How this instrument works
Body surface area estimates the total outer area of a person's skin in square metres, worked out from height and weight rather than measured directly with tape and plaster casts, which is how researchers first mapped it in the early twentieth century. Oncology and burn units read this figure to size chemotherapy doses and fluid resuscitation volumes, because drug clearance and heat loss both track with skin area more closely than with body weight alone — a short, dense person and a tall, lean person of the same mass can need noticeably different doses.
The formula here is Richard Mosteller's 1987 shortcut, published as a one-page letter in the New England Journal of Medicine. It approximates the older DuBois and DuBois equation from 1916 — built from surface measurements of a small number of people — replacing two fractional exponents with a single square root, trading a sliver of precision for arithmetic simple enough to check by hand. Both formulas still appear in clinical protocols; Mosteller's is the one most infusion pharmacies default to today because it is fast and rarely disagrees with DuBois by more than a fraction of a percent.
Like any surface-from-mass estimate, it assumes a build close to average. It cannot see whether the weight you enter is mostly muscle, fat or retained fluid, so it can misjudge the relationship between weight and true skin area for people with significant obesity, muscle loss or oedema — which is why clinical teams still check unusual doses by hand rather than trusting a formula alone. Treat the figure here as the same estimate a chart would show, not a substitute for one.
- Enter your Height — the field accepts centimetres, metres, inches or feet.
- Enter your Weight in kilograms, pounds or stone.
- Read Body surface area (m²) in the results panel; it updates as you type.
- Compare the figure to the typical adult range of roughly 1.6 to 2.0 m² to see where it falls.
- Check the formula box beneath the result to see the exact arithmetic behind your figure.
Worked example — 180 cm, 80 kg
Enter 180 centimetres for height and 80 kilograms for weight — this page's own default sheet. Height times weight gives 180 × 80 = 14,400; dividing by 3600 leaves exactly 4, and the square root of 4 is a clean 2.0 m², one of the rare cases where every step lands on a whole number.
That 2.0 m² sits near the upper half of the adult range, and it is a useful figure to sanity-check the formula against: the older DuBois equation gives 1.996 m² for the same 180 cm and 80 kg, a gap of under a fifth of a percent, which is exactly why Mosteller's shortcut replaced it in most infusion pharmacies rather than being treated as a lesser approximation.
Questions
How accurate is the Mosteller BSA formula?
Mosteller's shortcut typically comes within a few percent of directly measured body surface area and within roughly a fifth of a percent of the older DuBois equation for the same height and weight — the two rarely disagree by more than a decimal point. It loses accuracy for bodies far from an average build: significant obesity, muscle loss or fluid retention all change the weight-to-surface relationship the formula assumes.
Why does my BSA reading differ from what my doctor calculated?
Clinics use several competing equations — DuBois from 1916, Mosteller from 1987, Haycock from 1978 and others — and each gives a slightly different figure from the same height and weight. A chart may also use measurements taken days earlier, rounded inputs, or a capped BSA applied for safety. A few hundredths of a square metre of disagreement is normal and not a sign either number is wrong.
What is body surface area actually used for?
Mainly drug dosing. Chemotherapy, some antibiotics and burn-fluid resuscitation are calculated per square metre of body surface rather than per kilogram of weight, because clearance and heat loss track surface area more closely than mass. Cardiologists also divide cardiac output by BSA to compare heart performance across differently sized patients.
Can this formula be used for infants and young children?
Mosteller's equation was built from adult and older-child data and is still commonly applied down to infancy in practice, but very small bodies have a higher surface-to-mass ratio than the formula assumes. Many paediatric dosing protocols apply their own correction or a dedicated formula such as Haycock's — check a paediatric reference before relying on this for a young child.
Is body surface area the same thing as BMI?
No — they measure different things from the same two inputs. BMI is a weight-to-height ratio used to screen for weight categories; BSA estimates the actual area of skin and is used mainly for dosing and for comparing organ performance across body sizes. A tall, thin person and a short, heavy person can share a BMI while having noticeably different BSA.
Should I use my BSA figure to adjust my own medication dose?
No. This page produces the same estimate a clinician's formula would, but real dosing decisions weigh kidney and liver function, drug-specific caps and clinical judgement that a calculator cannot see. Bring the figure to a conversation with your prescriber rather than acting on it alone.
References
- PubMed — Mosteller, "Simplified calculation of body-surface area" (1987)
- NIH NCBI Bookshelf (StatPearls) — Body Surface Area
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.