Body Surface Area Calculator
Calculate body surface area using the Mosteller, Du Bois, Haycock and Gehan-George formulas, the basis for chemotherapy dosing and cardiac index.
Body Surface Area Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Body surface area is estimated from height and weight. The Mosteller formula — the square root of height in centimetres times weight in kilograms, divided by 3,600 — is the most widely used because it is simple and accurate. A typical adult BSA is 1.6 to 2.0 m².
Formula
Mosteller's 1987 simplification reduced earlier multi-term power equations to a single square root while matching them to within a few percent.
Worked Example
- Multiply height by weight. 178 × 82 = 14,596.
- Divide by 3,600. 14,596 ÷ 3,600 = 4.0544.
- Take the square root. √4.0544 = 2.014 m².
- Cross-check with Du Bois. 0.007184 × 1780.725 × 820.425 = 2.001 m², within 0.7%.
- Apply to dosing. At 500 mg/m² the dose is 1,007 mg.
Clinical note. The four formulas agree to within about 1.3% for this subject, which is typical for average adult builds. Divergence grows at the extremes of height and weight, and in paediatrics Haycock and Gehan-George are generally preferred as they were derived with children in the sample.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Chemotherapy and high-risk drug dosing
Cytotoxic agents are dosed per square metre because BSA correlates better with metabolic rate and organ clearance than weight alone. The narrow therapeutic index of these drugs makes the choice of formula clinically relevant. Compare with the Lean Body Mass Calculator, used for other agent classes.
Indexing cardiac and renal measurements
Cardiac output is normalised to BSA as the cardiac index, typically 2.5 to 4.0 L/min/m², so patients of different sizes can be compared. Glomerular filtration rate is conventionally indexed to 1.73 m² for the same reason.
Estimating burn extent and fluid resuscitation
Burn severity is assessed as a percentage of total body surface area, and fluid resuscitation formulas scale from it. Accurate BSA is therefore the foundation of the entire calculation in emergency care.
Methodology & Editorial Standards
Four published formulas are evaluated in parallel. Mosteller (1987) is presented as the headline figure because its simplicity has made it the clinical standard and it agrees closely with the older equations across normal adult ranges. Du Bois and Du Bois (1916) is the historical reference but was derived from only nine subjects and is known to underestimate in obesity. Haycock (1978) and Gehan and George (1970) were derived from larger samples including children and are generally preferred in paediatrics. Cardiac output is shown at a reference cardiac index of 3.0 L/min/m². Dose calculation is a straightforward product of BSA and the specified dose per square metre, and must never substitute for a prescriber's judgement. This engine implements the published formula exactly as specified in the source literature, with inputs validated for physiological plausibility before evaluation. Values outside the domain of the formula return an em-dash rather than a misleading figure.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Body Surface Area Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What is a normal body surface area?
Around 1.6 to 2.0 m² for most adults. The commonly quoted average is 1.73 m² for adults and 1.9 m² for adult men specifically, which is why 1.73 m² appears as the indexing standard for kidney function.
Which BSA formula is most accurate?
Mosteller is the clinical standard and agrees with the others to within a few percent across normal adult ranges. Haycock and Gehan-George are generally preferred in paediatrics because children were included in their derivation samples.
Why is chemotherapy dosed by body surface area?
Because BSA correlates better with metabolic rate, blood volume and renal clearance than weight alone. Given the narrow therapeutic index of cytotoxic drugs, the improvement in scaling is clinically significant.
Is BSA better than weight for all drug dosing?
No. It suits cytotoxic agents and some others, but drugs distributing primarily into lean tissue are better dosed on lean body mass or adjusted body weight. The right scalar depends on the pharmacokinetics of the specific drug.
How accurate are BSA formulas?
They estimate to within roughly 5% for average builds when compared with direct measurement. Accuracy falls at the extremes, particularly in severe obesity where Du Bois in particular tends to underestimate.
What is cardiac index?
Cardiac output divided by body surface area, normally 2.5 to 4.0 L/min/m². Indexing allows meaningful comparison between patients of very different sizes, since a large person naturally requires greater absolute output.
Why is GFR indexed to 1.73 m²?
It was the average BSA of young adults in the original normative studies from the 1920s. Indexing to it allows kidney function to be compared across body sizes, though the convention is increasingly questioned for very large or small patients.
Can BSA be used for children?
Yes, and it is routine in paediatric dosing. Haycock and Gehan-George are usually preferred as their derivation samples included children; Mosteller nonetheless performs acceptably across most paediatric ranges.
How is BSA used in burns?
Burn extent is expressed as a percentage of total body surface area, assessed with the rule of nines or Lund-Browder charts. Fluid resuscitation formulas scale directly from that percentage and the patient's weight.
Why does the Mosteller formula divide by 3,600?
It is a normalising constant fitted so that the square root of the height-weight product returns square metres. Mosteller derived it in 1987 as a simplification of earlier power-law equations, and it happens to be 60 squared.
Does BSA change with body composition?
Only through height and weight, since the formulas see nothing else. Two people of identical height and weight receive identical BSA regardless of how muscular either is.
Should I use BSA to assess my own health?
Not really — it is a clinical scaling parameter rather than a health metric. BMI, body fat percentage and waist measurements are far more informative for personal assessment.
Is this body surface area calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.