Body Surface Area Calculator: Mosteller Formula Explained

A body surface area calculator estimates the external surface area of the human body in square metres (m²). The widely used Mosteller formula calculates BSA from height and weight: BSA (m²) = √([height in cm × weight in kg] ÷ 3600). Although the calculation is simple, the result should be interpreted within the relevant clinical protocol rather than used as an independent treatment recommendation.

What Is Body Surface Area?

Body surface area, usually abbreviated as BSA, is an estimate of the total external surface area of the body. Unlike body mass index, which relates weight to height and is commonly used to classify weight status, BSA is primarily used to standardize selected physiological measurements and medication doses.

BSA cannot usually be measured directly during routine clinical care. Instead, it is estimated with mathematical equations based on height and weight. Several equations are available, including the Mosteller, Du Bois and Du Bois, Haycock, and Gehan and George formulas.

The Mosteller formula is popular because it is straightforward, requires only two readily available measurements, and can be calculated with a basic calculator.

Mosteller Body Surface Area Formula

The Mosteller formula is:

BSA (m²) = √([height in centimetres × weight in kilograms] ÷ 3600)

An equivalent way to write the equation is:

BSA (m²) = √(height in cm × weight in kg) ÷ 60

Both versions produce the same result.

Measurements required

• Height: Entered in centimetres.
• Weight: Entered in kilograms.
• Result: Reported in square metres.

Accurate and current measurements are important, especially when BSA is being used as part of medication dosing or serial clinical assessment.

How to Calculate BSA Step by Step

Consider an adult who is 170 cm tall and weighs 70 kg.

1. Multiply height by weight: 170 × 70 = 11,900.
2. Divide by 3600: 11,900 ÷ 3600 = 3.3056.
3. Calculate the square root: √3.3056 = 1.818.
4. Round appropriately: BSA ≈ 1.82 m².

For this example, the estimated body surface area is approximately 1.82 m².

Another example

For a child who is 110 cm tall and weighs 20 kg:

1. Multiply height by weight: 110 × 20 = 2,200.
2. Divide by 3600: 2,200 ÷ 3600 = 0.6111.
3. Calculate the square root: √0.6111 = 0.782.

The estimated BSA is approximately 0.78 m². This value is an estimate only; any medication calculation must still follow the relevant paediatric protocol and be checked by an appropriately qualified professional.

Converting Imperial Measurements

The standard Mosteller equation shown above requires height in centimetres and weight in kilograms. Imperial measurements should therefore be converted before calculating BSA.

• Inches to centimetres: multiply inches by 2.54.
• Pounds to kilograms: divide pounds by 2.2046.

For example, a height of 68 inches is 172.72 cm, while a weight of 154 pounds is approximately 69.85 kg. Using unconverted inches or pounds in the metric formula will produce an incorrect result.

What Is BSA Used For?

Body surface area is used in several areas of medicine, but its purpose depends on the clinical setting.

Medication dosing

Some medications are prescribed in units such as milligrams per square metre, written as mg/m². A protocol dose may be multiplied by the patient’s BSA to calculate an initial dose:

Calculated dose = prescribed dose per m² × BSA

For example, a protocol specifying 100 mg/m² for a patient with a BSA of 1.82 m² would produce a mathematical dose of 182 mg. This does not automatically mean that 182 mg should be administered. Clinical protocols may require dose rounding, maximum-dose limits, dose capping, organ-function adjustments, toxicity modifications, laboratory review, or other safety checks.

BSA-based dosing is particularly associated with oncology, although it is also used for selected medications in other specialties and in paediatric care.

Indexing physiological measurements

Some cardiovascular and renal measurements may be indexed to a standard body surface area so that results can be compared across people of different body sizes. Examples include cardiac index and certain reported kidney-function measurements.

Indexing can improve comparability, but a value normalized to a standard BSA is not always interchangeable with a patient’s absolute, non-indexed value. This distinction may matter when evaluating kidney function for medication dosing.

Paediatric calculations

BSA is sometimes used in paediatric medication protocols because children vary substantially in body size. However, not every paediatric medicine is dosed by BSA. Many are dosed by body weight, age, indication, or a combination of factors.

The prescribing guideline for the specific medicine should always determine which method is appropriate.

How to Interpret a BSA Result

A BSA result is not classified as normal, overweight, underweight, healthy, or unhealthy. It is a continuous estimate of body size used for a specific clinical purpose.

Interpretation therefore depends on questions such as:

• Why was BSA calculated?
• Which formula does the relevant protocol specify?
• Are the height and weight measurements current and accurate?
• Does the protocol use actual, ideal, adjusted, or another defined body weight?
• Is the reported clinical value indexed to a standard BSA?
• Are there maximum-dose, dose-rounding, or dose-adjustment rules?

A larger BSA does not by itself indicate illness, and a smaller BSA does not by itself indicate poor health.

Mosteller Formula Versus Other BSA Equations

The Mosteller formula is one of several equations used to estimate body surface area.

For many people, different equations produce broadly similar results. Differences may become more relevant at extremes of body size, in infants, or when a protocol was validated using a particular formula.

When a study, hospital policy, prescribing guideline, or treatment protocol names a specific BSA equation, that equation should generally be used consistently.

Limitations of the Mosteller Formula

BSA is an estimate

The formula estimates surface area indirectly from height and weight. It does not measure the body’s external surface directly and does not account for all variation in body shape or composition.

Body composition is not represented

Two people with the same height and weight can have different proportions of fat, muscle, fluid, and lean tissue. Their calculated BSA may be identical even though the way they distribute or eliminate a medication differs.

Extremes of body size require caution

Formula differences and dosing uncertainty may be more clinically relevant in severe obesity, very low body weight, marked oedema, major fluid shifts, amputations, or unusually small or large patients. The appropriate weight to enter may also depend on the protocol.

BSA does not predict drug exposure precisely

Drug exposure and toxicity can be influenced by kidney function, liver function, genetics, age, interactions, disease characteristics, previous treatment, and other factors. BSA-based dosing may reduce some body-size variation, but it does not remove interpatient pharmacokinetic variability.

Input errors can affect the result

Common mistakes include entering height in metres instead of centimetres, using pounds without conversion, reversing height and weight fields, or using an outdated weight after a clinically significant change.

Important Safety Checks for BSA-Based Dosing

A BSA calculation should be treated as one component of a wider prescribing process. Before a BSA-based medicine is prescribed or administered, the clinical team may need to confirm:

• The patient’s identity, current height, and current weight.
• The correct BSA equation and measurement units.
• The prescribed dose expressed in mg/m² or another appropriate unit.
• The indication, treatment cycle, and protocol version.
• Kidney and liver function.
• Blood counts and other relevant laboratory results.
• Previous adverse effects or dose reductions.
• Maximum doses, dose caps, and rounding policies.
• Potential interactions and contraindications.
• An independent calculation check where required.

A calculator can reduce arithmetic effort, but it cannot confirm whether a prescription is appropriate for a particular patient.

Common BSA Calculation Errors

• Using metres instead of centimetres: Entering 1.70 instead of 170 will produce a major error.
• Using pounds as kilograms: A weight of 154 pounds must not be entered as 154 kg.
• Rounding too early: Keep additional decimal places during intermediate calculations and round the final result according to local policy.
• Confusing BSA with BMI: They are different measurements used for different purposes.
• Applying the result to every medication: Only use BSA dosing when the specific protocol calls for it.
• Ignoring clinical adjustments: The calculated mathematical dose may require modification based on laboratory results, organ function, toxicity, or protocol limits.

BSA Versus BMI

Body surface area and body mass index both use height and weight, but they answer different questions.

A BSA value should not be interpreted using BMI categories, and a BMI result cannot replace BSA in a protocol that specifically requires dosing per square metre.

Frequently Asked Questions

What is the Mosteller formula for BSA?

The formula is BSA in m² = √([height in centimetres × weight in kilograms] ÷ 3600).

What unit is used for body surface area?

Body surface area is normally reported in square metres, written as m².

Can BSA determine the correct medication dose?

BSA can provide one input for medicines prescribed per square metre, but it cannot independently determine a safe dose. The medicine, indication, protocol, organ function, laboratory results, dose limits, and individual patient factors must also be considered.

Is the Mosteller formula suitable for children?

It is used in many paediatric settings, but the relevant paediatric guideline should specify whether BSA, body weight, age, or another dosing method is required. Extra caution is necessary for infants and very small children.

Should actual body weight always be used?

Not necessarily. The correct weight definition depends on the treatment protocol and clinical situation. Clinicians should not substitute ideal or adjusted weight unless the relevant guidance directs them to do so.

Why do different BSA calculators give different answers?

They may use different equations, measurement units, conversion factors, or rounding rules. Confirm the formula and units before comparing results.

Summary

The Mosteller body surface area calculator estimates BSA with the equation √([height in cm × weight in kg] ÷ 3600). It is simple and widely used for selected medication calculations and indexed physiological measurements. However, BSA is an estimate rather than a direct measurement, and it does not account for organ function, body composition, pharmacokinetics, toxicity, or all factors that influence treatment decisions. Always follow the formula, units, and adjustment rules specified by the relevant clinical protocol.

Medical disclaimer: This article is for education only and does not provide an individual diagnosis, prescription, or dosing recommendation. Medication doses, particularly chemotherapy and paediatric doses, must be calculated and verified by appropriately qualified healthcare professionals using the applicable protocol and current patient information.