primary care & preventionformula

Body Surface Area (Mosteller)

Calculates body surface area using the Mosteller simplified formula: BSA (m²) = √(height (cm) × weight (kg) / 3600). BSA is primarily used for drug dosing (especially chemotherapy) and physiological normalisation.

inputs

✓ when to use

Use when drug dosing is specified per m² of body surface area (most chemotherapy protocols, some cardiac and paediatric drug doses). Also used for normalising physiological measurements (cardiac output, renal clearance) and calculating body surface area-adjusted eGFR.

✗ when not to use

BSA-based dosing is an approximation. In obese patients, BSA calculated from actual weight may overestimate lean body mass, leading to overdosing of hydrophilic drugs. Some oncology protocols use adjusted BSA or cap BSA at 2.0 m². Always follow the specific dosing protocol for the drug being administered.

clinical pearls

  • Average adult BSA is approximately 1.7 m² (range 1.5–2.2 m²). This is a useful mental check — if your calculation gives a BSA outside 1.0–3.0 m², check the inputs.
  • In obese patients, discuss with pharmacy/oncology whether to use actual, ideal, or adjusted body weight for BSA calculation. Protocols vary by drug and institution.
  • BSA is used to 'normalise' physiological measurements: cardiac index = cardiac output / BSA; eGFR is normalised to 1.73 m² BSA. Awareness of the patient's actual BSA is important when interpreting these normalised values.
  • The formula requires height in cm and weight in kg. If inputs are in other units, convert first. The most common calculation error is using the wrong units.
  • Alternative BSA formulae (DuBois, Haycock, Gehan & George) exist and may give slightly different results. Most clinical protocols do not specify which formula — Mosteller is the default standard.

related tools

bmiWeight classificationcockcroft-gaultCrCl for drug dosing