The formula
How it works
Body surface area (BSA) is the total area of the outside of your body. It scales better with metabolism than weight alone, which is why doctors use it to size drug doses. This calculator uses the simple, widely used Mosteller formula.
FAQ
Why is BSA used for drug doses?
Many processes in the body, like blood flow and metabolism, track surface area more closely than weight. Chemotherapy and some other drugs are dosed per square metre of BSA for this reason.
Which formula is this?
The Mosteller formula, chosen because it is simple enough to do on a basic calculator yet agrees closely with older, more complex equations like Du Bois.
How does Mosteller differ from the Du Bois formula?
Du Bois uses height and weight raised to separate exponents and was derived from a small 1916 study, while Mosteller is a simpler square-root formula that gives very similar results for most adults. Hospitals still use several BSA formulas, and they typically agree within a few percent of each other.
Does this work for children?
Yes, the Mosteller formula applies to both adults and children as long as accurate height and weight are entered, which is why it’s common in pediatric drug dosing. Children’s smaller size just produces a proportionally smaller BSA.
Does BSA change if I gain or lose weight?
Yes, but only modestly, since BSA scales with the square root of weight rather than directly with it. A 10% change in weight shifts BSA by roughly 5%, which is exactly why it tracks body size more smoothly than weight alone.
Is BSA the same as BMI?
No — BMI relates weight to height to flag under- or overweight status, while BSA estimates total skin area and is used mainly for medical dosing rather than weight assessment. A person can have a high BMI and an average BSA, or vice versa.
Is BSA used outside drug dosing?
Yes. Burn severity is often reported as a percentage of total BSA affected, and BSA also shows up in calculations of metabolic rate, cardiac output index and fluid needs. It is a general-purpose way to normalise measurements to body size, not just a dosing tool.
About the body surface area calculator
This calculator estimates your body surface area — the total area of skin covering your body — from your height and weight. BSA might sound abstract, but it is one of the most useful body measurements in medicine, because many of the body’s functions scale with surface area rather than with weight. It gives a single number, in square metres, that summarises overall body size in a physiologically meaningful way.
How to use it
Choose metric or imperial units and enter your height and weight. The calculator returns your body surface area in square metres. For example, someone 180 cm tall weighing 80 kg has a BSA of about 2.0 m². The result rises with both height and weight, but more slowly than weight alone, which is exactly why it is preferred when a dose or a rate needs to match body size rather than raw mass.
The formula
This uses the Mosteller formula, , where is height in centimetres, is weight in kilograms and the result is in square metres. Imperial inputs are converted to metric first. The square root is what makes BSA grow more gently than weight: doubling weight does not double surface area, which mirrors how real bodies scale as they get larger.
Where it is used
BSA is used across medicine. Chemotherapy doses are almost always calculated per square metre of BSA, and it also guides doses of other potent drugs, fluid replacement in burns, and settings for dialysis and heart-lung machines. Kidney function is often reported adjusted to a standard BSA. Outside dosing, it appears in physiology whenever heat loss, metabolic rate or blood flow need to be compared between people of different sizes.