🧠 The theory: why surface area
Many body processes, including drug clearance and heat loss, scale more closely with surface area than with weight, because they happen across surfaces and membranes. Surface area cannot be measured directly, so formulas such as Mosteller estimate it from height and weight. Two people of the same weight but different heights have different surface areas, which is why both measurements are needed.
BSA (m2) = square root of ((height in cm x weight in kg) / 3600)
Worked example
Calculate the BSA for a patient 170 cm tall and 70 kg.
- Step 1 — Multiply height by weight: 170 x 70 = 11,900.
- Step 2 — Divide by 3600: 11,900 / 3600 = 3.306.
- Step 3 — Take the square root: square root of 3.306 = 1.82.
Answer: 1.82 m2
Worked example
Calculate the BSA for a patient 180 cm tall and 80 kg.
- Step 1 — Multiply: 180 x 80 = 14,400.
- Step 2 — Divide by 3600: 14,400 / 3600 = 4.
- Step 3 — Square root: square root of 4 = 2.
Answer: 2.0 m2
Using BSA for a dose
Worked example
A drug is dosed at 100 mg/m2. What dose suits a BSA of 1.82 m2?
- Step 1 — The dose is 100 mg for every square metre of surface area.
- Step 2 — Multiply the per-m2 dose by the BSA: 100 x 1.82 = 182.
Answer: 182 mg
Body mass index
BMI = weight in kg / (height in metres squared)
Worked example
Calculate the BMI of a patient who is 80 kg and 1.75 m tall.
- Step 1 — Square the height (in metres, not cm): 1.75 x 1.75 = 3.0625.
- Step 2 — Divide the weight by that: 80 / 3.0625 = 26.1.
Answer: BMI of about 26 kg/m2
💡 Use the formula given
Several BSA formulas exist (Mosteller, Du Bois). The exam will state which to use, so always work from the formula in the question rather than one you have memorised.
Test yourself
Calculate the BSA (Mosteller) for a patient 150 cm and 50 kg.
Answer: (150 x 50) / 3600 = 2.083; square root = 1.44 m2
A drug is dosed at 50 mg/m2. What dose suits a BSA of 2.0 m2?
Answer: 50 x 2.0 = 100 mg
Calculate the BMI of a patient 100 kg and 2.0 m tall.
Answer: 2.0 x 2.0 = 4; 100 / 4 = 25 kg/m2