Body Surface Area Calculator

Current result

{{ 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 }}

Embed:

Customize

Include current inputs

Size

Advanced

Width

Height

Aspect ratio

Max height

Collapsible embed

Allow fullscreen

Referrer policy

Sandbox tokens

A height and weight measurement can answer more than one body-size question. Body mass index describes weight relative to height, but body surface area, or BSA, estimates the external area of the body in square meters. That surface-area estimate is used when a value needs to be scaled to body size instead of read as a raw weight or height number.

BSA matters most in settings where "average adult" is too rough a comparison. Medication protocols may express a regimen in mg per m². Kidney and cardiac reports may index measurements to a reference body size. Pediatric review often needs a way to compare bodies that are changing quickly during growth. In each case, the BSA number is a scaling aid, not a direct observation of skin area.

Body surface area use contexts
Situation	Why BSA appears	What still needs judgment
Dose review	Converts a mg/m² regimen into a total dose.	The source protocol decides formula, cap, rounding, and weight basis.
Physiology indexing	Compares a value with a reference body size such as 1.73 m².	The index is a comparison anchor, not a diagnosis by itself.
Pediatric sizing	Accounts for large body-size differences across ages.	Formula choice matters more for infants and small children.
Obesity review	Lets actual, adjusted, and ideal-weight scenarios be compared.	Lower arithmetic output is not automatically the clinically correct one.

The common equations are empirical. Researchers measured surface area in sample groups, then fitted formulas from height and weight. That history explains why Mosteller, Du Bois, Haycock, Gehan and George, and Boyd can give slightly different answers for the same person. Near average adult sizes, the gap is often small. At very small size, high body weight, unusual body proportions, edema, pregnancy, amputation, or severe wasting, the gap can become large enough to change a dose review or reference comparison.

BSA turns body measurements into a square-meter estimate, then protocol and reference rules decide how that estimate is used.

The 1.73 m² reference value is easy to misread. It is a conventional adult indexing size, not a desirable body size. Likewise, a BMI category can help decide whether an obesity dosing rule should be considered, but BMI is not the BSA result. BSA supplies the scaling arithmetic; the clinical or reporting protocol supplies the rule for using it.

Health note: BSA arithmetic can support a review, but it cannot choose a medication, prove a regimen is safe, replace organ-function checks, or override the prescribing protocol, lab results, toxicity history, or clinician judgment.

How to Use This Tool:

Start with the measured body size, then add formula, reference, and dosing choices only when they match the protocol or report you are checking.

Enter Height and Weight with the matching units. The summary appears after both values are positive; if a unit warning appears, fix that before reading tables.
Choose Formula. Use the formula named by the source protocol when one is specified; otherwise compare the default Mosteller value against the other rows in Formula Bench.
Select Patient profile to tune the guidance for adult oncology, pediatric, or obesity review. The profile changes the recommendation text, not the equations.
Set Sex for IBW/AdjBW before using ideal or adjusted-weight scenarios. The Devine ideal-weight calculation needs that choice.
Choose Dose weight basis only when a protocol asks for actual, adjusted, or ideal body weight. Adjusted or ideal weight cannot become active until BMI is greater than or equal to the Obesity gate BMI.
Use Cap profile and Cap BSA only when the regimen states a ceiling. A cap lowers Protocol BSA when the selected dosing BSA exceeds that ceiling.
Enter Dose per m² for a medication arithmetic check, then set Dose rounding increment if the protocol rounds to 5, 10, 25, or 50 mg. Keep it at no rounding for BSA-only review.
Check Formula Bench, Reference Lane, and Weight-Basis Protocol. The result is ready to use only when input warnings are cleared and the active formula, weight basis, cap, and reference row match the source protocol.

For a common recovery path, a height of 67 entered as centimeters will trigger an unusually low height warning. Switch the unit to inches or ft/in, then confirm that BMI, BSA, and the reference index return to the expected range before using any dose rows.

Interpreting Results:

Body Surface Area is the selected effective BSA. It reflects the chosen formula, the active dosing weight, and any cap that reduces the dosing BSA. When a Capped badge appears, read Raw BSA and Protocol BSA side by side before carrying the value into a regimen worksheet.

Formula spread is the main arithmetic confidence check. Tight agreement means the five formulas are close for the entered height and weight. Wide agreement means formula choice has become a meaningful part of the result, especially when Dose per m² turns that spread into a visible mg window.

Body surface area result checks
Output cue	Boundary	How to read it
Tight agreement	Formula spread is at or below 60% of the selected consensus tolerance.	Formula choice is unlikely to be the main arithmetic issue.
Moderate agreement	Formula spread is above 60% of tolerance but still at or below the tolerance.	Check the table before using the selected formula alone.
Wide agreement	Formula spread is greater than the selected consensus tolerance.	Confirm the protocol formula and consider the dose window.
Reference index	Selected effective BSA divided by the reference BSA, multiplied by 100%.	An index above or below 100% is a size comparison, not a health grade.

A polished dose value can still be wrong if the wrong protocol rule was selected. Verify the source document first, then use the Weight-Basis Protocol status row and Reference Lane comparison to catch false confidence from a preferred-looking number.

Technical Details:

BSA formulas model an external surface from two easy measurements. Height changes the length scale, weight changes the mass-size relationship, and each equation assigns different exponents to those quantities. Because the formulas were fitted from measurement data rather than derived from anatomy, agreement depends on how closely the current body size resembles the source populations.

All formulas use normalized height and weight before comparison. Height is converted to centimeters, weight is converted to kilograms, and Boyd also converts weight to grams for its weight-dependent exponent. Protocol rows can recalculate BSA with actual, ideal, or adjusted body weight, but the chosen formula stays the same within a row comparison.

Formula Core:

Mosteller is the default adult baseline. With height in centimeters and weight in kilograms:

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

For 170 cm and 70 kg, the substitution is sqrt(170 x 70 / 3600), which is about 1.82 m². Display precision is controlled separately by Round BSA to, so the same stored value can appear with fewer or more decimals in summaries and tables.

The formula comparison span is expressed relative to the mean of the formula outputs:

spread percent = \frac{max BSA - min BSA}{mean BSA} \times 100

When a cap exists, the protocol value is the smaller of the dosing BSA and the cap. A dose check multiplies that effective value by the entered mg/m² regimen and then rounds to whole milligrams or to the selected increment:

effective BSA = \min (dosing BSA, cap)

dose total = effective BSA \times mg per m2

Body surface area formulas used in this calculator
Formula	Equation form	Practical role
Mosteller	sqrt(height cm x weight kg / 3600)	Default adult baseline and quick cross-check.
Du Bois	0.20247 x height m^0.725 x weight kg^0.425	Historic formula useful for legacy protocols.
Haycock	0.024265 x height cm^0.3964 x weight kg^0.5378	Pediatric-friendly comparison from infant, child, and adult validation.
Gehan and George	0.0235 x height cm^0.42246 x weight kg^0.51456	Direct-measurement alternative from a larger source sample than Du Bois.
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 and validation rules for body surface area review
Rule	Exact boundary or calculation	Effect
BMI	weight kg / height m squared	Shows underweight, healthy weight, overweight, or obesity category.
Adult BMI bands	<18.5, 18.5 to <25, 25 to <30, and >=30	Supports the BMI badge and obesity gate review.
Obesity gate	BMI must be >= the selected gate; allowed gate range is 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 protocol row.
Adjusted body weight	IBW + 0.4 x max(0, actual weight - IBW)	Feeds the adjusted-weight protocol row.
Input warnings	height <45 cm or >250 cm, weight <2 kg or >350 kg, dose >2000 mg/m², or nonpositive manual reference BSA	Prompts a unit, range, or protocol check before relying on outputs.

A 2.28 m² selected dosing BSA with a 2.0 m² cap becomes an effective BSA of 2.0 m². At 75 mg/m², the dose total is 150 mg before any selected dose increment changes the display. Without the cap, the same regimen would calculate from 2.28 m², or 171 mg before rounding.

Limitations and Accuracy Notes:

BSA is useful because it is reproducible, but it is still a formula estimate. In high-risk medication review, a small BSA change can matter, while a large decision still depends on the protocol and the patient record.

Formula agreement does not prove that BSA-based dosing is appropriate for the patient or drug.
Routine dose capping or substituting ideal weight may conflict with oncology guidance unless the regimen or institution specifies that rule.
BSA does not include kidney function, liver function, pharmacogenomics, performance status, drug exposure targets, recent toxicity, or concentration limits.
Edema, amputation, pregnancy, severe cachexia, and unusual body proportions can make height-weight formulas less representative.
No names, identifiers, or record numbers are needed for the arithmetic. Avoid putting patient-identifying details in shared URLs, copied JSON, or downloaded files.

Worked Examples:

Average adult formula check

Enter 170 cm and 70 kg, keep Formula on Mosteller, and leave the cap at none or 0. Body Surface Area is about 1.82 m². In Formula Bench, a narrow Formula spread suggests the formula choice has little arithmetic effect for this body size.

Cap-sensitive dose review

A selected dosing BSA of 2.28 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 cap badge is the cue to confirm that the source regimen really uses a 2.0 m² ceiling.

Obesity gate comparison

If BMI is 32 and Dose weight basis is adjusted body weight, the Weight-Basis Protocol table can mark adjusted body weight as Active basis. If the same patient is below the selected gate, actual body weight remains active and the adjusted or ideal rows show Gate not met.

Unit warning recovery

A value of 67 entered as centimeters produces an unusually low height warning. Change Height to inches or ft/in, then re-read BMI, Body Surface Area, and Reference Lane before using the dose table.

FAQ:

Which BSA formula should I use?

Use the formula named by the protocol or report. If no formula is named, Mosteller is a common adult starting point, and Haycock is the key pediatric cross-check in the comparison set.

Why do five formulas produce different BSA values?

They were fitted from different measurement data and use different height and weight exponents. Formula Bench and Consensus Drift Plot show whether the difference is small or dose-relevant for the entered measurements.

Is 1.73 m² a normal BSA target?

No. The 1.73 m² value is a reference size used for indexing. In Reference Lane, Index compares the selected BSA with that anchor; it does not grade health.

Can this decide actual, adjusted, or ideal weight dosing?

No. The calculator compares those scenarios and enforces the selected BMI gate, but the medication protocol decides which weight basis is appropriate.

Why did an input warning appear?

Warnings appear for unusual height or weight, very high mg/m² entries, or a manual reference BSA that is not positive. Check units first, then confirm that the summary and tables look reasonable 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 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 body weight can become active.
Adjusted body weight: Ideal body weight plus 40% of the positive difference between actual and 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, March 19, 2024.