kg and m² — drug calculations
Body surface area and dose per kg and per m²
How to work out a dose by weight and by body surface area, with formulas and worked examples.
Dose per kg
Many doses are given per kilogram of body weight.
Then work out how much to give using an ordinary dosage calculation.
Body surface area and dose per m²
Some medicines are dosed by body surface area, measured in square metres (m²).
Body surface area is estimated from height and weight. A common formula is Mosteller:
170 × 70 = 11,900. 11,900 ÷ 3600 = 3.31. √3.31 ≈ 1.82 m²
How to do it
Identify how the dose is given
Per kg, per m² or as a fixed dose.
Work out the total dose
Multiply the dose by the weight or the body surface area.
Work out the amount
Use the strength of the product, for example mg/mL, to find the mL or number of tablets.
Frequently asked questions
How do you calculate a dose per kg?+
Multiply the dose per kg by the patient's weight. 5 mg/kg and 60 kg gives 300 mg.
How do you calculate body surface area?+
A common formula is Mosteller: the square root of (height in cm × weight in kg ÷ 3600). Use the formula your course specifies.
What does mg/m² mean?+
It means mg per square metre of body surface area. You get the dose by multiplying by the patient's body surface area.
Is body surface area the same as weight?+
No. Body surface area is calculated from both height and weight and is given in m².
Practise dose per kg and per m²
Practise calculating doses by weight and body surface area, and use the statistics to see which steps need more practice. Dos is a practice app for drug calculations, with practice tests of ten questions, a built-in calculator next to each question and statistics per topic.
