Body Surface Area Calculator: How BSA Is Worked Out

Nobody measures skin area directly. Every figure you will ever see is an estimate from height and weight, produced by a formula that was fitted to a small group of people decades ago and has stayed in use because it works well enough.

Why anyone calculates skin area at all

Weight alone is a poor guide to how a body processes things. Surface area tracks metabolic rate and blood volume more closely than weight does, so it became the standard denominator for a handful of clinical numbers.

Drug dosing

Chemotherapy doses are written per square metre. A drug prescribed at 60 mg/m² gives 110 mg to a patient with a BSA of 1.84 m² and 94 mg to one at 1.56 m², which is a 17% difference between two adults who look fairly similar on paper. That is why the calculation gets checked twice.

Cardiac index

Cardiac output on its own says little without knowing the size of the person. Divide it by BSA and you get cardiac index. Five litres per minute in someone with a BSA of 1.84 m² is 2.7 L/min/m², which sits inside the usual reference band of roughly 2.5 to 4.0. The same 5 L/min in a much larger person would be a worse result.

Mosteller, Du Bois, and why they disagree

Mosteller is the square root version above, and it is popular because you can do it on any calculator. Du Bois is older and uses exponents: 0.007184 multiplied by height in cm to the power 0.725, multiplied by weight in kg to the power 0.425.

For our 175 cm, 70 kg example Mosteller gives 1.84 m² and Du Bois gives 1.85 m². Under 1% apart, which does not matter. The two drift further apart at the extremes of body size, particularly at high weights, and that gap can matter for dosing. Hospitals handle it by fixing one formula in the protocol rather than letting each department choose.

Running your own numbers

The calculator needs two inputs. Height can be entered in centimetres or in feet and inches; 5 ft 9 in is 175.3 cm. Weight takes kilograms, pounds or stones and pounds; 11 stone exactly is 69.9 kg. Use the unit toggle rather than converting by hand, since a conversion slip is the most common source of a wrong answer.

Use the same measurements you would use for a BMI calculation, taken without shoes and, for weight, first thing in the morning. If you want a sense of whether your weight sits in a typical range for your height, a healthy weight estimate answers a different question than BSA does, and the two are not interchangeable.

Common questions

What is a normal body surface area? Most adults land between about 1.5 and 2.0 m². There is no healthy or unhealthy value. BSA is a scaling number, not an assessment, so a figure of 1.6 says nothing about your health on its own.

How much does BSA change if I lose weight? Less than you would guess. At 175 cm, dropping from 70 kg to 65 kg moves BSA from 1.84 to 1.78 m², a change of about 3%. The square root flattens the effect of weight, which is one reason BSA is more stable than weight for dosing.

Is BSA better than weight for working out a dose? For some drugs, yes, and for others the evidence is thin and weight-based or flat dosing is used instead. It depends entirely on the drug. The calculator gives you the number; it does not decide which drugs should use it.

Can I use this to check my own prescription? You can work out your BSA and see it match what the clinic has. Do not use it to question or adjust a dose on your own. Rounding rules, capped doses for very large patients and adjustments for kidney or liver function all sit on top of the raw figure.