Body Surface Area (BSA) Calculator and mg/m² Dosing

Calculate body surface area with the Mosteller formula and a dose ordered in mg/m², from height and weight in metric or US units.

Formula

BSA (m²) = √(height (cm) × weight (kg) ÷ 3,600)

Dose = dose per m² × BSA

How to calculate it step by step

Convert the height to cm (inches × 2.54) and the weight to kg (lb ÷ 2.2).
Multiply height by weight, divide by 3,600 and take the square root.
Round the BSA to the hundredth.
Multiply the dose in mg/m² by the BSA.

Worked examples

Example 1. A patient is 164 cm tall and weighs 86 kg. The order is 100 mg/m². Using the Mosteller formula and a BSA rounded to the hundredth, what is the dose in mg? Round to the tenth.

BSA = √(164 × 86 ÷ 3,600) = 1.98 m².
Dose = 100 mg/m² × 1.98 m² = 198 mg.

Answer: 198 mg

Example 2. A child is 95 cm tall and weighs 14 kg. The order is 75 mg/m². Using the Mosteller formula and a BSA rounded to the hundredth, what is the dose in mg? Round to the tenth.

BSA = √(95 × 14 ÷ 3,600) = 0.61 m².
Dose = 75 mg/m² × 0.61 m² = 45.8 mg.

Answer: 45.8 mg

Rounding

BSA is usually rounded to the hundredth (for example 1.79 m²) before calculating the dose.

Common mistakes

Forgetting the square root.
Using inches and pounds in the metric formula: the US formula is √(inches × lb ÷ 3,131).

Nursing safety points

BSA-based doses are often chemotherapy: they require an independent double check, and the height and weight must be current.
Some protocols cap the BSA or use a different formula: follow the order and the protocol.

Frequently asked questions

What is the Mosteller formula?

BSA = √(height in cm × weight in kg ÷ 3,600). For 165 cm and 70 kg: √(165 × 70 ÷ 3,600) = 1.79 m².

How do you calculate a dose in mg/m²?

Multiply the dose per m² by the BSA. 100 mg/m² × 1.79 m² = 179 mg.