How to Use This Tool
Enter height and weight. Every formula is shown, because the disagreement between them is the useful output.
Why surface area rather than weight
Metabolic rate, cardiac output, blood volume and glomerular filtration all scale more closely with surface area than with mass. So for a small number of drugs — most visibly cytotoxic chemotherapy — the dose is prescribed per square metre.
The convention is old and imperfect. It was adopted partly because it seemed more physiological than weight and partly because it happened to work in early trials, and there is a long-running argument in pharmacology about how much variation in drug exposure it actually removes. It remains standard practice for particular agents.
Which formula
- Mosteller (1987) — √(height × weight ÷ 3600). The usual default, mainly because it can be checked mentally and agrees closely with the others in the normal range.
- Du Bois and Du Bois (1916) — still widely used and fitted on nine subjects, none of them obese. It is known to underestimate at high body mass.
- Haycock (1978) — developed with children in the sample, so often preferred in paediatrics.
- Gehan and George (1970) — fitted on a much larger dataset.
- Boyd (1935) — the most complex, with a weight-dependent exponent.
For 175 cm and 70 kg they land between 1.8447 and 1.8539 — a spread of half a per cent, which is clinically irrelevant. For 185 cm and 120 kg they range from 2.4195 to 2.5227, a spread of 4.27%, and at 350 mg/m² that is 36 mg of difference produced by nothing but the choice of equation.
The 2 m² cap
Some protocols cap calculated BSA at 2.0 m² regardless of the patient, historically out of concern that dosing a large patient on true BSA would produce excessive toxicity.
The practice is contested. Capping reduces the dose by 19.5% for a patient calculated at 2.483 m², and several bodies now advise against automatic capping for many regimens on the grounds that it under-treats. Whether it applies is set by the protocol, not by the arithmetic, which is why it is a checkbox here rather than a default.
Read this as arithmetic, not as a dose
This page multiplies two numbers you typed. It knows nothing about renal or hepatic function, prior toxicity, age, performance status, drug interactions, the specific protocol, or whether BSA is even the right basis for the agent in question — several are dosed by weight, by a flat amount, or by target AUC instead.
Dose calculations in practice are performed and independently checked by clinicians against a specific protocol. Use this to understand where the numbers come from, and never as a source for a dose. This is general information, not medical advice.
