Dose by weight calculator (mg/kg)
Enter the body weight, the dose in mg/kg as written in the prescription or the reference you use, how many times a day it is given and the strength of the liquid. The calculator shows the dose in mg and the volume in mL. It does the arithmetic only: it does not know any medicine's safe dose or maximum.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose kg or lb and enter the weight.
- Enter the dose in mg/kg and say whether it is per dose or per day.
- Choose how many times a day the dose is given.
- For a liquid, enter its strength as on the bottle, for example 100 mg in 5 mL. Enter 0 mg for tablets or when you only need milligrams.
- Weight (kg) = weight (lb) × 0.45359237
- Per-dose figure: dose (mg) = weight (kg) × mg/kg, daily total = dose × doses a day
- Per-day figure: daily total (mg) = weight (kg) × mg/kg/day, dose = daily total ÷ doses a day
- Concentration (mg/mL) = strength mg ÷ strength mL; volume (mL) = dose (mg) ÷ concentration
Worked example
A child weighs 44 lb and the dose is 10 mg/kg three times a day, with a liquid of 100 mg in 5 mL. 44 × 0.45359237 = 19.96 kg; 19.96 × 10 = 199.6 mg a dose, 598.7 mg a day. The liquid holds 100 ÷ 5 = 20 mg/mL, so each dose is 199.6 ÷ 20 = 9.98 mL, practically 10 mL.
Tips and common mistakes
- Use a recent weight. An old weight is the most common source of a wrong dose in children.
- Check the strength on the bottle: the same medicine is often sold in more than one strength.
- Measure liquids with an oral syringe or the spoon supplied, not a kitchen spoon.
- This calculator does arithmetic only and does not replace a prescription, the leaflet or a pharmacist.
Frequently asked questions
How do I calculate a dose by weight?
Multiply the weight in kilograms by the dose in mg/kg. A 20 kg child and a dose of 10 mg/kg need 20 × 10 = 200 mg. To turn milligrams into millilitres, divide by the concentration: with 100 mg in 5 mL (20 mg/mL), 200 mg is 10 mL.
What is the difference between mg/kg per dose and mg/kg per day?
A per-dose figure is given each time; a per-day figure is the total for 24 hours, split into equal doses. 30 mg/kg/day for a 15 kg child is 450 mg a day, which three times a day is 150 mg a dose. Mixing them up gives three or four times too much or too little, so check which one your source means.
How do I convert pounds to kilograms?
Multiply by 0.45359237, or roughly divide by 2.2. 44 lb is 19.96 kg. Choose lb and the calculator converts for you and shows the weight in kg.
Does the calculator check the maximum dose?
No. It knows nothing about any medicine. Many medicines have a maximum single or daily dose that applies whatever the weight, which matters for heavier children and adults. Check the leaflet or ask a pharmacist.