Doses by body weight and body surface area
Some doses depend on the patient, not the tablet. This guide covers mg/kg and mg/m² doses, the per-day versus per-dose trap, and how to turn a dose into a volume.
Not affiliated with the General Pharmaceutical Council. This guide is for revision only. It is not clinical or legal advice, and its worked examples use invented numbers. Check a current source before you rely on any point.
Two ways to scale a dose
By weight is written as mg/kg, or as a volume per kg such as mL/kg. Multiply by the patient's weight in kilograms.
By body surface area (BSA) is written as mg/m². Multiply by the patient's BSA in square metres. A question will give you the BSA, or give you the height, weight and the formula to find it. Take the formula from the question instead of relying on memory.
Before any sum, ask three things. Is the dose per dose or per day? How many doses make up the day? Is there a maximum that caps the answer?
What product information does with weight
The SmPC for an intravenous paracetamol and ibuprofen product shows both ideas in one place. For adults weighing more than 50 kg, it says to give one vial (100 mL). For adults weighing 50 kg or under, the dose is 1.5 mL/kg, as a 15-minute infusion every 6 hours. This equates to a maximum single dose of 75 mL, and the remaining medicine in the vial is discarded.
So the weight sets which rule applies, as well as the size of the dose.
Example 1: weight-based, from the SmPC rule above
An adult weighs 46 kg. Using the rule of 1.5 mL/kg for adults of 50 kg or under, what volume is one dose?
46 × 1.5 = 69 mL. That is below the 75 mL maximum single dose, so no cap applies. An adult weighing 52 kg is over 50 kg and gets the whole 100 mL vial, not 52 × 1.5 = 78 mL.
Example 2: a dose per kg, then a volume
Drug K is prescribed at 15 mg/kg for one dose. The child weighs 18 kg. The suspension is 120 mg in 5 mL. What volume?
Dose = 15 × 18 = 270 mg. Strength per mL = 120 ÷ 5 = 24 mg/mL. Volume = 270 ÷ 24 = 11.25 mL. Whether to round this depends on the oral syringe and what the question asks, so read the instruction.
Example 3: a daily dose split into portions
Drug L is prescribed at 40 mg/kg/day in 3 divided doses. The child weighs 14.5 kg. What is each dose?
Daily dose = 40 × 14.5 = 580 mg. Each dose = 580 ÷ 3 = 193.3 mg, so 193 mg to the nearest milligram. If the question says "divided" you divide by the number of doses. If it says "per dose", you do not.
Example 4: body surface area
The SmPC for a docetaxel 20 mg/mL concentrate lists, for one breast cancer regimen, a recommended dose of 75 mg/m². That is a standard BSA-based dose. The figures below use that dose with an invented patient.
A patient has a BSA of 1.79 m². The dose is 75 mg/m². The concentrate is 20 mg/mL. What dose and what volume?
Dose = 75 × 1.79 = 134.25 mg, so 134 mg to the nearest milligram. Volume = 134.25 ÷ 20 = 6.71 mL, so 6.7 mL. Chemotherapy doses are set, checked and adjusted by specialist teams. This is an arithmetic exercise, nothing more.
Example 5: when a cap applies
Drug M is 5 mg/kg per dose, maximum 300 mg per dose. A patient weighs 72 kg. What is one dose?
5 × 72 = 360 mg, which is over the maximum. The dose is 300 mg. Check for a cap every time. It is the easiest mark to lose in a weight-based question.
Habits
- Write the units on each line, and carry them through (mg/kg × kg = mg).
- Do the dose first and the volume second.
- Round at the stage the question tells you to, and no earlier.
- Ask if the answer makes sense for the person. A dose that is larger for a small child than for an adult is a warning sign.
For turning a dose into a pump rate, see the infusion rates guide. For strengths and dilutions, see the strengths guide.
Sources
- Combogesic IV 10 mg/ml + 3 mg/ml solution for infusion, Summary of Product Characteristics (emc), section 4.2. https://www.medicines.org.uk/emc/product/15054/smpc. Accessed 22 October 2026.
- Docetaxel 20 mg/ml concentrate for solution for infusion, Summary of Product Characteristics (emc), section 4.2. https://www.medicines.org.uk/emc/product/7206/smpc. Accessed 22 October 2026.
Not affiliated with the General Pharmaceutical Council. Revision only, not clinical or legal advice. Worked examples use invented numbers and products. Every source above was read on 22 October 2026; check anything that matters against a current source. See our terms of use and privacy notice.