Body Surface Area (Mosteller)

What the Body Surface Area Calculator Does

This calculator estimates your body surface area (BSA) — the total external surface of the human body, expressed in square metres (m²) — using the Mosteller formula. You enter your height in centimetres and weight in kilograms, and it returns a single BSA value.

BSA is widely used in medicine because it correlates better with metabolism, blood volume, and organ size than body weight alone. The tool is most useful for clinicians, pharmacists, oncology and pediatric staff, and students who need a quick, repeatable figure for dosing or physiological calculations.

How the Mosteller Formula Works

The Mosteller formula, published by R.D. Mosteller in 1987, is popular because it is simple enough to compute by hand yet agrees closely with older, more complex equations. It uses only height and weight.

Written out, the formula is:

BSA (m²) = square root of [ (height in cm × weight in kg) ÷ 3600 ]

The constant 3600 normalises the units so the result comes out in square metres. Because both inputs sit inside a square root, the calculator is forgiving of small input errors — a 1% change in weight shifts BSA by only about 0.5%.

Worked Example With Real Numbers

Take an adult who is 175 cm tall and weighs 70 kg.

Step 1: Multiply height by weight: 175 × 70 = 12,250.

Step 2: Divide by 3600: 12,250 ÷ 3600 = 3.4028.

Step 3: Take the square root: square root of 3.4028 = 1.845.

So the BSA is about 1.85 m². For comparison, the often-quoted 'average' adult BSA of 1.73 m² corresponds to roughly 165 cm and 65 kg, which is why this person sits slightly above that reference.

How BSA Is Used for Drug Dosing

Many drugs — chemotherapy agents in particular — are dosed in milligrams per square metre (mg/m²) rather than per kilogram, because BSA tracks cardiac output and renal function more reliably across different body sizes.

To get a dose, multiply the prescribed rate by the patient's BSA. For instance, a drug ordered at 100 mg/m² for the 1.85 m² patient above would be 100 × 1.85 = 185 mg.

Always treat the calculator as a starting point. Final doses must be checked against the prescribing protocol, capping rules, and the patient's clinical status by a qualified professional.

Tips, Common Mistakes, and Factors That Affect the Result

The most frequent errors come from units and inputs rather than the maths itself. Keep these points in mind:

• Use metric units: height in centimetres, weight in kilograms. Entering inches or pounds without converting will give a meaningless figure.
• Mosteller assumes actual body weight. In obesity, some protocols use adjusted or ideal body weight, or cap BSA at 2.0 m² — follow your local policy.
• Different formulas (Du Bois, Haycock, Boyd) give slightly different results. For consistency, use the same formula every time, especially when re-dosing a patient.
• Double-check decimal points. A weight typed as 700 instead of 70 inflates BSA dramatically.
• BSA is an estimate, not a measurement. Body shape, amputation, or significant edema can make any height-weight formula less accurate.