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:

Examples: 170 cm, 1.70 m, 67 in, or 5 ft 7 in.

Weight:

Examples: 65 kg or 143 lb; enter current actual body weight.

Formula:

Default: Mosteller; compare Du Bois, Haycock, Gehan and George, and Boyd in Formula Bench.

Patient profile:

Accepted values: adult oncology, pediatric, or obesity review.

Sex for IBW/AdjBW:

Select the coefficient set required by the dosing protocol.

Dose weight basis:

Options: actual, adjusted, or ideal body weight; actual stays active below the gate.

Cap profile:

Choose No cap, Manual, 2.0 m², or 2.2 m² according to the active protocol.

Cap BSA:

Enter 0 for no ceiling; otherwise use the protocol cap in m².

m²

Input check

{{ message }}

Obesity gate BMI:

Allowed range: 25.0-45.0 BMI; default is 30.0.

BMI

Reference lane:

Preset anchors include adult 1.73 m² and pediatric examples; Manual unlocks custom BSA.

Reference BSA:

Enter a positive m² value; this field is editable only when Reference lane is Manual.

m²

Consensus tolerance:

Range: 1-12%; lower values flag formula disagreement sooner.

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

Spread alert threshold:

Range: 0-12%; set 0 to disable the extra spread alert.

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

Dose per m²:

Use 0 for BSA-only review; enter the regimen strength in mg/m² for dose totals.

mg/m²

Dose rounding increment:

Options: no rounding, 5, 10, 25, or 50 mg increments.

Round BSA to:

Allowed range: 0-5 decimal places; 2-3 dp is typical for review.

{{ 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 }}

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

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 }}

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Medication orders, indexed physiology reports, cardiac measurements, and pediatric comparisons often need a body-size adjustment before the number is fair to interpret. Body surface area, or BSA, turns height and weight into a single square-meter estimate for that scaling step.

Routine BSA is an estimate, not a literal tape-measure count of skin area. The common formulas were fitted from groups of measured people, so they assign slightly different weight to height and body mass. Those differences are usually modest near average adult sizes and more important for infants, very small adults, severe obesity, edema, pregnancy, amputation, or body proportions far from the original study groups.

The result is normally expressed in m². Many adults fall near 1.6 to 1.9 m², while infants and young children are much lower. The well-known 1.73 m² value is a reference size used to index some physiologic measurements. It is not a health target, a diagnosis, or proof that a medication plan is appropriate.

Height and weight feeding an empirical body surface area estimate before indexed measurement or dose review. — BSA is a size-scaling estimate. Formula choice and protocol limits can change how the number is used.

Common uses and cautions for body surface area
Use context	What BSA helps scale	Common caution
Medication review	mg/m² regimens and dose caps	The active protocol decides whether actual, adjusted, capped, or rounded values are allowed.
Physiology reports	Indexed measures such as values normalized to 1.73 m²	The reference number supports comparison, not a personal goal.
Pediatric checks	Body-size comparison across growth stages	Adult-derived formulas can behave differently at very small body sizes.
Obesity review	Scenario checks using actual, ideal, or adjusted weight	Lower arithmetic doses are not automatically safer or guideline-concordant.

BSA and body mass index are easy to confuse because both use height and weight. BMI is a weight-for-height ratio used for broad weight-category screening. BSA is a surface-area estimate used to scale physiologic quantities or a dose calculation. A BMI category may trigger a protocol rule, but the BSA value supplies the size-scaling arithmetic.

Health note: BSA arithmetic supports review; it does not choose a drug, confirm a regimen, replace organ-function checks, or override prescribing protocols, toxicity history, laboratory results, or clinician judgment.

How to Use This Tool:

Start with measured height and current actual body weight, then add protocol choices only when they apply to the review you are doing.

Enter Height in cm, m, in, or ft/in and Weight in kg or lb. The summary appears once both values are positive.
Choose the required Formula. Mosteller is the default, and the Formula Bench keeps Du Bois, Haycock, Gehan and George, and Boyd visible for comparison.
Select a Patient profile so the guidance text matches adult oncology, pediatric, or obesity-review use. The profile changes notes and recommended cross-checks, not the equations themselves.
Set Sex for IBW/AdjBW before comparing ideal or adjusted body weight. That choice is used only for the Devine ideal-weight and adjusted-weight scenarios.
Choose the Dose weight basis. Actual body weight remains active until BMI meets the Obesity gate BMI; the Weight-Basis Protocol table marks the active row with Status.
Apply a Cap profile only when the protocol calls for one. Manual cap values, 2.0 m², and 2.2 m² reduce Protocol BSA only when the selected dosing BSA is above the cap.
Enter Dose per m² when you want a dose check. The result shows Protocol dose as a whole-mg value, with optional rounding to the selected 5, 10, 25, or 50 mg increment.

If an input warning appears, fix the unit or range before using the tables. The most common corrections are switching height to ft/in, confirming kg versus lb, lowering an unusually high mg/m² entry, or giving Manual reference BSA a positive value.

Interpreting Results:

The large Body Surface Area value is the selected effective BSA. It reflects the chosen formula, the active dose weight basis, and any cap that lowers the dosing BSA. If the Capped badge appears, compare Raw BSA and Protocol BSA in the Formula Bench before carrying the value into a dose review.

Formula spread is the main confidence check. Tight agreement means the five empirical formulas give similar BSA values for the entered height and weight. Wide agreement means formula choice has a meaningful arithmetic effect, especially if a mg/m² dose turns that spread into a visible dose window.

Reference Lane results compare the selected effective BSA with preset anchors such as newborn, child, adult female, adult reference, and adult male values. Read Index as a scaling comparison. A value above or below 100% is not a health grade.

The Weight-Basis Protocol table is a scenario check. Actual, ideal, and adjusted body weight rows can show different Selected BSA, Protocol BSA, and Protocol dose. Use the row required by the source protocol, not the row that produces the preferred number.

Technical Details:

BSA equations are empirical height-weight models. They estimate total external surface area from body dimensions because direct measurement is impractical in routine care. Agreement is usually closest near the body sizes represented in the formula's source data and weaker when body composition, age, edema, amputation, pregnancy, or extreme size changes the relationship between height, weight, and surface area.

The calculation first normalizes height to centimeters and weight to kilograms. Formula outputs are compared on the same normalized inputs, while protocol scenarios recalculate BSA with the active dosing weight when ideal or adjusted weight is allowed.

Formula Core:

Mosteller is the default formula. With height in centimeters and weight in kilograms, the square-root form is:

BSA = \sqrt{\frac{H_{cm} \times W_{kg}}{3600}}

Formula spread measures how far the five formula results separate from one another:

spread % = \frac{maximum BSA - minimum BSA}{mean BSA} \times 100

When a cap is active, the effective BSA is the lower of the selected dosing BSA and the cap:

effective BSA = \min (dosing BSA, cap)

A dose check multiplies the effective BSA by the entered regimen strength, then displays a whole-mg value or the nearest selected increment:

dose = effective BSA \times mg per m2

Body surface area formulas included in the calculator
Formula	Equation form	Useful interpretation
Mosteller	square root of height cm times weight kg divided by 3600	Simple adult baseline and default starting point.
Du Bois	0.20247 x height m^0.725 x weight kg^0.425	Historic comparison formula with height and weight exponents.
Haycock	0.024265 x height cm^0.3964 x weight kg^0.5378	Important pediatric cross-check because it was validated from infants through adults.
Gehan and George	0.0235 x height cm^0.42246 x weight kg^0.51456	Direct-measurement alternative from a larger sample than the original Du Bois work.
Boyd	0.0003207 x height cm^0.3 x weight g^{0.7285 - 0.0188 log10(weight g)}	Weight-dependent exponent that can separate more at larger sizes.

Protocol scenario rules used by the body surface area calculator
Rule	Boundary or equation	Effect on displayed result
BMI category	<18.5 underweight, 18.5 to <25 healthy weight, 25 to <30 overweight, 30 or greater obesity	Shown as the BMI badge and used to judge the obesity gate.
Obesity gate	BMI must be greater than or equal to the selected gate, range 25.0 to 45.0	Below the gate, actual body weight remains the active dose basis.
Ideal body weight	Female 45.5 kg or male 50 kg, plus 2.3 kg for each inch above 60 inches	Feeds the ideal-weight scenario row.
Adjusted body weight	IBW + 0.4 x the positive difference between actual weight and IBW	Feeds the adjusted-weight scenario row when actual weight exceeds IBW.
Manual reference BSA	Must be positive to make a meaningful index	Controls the custom reference comparison.
Input warnings	Height below 45 cm or above 250 cm, weight below 2 kg or above 350 kg, dose above 2000 mg/m²	Prompts a unit or range check before using the output.

For a 170 cm and 70 kg adult, Mosteller gives sqrt(170 x 70 / 3600), about 1.82 m². A 75 mg/m² dose check uses 1.82 x 75, or about 137 mg before whole-mg display rounding. If a selected formula and weight basis produced 2.28 m² with a 2.0 m² cap, the effective BSA would be 2.0 m² and the same 75 mg/m² check would display 150 mg.

Limitations and Accuracy Notes:

BSA is useful for scaling, but it remains an estimate. Small arithmetic differences can become important in high-risk medication review, while large clinical decisions still require the source protocol and patient-specific assessment.

Formula agreement does not prove that BSA-based dosing is clinically appropriate for the patient or medication.
Routine capping or substituting ideal weight can conflict with some oncology guidance unless the regimen or institution specifically requires it.
BSA does not account for kidney function, liver function, pharmacogenomics, performance status, drug exposure targets, recent toxicity, or drug concentration limits.
Edema, amputation, pregnancy, severe cachexia, and unusual body proportions can make height-weight formulas less representative of measured surface area.
No names, identifiers, or clinical record details are needed for the arithmetic. Avoid placing patient-identifying information in notes, shared URLs, or downloaded files.

Worked Examples:

Average adult comparison: Enter 170 cm and 70 kg, keep Formula on Mosteller, and leave Cap profile uncapped or at 0. The summary Body Surface Area is about 1.82 m². In Formula Bench, check Raw BSA and Δ vs mean; small percentage differences suggest formula choice has little arithmetic effect for this body size.

Cap-sensitive dose review: A selected BSA of 2.31 m² with Cap profile set to 2.0 m² changes Protocol BSA to 2.00 m². With Dose per m² set to 75, Protocol dose displays 150 mg. The Capped badge is the cue to verify that the cap belongs to the regimen.

Obesity gate scenario: If BMI is 32 and Dose weight basis is adjusted body weight, the Weight-Basis Protocol table marks adjusted body weight as Active basis. If the same patient has BMI 29 with a gate of 30, actual body weight remains active and adjusted or ideal rows show Gate not met.

Troubleshooting a warning: A 67 value entered as centimeters produces an unusually low height warning. Change Height to inches or ft/in, then confirm that BMI, Body Surface Area, and the reference Index return to expected ranges before reading the dose table.

FAQ:

Which BSA formula should I choose?

Use the formula named by the protocol or source document. If no formula is specified, Mosteller is a common starting point, while Haycock deserves attention for pediatric review.

Why do the formulas disagree?

Each equation came from different measurement data and uses different height and weight exponents. The Formula Bench and Consensus Drift Plot show whether that difference is small or dose-relevant for the entered body size.

Does a 1.73 m² reference mean normal?

No. The 1.73 m² adult reference is an indexing anchor. In Reference Lane, the Index tells you how the selected BSA compares with that anchor, not whether the person is healthy.

Should obesity dosing use actual, adjusted, or ideal weight?

Follow the medication protocol. The table can compare actual, adjusted, and ideal body weight scenarios, but it cannot decide which basis is clinically appropriate.

Why did I get an input warning?

Warnings appear for unusual height, weight, very high mg/m² entries, or a nonpositive manual reference BSA. Check the unit first, then re-read the summary and table rows after the warning clears.

Glossary:

BSA: Body surface area, an estimated external body surface reported in square meters.
Formula spread: The percent range between the highest and lowest formula estimates relative to their mean.
Protocol BSA: The BSA value after the selected formula, active weight basis, and cap rules are applied.
Reference Lane: A comparison between the selected BSA and preset or manual reference BSA values.
Obesity gate: The BMI threshold that must be met before adjusted or ideal weight can become the active dose basis.
Adjusted body weight: A protocol scenario that adds 40% of the positive difference between actual and ideal body weight to ideal body weight.

References:

Simplified calculation of body-surface area, New England Journal of Medicine, 1987.
A height-weight formula to estimate the surface area of man, Proceedings of the Society for Experimental Biology and Medicine, 1916.
Geometric method for measuring body surface area, Journal of Pediatrics, 1978.
Estimation of human body surface area from height and weight, Cancer Chemotherapy Reports, 1970.
Appropriate Systemic Therapy Dosing for Obese Adult Patients with Cancer: ASCO Guideline Update, Journal of Clinical Oncology, 2021.
Adult BMI Categories, Centers for Disease Control and Prevention, 2024.