What body surface area is, and why doses are scaled to it
Body surface area is the total external area of the skin, expressed in square metres. You cannot measure it directly on a living patient, so every clinical BSA figure is an estimate produced by a regression equation fitted to a small set of people whose surface area was measured directly — historically by wrapping the body in paper moulds or coating it in strips and measuring the material.
BSA earned its place in dosing because several determinants of drug handling track surface area more closely than they track body weight. Basal metabolic rate, cardiac output, glomerular filtration rate and extracellular fluid volume all scale roughly with a power of body mass between 0.6 and 0.75 rather than with mass itself, and surface area is a convenient proxy for that exponent. A 100 kg adult does not clear a cytotoxic drug twice as fast as a 50 kg adult; the BSA ratio between them is closer to 1.3 than to 2, and dosing on BSA rather than weight reflects that.
Where BSA is used today: cytotoxic chemotherapy protocols almost universally, several antimicrobials and immunosuppressants, paediatric dosing when weight-based rules are unreliable, and indexing — cardiac index is cardiac output divided by BSA, and estimated GFR is reported per 1.73 m², the surface area of a notional average adult. If your dose is written in mg/kg instead, use the weight-based drug dosage calculator.
Why there are five formulas and how they differ
All five equations solve the same problem — predict a measured surface area from height and weight — and all five are power laws. They differ because each author fitted a different sample.
Du Bois & Du Bois (1916) is the original, fitted to nine subjects, and remains embedded in older nomograms and in some cardiology software. Boyd (1935) used a much larger compilation and is the only common formula whose weight exponent itself depends on weight, which is why its algebra looks unlike the others. Gehan & George (1970) refitted 401 direct measurements. Haycock (1978) was fitted across infants, children and adults, which is why paediatric services often prefer it. Mosteller (1987) is not a new fit at all: it is a deliberately simple square-root approximation to the earlier equations, published as a one-line formula you can evaluate on a pocket calculator, and it has become the default in most oncology pharmacy systems for exactly that reason.
Read the Mosteller form as a statement about geometry. Multiply height in centimetres by weight in kilograms, divide by 3600, take the square root. The constant 3600 is chosen so that a person 100 cm tall weighing 36 kg comes out at exactly 1.000 m², which anchors the scale. Because the answer is a square root, a 1% error in either height or weight moves BSA by only about 0.5% — a useful robustness property when a patient is weighed in outdoor clothing.
Worked example: 180 cm, 70 kg, docetaxel at 75 mg/m²
Take an adult 180 cm tall weighing 70 kg, with a protocol dose of 75 mg/m².
- Multiply height by weight. 180 × 70 = 12,600.
- Divide by 3600. 12,600 ÷ 3600 = 3.5.
- Take the square root. √3.5 = 1.8708 m² by Mosteller.
- Multiply by the ordered dose. 1.8708 × 75 = 140.3 mg.
Now run the same patient through the other four formulas so you can see the disagreement:
- Du Bois: 0.007184 × 1800.725 × 700.425 = 0.007184 × 43.153 × 6.0839 = 1.8861 m²
- Haycock: 0.024265 × 1800.3964 × 700.5378 = 0.024265 × 7.8342 × 9.8243 = 1.8676 m²
- Gehan & George: 0.0235 × 1800.42246 × 700.51456 = 0.0235 × 8.9694 × 8.8995 = 1.8758 m²
- Boyd: exponent = 0.7285 − 0.0188 × log10(70,000) = 0.7285 − 0.0911 = 0.63741; BSA = 0.0003207 × 1800.3 × 70,0000.63741 = 0.0003207 × 4.7483 × 1,225.9 = 1.8666 m²
The five values run from 1.8666 to 1.8861 m². Their mean is 1.8734 m², so the spread is 0.0195 ÷ 1.8734 = 1.04% — about 1.5 mg of docetaxel on a 140 mg dose. For this patient the choice of formula is clinically irrelevant. It is not always so small, which is why the calculator reports the spread rather than hiding it.
How to read the number you get
An average adult sits near 1.7 m². The conventional reference figure used for indexing renal function is 1.73 m², which is why the calculator also reports your patient as a percent of that reference: a result of 108% means the patient presents 8% more surface area than the notional adult behind every eGFR result reported per 1.73 m². That matters when you take an indexed eGFR and try to use it for drug dosing — for a large or small patient you have to de-index it by multiplying by BSA ÷ 1.73, or use an unindexed clearance such as Cockcroft-Gault, which most renal dosing tables were actually derived against.
Typical ranges worth carrying in your head: a term newborn is near 0.25 m², a one-year-old near 0.5 m², a ten-year-old near 1.1 m², and adults mostly fall between 1.5 and 2.1 m². A value outside those bands is usually a data-entry error — a weight typed in pounds while the field expects kilograms inflates BSA by roughly half, which is exactly the kind of mistake that produces a catastrophic chemotherapy overdose.
The spread output tells you how much the formula choice is worth. It is small in mid-range adults and grows at the extremes of size, because that is where the fitted samples ran out of data. When the spread exceeds a couple of percent, the honest answer is that BSA itself is uncertain at that body size, and the protocol formula should be applied consistently from cycle to cycle rather than switched.
Mosteller body surface area by height and weight
| Weight | 150 cm | 160 cm | 170 cm | 180 cm | 190 cm |
|---|---|---|---|---|---|
| 50 kg | 1.443 | 1.491 | 1.537 | 1.581 | 1.624 |
| 60 kg | 1.581 | 1.633 | 1.683 | 1.732 | 1.780 |
| 70 kg | 1.708 | 1.764 | 1.818 | 1.871 | 1.922 |
| 80 kg | 1.826 | 1.886 | 1.944 | 2.000 | 2.055 |
| 90 kg | 1.936 | 2.000 | 2.062 | 2.121 | 2.179 |
| 100 kg | 2.041 | 2.108 | 2.173 | 2.236 | 2.297 |
Every cell is √(height × weight ÷ 3600) evaluated directly; the 180 cm / 80 kg and 160 cm / 90 kg cells are both exactly 2.000 m² because both products equal 14,400.
Mistakes that produce a wrong BSA
- Mixing units. Pounds entered as kilograms, or inches as centimetres. Both errors survive the arithmetic silently and produce a plausible-looking number. Use the unit selectors rather than converting in your head.
- Switching formulas between cycles. A patient dosed on Du Bois in cycle 1 and Mosteller in cycle 2 gets a different dose from an identical order. Record which formula the pharmacy system uses.
- Using a stale weight. BSA is recalculated at each cycle in most oncology protocols precisely because weight changes. A weight taken at diagnosis is not the dosing weight three months later.
- Assuming the cap is universal. Some institutions cap the BSA used for cytotoxic dosing; others dose on the uncapped value. The ASCO guideline on chemotherapy dosing in obese adults advises against automatic dose reduction. Follow the local protocol and document which you used.
- Rounding too early. Round the final dose, not the BSA. Rounding BSA to one decimal place before multiplying can shift a dose by several percent.
- Applying BSA where weight is the right scale. Carboplatin is dosed by target AUC and renal function, not BSA. Heparin, vasopressors and most paediatric antibiotics are dosed per kilogram.
What BSA dosing does and does not account for
BSA is a size correction, not a physiology correction. It knows nothing about hepatic function, renal function, protein binding, pharmacogenomics or body composition. Two patients with identical height and weight — one muscular, one with a large fat mass and low lean mass — receive the same BSA-based dose despite quite different volumes of distribution for lipophilic and hydrophilic drugs. That is the standing criticism of BSA dosing, and it is why therapeutic drug monitoring, AUC-targeted dosing and pharmacogenomic dose adjustment are displacing BSA for individual agents.
The calculator also assumes the height and weight you enter are real measurements. In practice, recorded heights drift upward with age-related vertebral compression while patients continue to report their peak adult height, and this systematically inflates BSA in older patients. Measure, do not ask.
Amputation is a specific trap: none of these formulas subtract a missing limb, so BSA is overestimated after amputation. Published correction factors exist by limb segment, and your pharmacy should apply one rather than using the raw value. Similarly, in massive oedema or ascites the measured weight includes fluid that is not metabolically active tissue.
Clinical use requires an independent check
Cytotoxic chemotherapy doses derived from BSA are high-risk calculations. Every institution that dispenses them requires an independent double check of height, weight, BSA and final dose by a second qualified person, and most electronic prescribing systems enforce a hard stop when a recalculated BSA differs from the previous cycle by more than a set percentage. This calculator is a reference and teaching tool; it is not a substitute for your pharmacy verification workflow, and it does not know your protocol, your dose-banding table or your patient.
Where BSA sits among the other size metrics
Four size metrics compete for the same job, and they are not interchangeable. Total body weight is the right scale for most paediatric antibiotics and for anything dosed per kilogram. Ideal body weight, from the Devine equations, is used for drugs that do not distribute into fat — aminoglycosides being the classic example — and it is what the ideal body weight calculator produces. Lean body mass is a better predictor for anaesthetic induction agents; the lean body mass calculator covers it. Body surface area is the oncology and indexing standard.
A useful sanity check is the relationship between BSA and body mass index. BMI and BSA answer different questions: BMI (weight ÷ height²) describes body shape and is independent of overall size, whereas BSA (roughly proportional to weight0.5 × height0.5 in the Mosteller form) grows with total size. Two people can share a BMI of 24 and differ by 40% in BSA. If you are converting a BSA-based dose into an infusion, take the resulting milligrams to the infusion rate calculator to get a pump rate.
Key terms
- BSA
- Body surface area, in square metres, estimated from height and weight by a fitted regression equation.
- mg/m²
- A dose expressed per square metre of body surface area. Multiply by BSA to get the milligrams to give.
- Indexing to 1.73 m²
- Reporting a physiological rate per 1.73 m² of surface area — the assumed average adult — so that values are comparable across body sizes. Used for eGFR and cardiac index.
- Dose banding
- Rounding a calculated dose to a standard band so that pre-made syringes and bags can be used. Bands are typically set within a few percent of the exact dose.
- AUC dosing
- Dosing to a target area under the concentration-time curve, using renal function rather than BSA. Standard for carboplatin.