changes
BMI band	< 18.5 underweight 18.5 to < 25 healthy weight 25 to < 30 overweight ≥ 30 obesity	Provides size context beside the BSA estimate.
Obesity gate	Adjusted or ideal body weight becomes active only when BMI is at or above the selected gate. The default gate is 30, and it can be set from 25 to 45.	Controls whether actual weight stays active or a different dosing basis is applied.
Consensus status	Tight agreement: spread ≤ 60% of the tolerance Moderate agreement: spread > 60% of the tolerance and ≤ tolerance Wide agreement: spread > tolerance	Shows whether formula choice is likely to be a minor detail or a real decision point.
Reference index	Reference index = selected effective BSA ÷ chosen reference BSA × 100	Shows whether the selected BSA sits above, below, or equal to the chosen anchor body.
Cap rule	If a cap is active and selected dosing BSA is higher than that cap, the effective BSA becomes the cap value.	Changes the dose basis even when the raw selected formula is higher.

Everyday Use & Decision Guide:

A sensible first pass is to enter height and weight in the units you already trust, leave Mosteller selected for a routine adult check, and read the Formula Bench before looking at the headline dose total. When the five equations stay tightly grouped, the BSA estimate is stable enough that protocol details usually matter more than formula shopping.

The profile setting is there to change your reading, not just the label. Adult oncology keeps Mosteller in the foreground. Pediatric makes Haycock the first cross-check. Obesity review is the right path when the real question is whether actual, adjusted, or ideal body weight changes the protocol meaningfully once the BMI gate is met.

Use Reference Lane when you want scale. Built-in anchors range from newborn 0.25 m² and two-year-old 0.50 m² up to adult reference 1.73 m² and adult male 1.90 m², and Manual reference lets you set your own comparator.
Use Weight-Basis Protocol when body composition matters more than formula family. That table shows whether switching away from actual weight changes BSA enough to matter.
Use BSA Maps when a table is not enough. The formula bar chart, drift plot, and reference map make it easier to see whether disagreement is trivial or worth reviewing.

The most common mistake is to treat the reference index like a normal range or to treat a cap like a recommendation. Neither is true. The reference value is only an anchor for comparison, and the cap settings only show what happens if a protocol uses that ceiling. They do not tell you that a patient should be capped.

Before you trust the result, check four things: the units look right, the selected weight basis is actually active, the cap status matches the protocol you mean to model, and the formula spread is not hiding a wider dose window than the headline BSA suggests. If you need to hand the calculation to someone else, the JSON view preserves the inputs, active settings, and full output set in one place.

Step-by-Step Guide:

Use the shortest path that still matches the question you are trying to answer.

Enter height and weight in any supported unit. If the page warns that height or weight is unusually low or high, stop and fix the units before comparing formulas.
Choose the formula you want to headline. Mosteller is the default adult baseline, while Haycock is the main pediatric cross-check.
Select the patient profile that matches the task: Adult oncology, Pediatric, or Obesity review.
Open Advanced only when you need more than a straight BSA estimate. That is where sex for IBW or AdjBW, dose weight basis, obesity gate, reference lane, cap, dose per m², and rounding live.
Read Formula Bench first. Focus on raw BSA, delta versus mean, protocol BSA, and any dose window that appears across formulas.
If body composition is part of the decision, open Weight-Basis Protocol and compare actual, adjusted, and ideal rows. If scale matters, open Reference Lane. If you need visuals or a sharable payload, use BSA Maps or JSON.

Interpreting Results:

The first number to judge is not always the selected BSA. Often it is the formula spread. A spread near zero means the equations are telling nearly the same story. Once spread rises above the tolerance you set, the summary changes to Wide agreement, which is a cue to slow down and decide whether formula choice itself needs review.

The BMI badge is context, but the obesity gate is operational. If BMI is below the current gate, actual body weight remains the active basis even if you selected adjusted or ideal weight in the form. If BMI meets or exceeds the gate, the chosen alternative basis can take over. That distinction matters because the protocol table and dose total follow the active basis, not just the menu choice.

How to read the main outputs of the calculator
Output	What it means	When to stop and verify
Selected BSA	The value from the currently selected formula using the active dosing weight.	Verify the formula choice if the spread is wide or if pediatric or obesity context applies.
Reference index	100% means the selected effective BSA exactly matches the chosen reference anchor. More than 100% means above that anchor. Less than 100% means below it.	Verify that the chosen reference body is the one you meant to compare against. The index is descriptive, not a normality score.
Protocol dose	The dose based on effective BSA after any active cap and any rounding increment.	Verify cap status, weight basis, and dose-per-square-metre units before using the number outside the page.
Decision notes	Short plain-language cues about formula spread, reference comparison, and whether actual or alternative weight is active.	Read them when the summary looks surprising. They usually explain whether the surprise came from spread, weight basis, or a cap.

The reference lane is easy to overread. Being close to 1.73 m² or close to 100% of an anchor does not mean the result is automatically correct, normal, or appropriate for dosing. It only means the selected BSA is near that comparison body. The same caution applies to caps. A capped result is a modeled scenario, not evidence that a cap should be used.

A final trust check is to compare the selected result with the full table, not just the badge row. If the formula bench shows a larger milligram window than you expected, or if the active basis is different from the weight basis you chose, that is the moment to recheck settings before carrying the estimate forward.

Worked Examples:

Routine adult baseline

A height of 170 cm and weight of 65 kg produces a Mosteller BSA of about 1.752 m². Across all five formulas, the values cluster from about 1.752 to 1.763 m², which is only about 0.6% spread. That is a stable result. Against the built-in adult reference of 1.73 m², the selected BSA is about 101%, so the comparison says “near the adult anchor,” not “automatically normal.”

Pediatric cross-check

A child at 95 cm and 14 kg gives a Haycock BSA of about 0.610 m². The five formulas span roughly 0.599 to 0.626 m², which is about 4.5% spread. That is still a workable estimate, but the wider gap is exactly why the pediatric profile highlights Haycock and why the full formula table matters more in smaller bodies.

Obesity review with alternative weight bases

A woman at 170 cm and 110 kg has a BMI of about 38.1, so a default obesity gate of 30 is met. Using Mosteller with actual weight gives about 2.279 m². Switching the active basis to adjusted body weight drops the selected BSA to about 1.954 m². If the regimen is 75 mg/m² with 25 mg rounding and a 2.0 m² cap, both the capped actual-weight path and the adjusted-weight path round to 150 mg. That is a good reminder that different raw BSA values can converge at the final dose step once cap and rounding rules are applied.

Unit error recovery

If someone types 1.70 while centimetres are selected, the page reads that as 1.70 cm and raises an unusually low height warning. The fix is simple: switch the unit to metres or enter 170 cm before reading the formula bench. The same idea applies to a manual reference of 0. A zero or near-zero reference produces a meaningless index and should be corrected before comparison.

FAQ:

Why do the five formulas disagree?

They were derived from different study populations and use different mathematical weightings for height and weight. In many routine adult measurements the difference is small. In children, obesity, and very large or very small bodies, the spread can become large enough that formula choice deserves an explicit review.

Does 1.73 m² mean a normal result?

No. In this calculator, 1.73 m² is one reference anchor called Adult reference. Matching it means the selected effective BSA is similar to that anchor body. It does not diagnose health status and it does not confirm that a dose is correct.

When do adjusted or ideal body weight rows actually become active?

Only when BMI is at or above the current obesity gate. Below that threshold, actual body weight stays active even if you selected adjusted or ideal weight in the form. The protocol table still shows the alternative rows, but they remain comparison-only rows until the gate is met.

Why is the dose lower than the raw BSA would suggest?

Usually one of three things changed the final number: a cap was active, a non-actual weight basis became active, or the dose was rounded to 5, 10, 25, or 50 mg. Compare Selected BSA, Protocol BSA, and Protocol dose in the tables to see which rule made the difference.

What should I do if I see an input warning?

Treat it as a unit or setup check, not a harmless decoration. Very low or very high height and weight values often mean the wrong unit was selected. A very high dose per square metre can mean the regimen was entered in the wrong unit. A manual reference of 0 means the reference index cannot be trusted.

Is the calculation sent to a server?

No server-side calculation step is used for the BSA math, weight-basis comparison, reference indexing, or dose conversion in this tool. The estimate is generated in your browser, which makes it easier to test scenarios without sending the body measurements away for processing.

Glossary:

BSA: Body surface area, expressed here in square metres and estimated from height and weight.
Formula spread: The percentage gap between the highest and lowest formula results relative to the mean of all five formulas.
Reference index: The selected effective BSA divided by the chosen reference BSA, shown as a percentage.
Ideal body weight: A height-based comparison weight used for dosing review when the tool is asked to test an ideal-weight scenario.
Adjusted body weight: A blended weight built from ideal and actual body weight for obesity-related scenario testing.
Effective BSA: The final BSA used for dose conversion after any active weight-basis rule and any active cap.
Cap: A ceiling placed on the selected dosing BSA before the page converts a mg/m² regimen into milligrams.

References:

Flint B, Das JM, Hall CA. Body Surface Area. StatPearls. Updated February 6, 2025.
Mosteller RD. Simplified calculation of body-surface area. New England Journal of Medicine. 1987.
Haycock GB, Schwartz GJ, Wisotsky DH. Geometric method for measuring body surface area: a height-weight formula validated in infants, children, and adults. Journal of Pediatrics. 1978.
Sigurdsson TS, Lindberg L. Six commonly used empirical body surface area formulas disagreed in young children undergoing corrective heart surgery. Acta Paediatrica. 2020.
Pai MP. Drug dosing based on weight and body surface area: mathematical assumptions and limitations in obese adults. Pharmacotherapy. 2012.