This tool is for practice only, and that rule frames everything that follows. It is a study calculator for students learning body surface area, using fictional examples such as a practice person who is 170 cm tall and weighs 70 kg. It is never for real patients, real clinical decisions, or real medical advice. Real clinical use belongs to qualified clinicians with real measurements and real protocols. Use this page to learn the formulas and the arithmetic, and keep it strictly in the classroom.
Body surface area, usually shortened to BSA, is an estimate of the outside area of the body, measured in square metres. It appears in textbook problems because some doses and some physiology questions scale better with surface area than with weight alone. Two people can weigh the same and still have different surface areas if their heights differ, and BSA captures that difference. That is why exam questions give you both height and weight: you need both numbers, and a weight-only shortcut will give the wrong answer.
The Mosteller formula is the one most students meet first, because it is short and easy to remember. Take height in centimetres, multiply it by weight in kilograms, divide by 3600, and take the square root. For the practice example, 170 times 70 is 11900, divided by 3600 is about 3.306, and the square root is about 1.818 square metres. Write every step down in an exam, because a marker can follow a method even when the final number slips, but a bare answer with no working earns nothing if it is wrong.
The DuBois formula is older and more complicated, and this tool shows it for comparison only. It raises height to the power 0.725 and weight to the power 0.425, then multiplies by 0.007184. For the same practice example it gives about 1.809 square metres, very close to Mosteller, and the tool shows the small difference between them. In practice problems, the question usually names the formula, and that name is an instruction: use Mosteller when the question says Mosteller, even if you know another formula by heart.
Units are the main source of errors in BSA problems. Height must be in centimetres for Mosteller, not metres. If you enter 1.70 instead of 170, the answer collapses to about 0.182, which is ten times too small, and that factor-of-ten error is a classic exam trap. Weight must be in kilograms, and the answer comes out in square metres. Before you trust any result, ask whether it looks plausible: most adults land between about 1.4 and 2.2 square metres, and a value far outside that range means a unit slipped somewhere.
Rounding should be modest and late. Three decimal places, as this tool shows, is plenty for a practice answer, and rounding earlier can shift the third decimal. In an exam, follow the question: if it asks for two decimal places, give two, and if it asks for a whole number, round at the end. Showing the unrounded working and then the rounded answer is good practice, because it lets a marker see that the method was right even if the final rounding differs by a hair from the expected value.
Practise with varied fictional figures to build confidence. Try a shorter practice person, a taller one, a lighter one, and a heavier one, and notice how BSA rises with both height and weight but more slowly than either alone, because of the square root. Compare Mosteller and DuBois each time and watch the gap stay small for ordinary sizes and grow slightly at the extremes. That pattern is worth remembering: formulas agree best in the middle range and disagree most where they are least tested, which is itself a useful lesson about models.