How to Calculate Medication Dosage by Body Weight
The mg/kg dose formula, converting doses to millilitres of liquid medicine, pounds to kilograms, maximum-dose caps and worked examples, with a calculator.
Published by Calculator.uk.com · Updated 21 October 2026 · Our methodology
Important safety note
This guide explains the arithmetic of weight-based dosing so you can check a dose that a doctor, pharmacist, vet or medicine label has given you. It does not tell you the right dose of any medicine. Never choose a dose yourself from this page. If a calculated amount doesn't match what you were told, stop and ask a pharmacist or doctor. In an emergency or suspected overdose, contact your local emergency number or poison information service.
Quick answer
Dose (mg) = prescribed dose (mg/kg) × body weight (kg). For a liquid medicine, volume (mL) = dose (mg) ÷ strength (mg/mL). Example: 10 mg/kg for an 18 kg child is 180 mg. With a 250 mg/5 mL liquid (50 mg/mL), that is 180 ÷ 50 = 3.6 mL. Convert pounds to kg first, divide per-day doses by the number of doses, and never exceed the stated maximum.
Dosage by weight calculator
Enter the weight and the mg/kg (or mcg/kg) dose you have been given. Add the maximum dose and liquid strength if you have them. No medicine doses are built in. The calculator only does the arithmetic.
Enter the weight and prescribed mg/kg dose to calculate
The formula, step by step
- Get the weight in kilograms. If you have pounds, divide by 2.2046.
- Check the dose basis. Is the mg/kg figure per dose or per day? Check the units too: mg or mcg?
- Multiply: dose per kg × weight in kg.
- If it is per day, divide by the number of doses a day.
- Apply the cap. If the result is above the stated maximum single or daily dose, use the maximum.
- Convert to volume for liquids: mg ÷ (mg per mL).
- Sense-check. Is the answer plausible? Would it fit in a normal syringe or tablet strength? A 10-fold error usually looks obviously wrong at this step.
Worked examples
The doses below are illustrative numbers chosen to show the method. They are not recommendations for any medicine.
1. Basic mg/kg dose
Prescribed 10 mg/kg per dose for a child weighing 18 kg: 10 × 18 = 180 mg per dose.
2. Converting to mL of liquid
The same 180 mg dose, with an oral suspension labelled 250 mg/5 mL. Strength = 250 ÷ 5 = 50 mg/mL. Volume = 180 ÷ 50 = 3.6 mL.
3. Weight in pounds
Prescribed 15 mg/kg for a child weighing 44 lb. Weight = 44 ÷ 2.2046 = 19.96 kg. Dose = 15 × 19.96 = 299.4 mg. Forgetting to convert (15 × 44) gives 660 mg, more than double the intended dose. This is one of the most common dosing errors, which is why hospitals record weight in kg only.
4. mg/kg/day in divided doses
Prescribed 30 mg/kg/day in 3 divided doses for a 20 kg child. Daily total = 30 × 20 = 600 mg. Each dose = 600 ÷ 3 = 200 mg, three times a day.
5. Applying a maximum dose
Prescribed 15 mg/kg with a maximum single dose of 1,000 mg, for a 70 kg adult. 15 × 70 = 1,050 mg, which is above the maximum, so the dose is 1,000 mg. Weight-based dosing in larger children and adults often hits the cap, which is why limits are stated alongside mg/kg doses.
6. Micrograms per kilogram
Prescribed 5 mcg/kg for a 12 kg child: 5 × 12 = 60 mcg = 0.06 mg. Mixing up mcg and mg is a 1,000-fold error, so keep the unit attached to every number you write down.
Unit conversions
| Conversion | Rule |
|---|---|
| Pounds to kilograms | lb ÷ 2.2046 (or × 0.4536) |
| Grams to milligrams | g × 1,000 |
| Milligrams to micrograms | mg × 1,000 |
| "X mg in Y mL" to mg/mL | X ÷ Y |
| Percentage strength (w/v) to mg/mL | % × 10 (1% = 1 g per 100 mL = 10 mg/mL) |
| Ratio strength 1:1,000 to mg/mL | 1 g in 1,000 mL = 1 mg/mL |
| 1 teaspoon (medicine spoon) | 5 mL. Use an oral syringe for accuracy. |
When weight-based dosing isn't the whole story
- Body surface area (mg/m²). Some medicines, including many chemotherapy drugs, are dosed by body surface area instead of weight. See the BSA calculator.
- Kidney or liver function. Reduced clearance can mean lower or less frequent doses. Prescribers often check kidney function (eGFR or creatinine clearance) first.
- Age bands. Many over-the-counter children's medicines are dosed by age band on the label rather than by mg/kg. Follow the label unless a clinician tells you otherwise.
- Newborns and premature babies handle drugs differently, so their doses must always come from a specialist.
- Animals. Vets also dose by mg/kg, but the doses differ from human doses, and some human medicines are toxic to pets. Use our veterinary dose calculators only with your vet's instructions.
Safety checklist
- Use a recent, accurate weight in kilograms.
- Write a zero before a decimal point (0.5 mg, not .5 mg), and never after a whole number (5 mg, not 5.0 mg).
- Check the strength on the actual bottle, because the same medicine comes in different strengths.
- Measure liquids with the oral syringe or cup supplied, not a kitchen spoon.
- Keep a written record of times and doses, so that two carers don't both give a dose.
- If the answer differs from the label or prescription, don't give it. Ask a pharmacist.
Frequently asked questions
How do I calculate a medication dosage by body weight?
Multiply the prescribed dose in mg per kg by the person’s weight in kilograms: dose (mg) = dose (mg/kg) × weight (kg). For example, 10 mg/kg for an 18 kg child is 180 mg. If the medicine is a liquid, divide the mg dose by the strength in mg per mL to get the volume: 180 mg ÷ 50 mg/mL = 3.6 mL. Convert pounds to kilograms first (divide by 2.2046), check whether the dose is per dose or per day, and never exceed the stated maximum dose.
How do I convert a dose in mg to mL?
Divide the dose in mg by the concentration in mg per mL. If the label says 250 mg in 5 mL, the concentration is 250 ÷ 5 = 50 mg/mL, so a 180 mg dose is 180 ÷ 50 = 3.6 mL. Use an oral syringe rather than a kitchen spoon to measure it.
What does mg/kg/day in divided doses mean?
It means the mg/kg figure is the total for 24 hours, not for each dose. Multiply by weight to get the daily total, then divide by the number of doses. For 30 mg/kg/day in 3 divided doses for a 20 kg child: 30 × 20 = 600 mg a day, so each dose is 200 mg. Treating a per-day figure as per-dose would give three times the intended amount.
Should I use actual or ideal body weight?
Most weight-based doses use actual body weight. For some medicines in people with obesity, prescribers use ideal or adjusted body weight instead, because the drug does not distribute into fat tissue. Children’s doses calculated by weight are normally capped at the adult dose. Which weight to use is a clinical decision for the prescriber or pharmacist.
What is the source for the weight-based dose calculation method?
The mg/kg method, concentration-to-volume conversion and maximum-dose capping follow standard pharmaceutical calculation practice, as taught in pharmacy calculation textbooks and set out in the dose-calculation guidance of national formularies such as the British National Formulary (BNF) and BNF for Children, and the WHO Model Formulary. The advice to record weight in kilograms only and to double-check decimal points reflects medication-safety recommendations from bodies such as the Institute for Safe Medication Practices.