This calculator checks arithmetic only. Always follow the dose on the prescription or the label, and ask a pharmacist or doctor if anything is unclear, especially for a child. Never exceed the maximum daily dose.
Weight-based doses
Many medicines for children, and some for adults, are prescribed per kilogram of body weight: “15 mg/kg every 6 hours”, for example. The same prescription then gives a bigger dose to a heavier child. The calculation is simple, but it has three classic traps: pounds instead of kilograms, a dose per day mistaken for a dose per intake, and the concentration of a liquid medicine.
The formulas
- Weight in kg = weight in lb × 0.4536
- Dose per intake (mg) = weight (kg) × dose (mg/kg), when the dose is per intake
- Dose per intake (mg) = weight (kg) × dose (mg/kg/day) ÷ number of intakes, when the dose is per day
- Volume (mL) = dose (mg) ÷ concentration (mg/mL)
Worked example: 15 mg/kg per dose, 4 times a day, for a 44 lb child
- Weight: 44 lb = 19.96 kg.
- Dose per intake: 19.96 × 15 = 299.4 mg.
- Liquid at 160 mg per 5 mL, which is 32 mg/mL: 299.4 ÷ 32 = 9.4 mL, almost 2 teaspoons.
- Per day: 299.4 × 4 = 1,197 mg, one dose every 6 hours.
Same prescription at other weights
| Weight | Dose per intake | Volume at 32 mg/mL |
|---|---|---|
| 22 lb | 149.7 mg | 4.7 mL |
| 33 lb | 224.5 mg | 7.0 mL |
| 44 lb | 299.4 mg | 9.4 mL |
| 66 lb | 449.1 mg | 14.0 mL |
| 88 lb | 598.7 mg | 18.7 mL |
Reading the concentration
The label of a liquid gives the strength as mg per mL or mg per 5 mL. Divide to get mg/mL: 160 mg/5 mL = 32 mg/mL, 100 mg/5 mL = 20 mg/mL. “Infant” and “children’s” versions of the same medicine can have different strengths, so always check the bottle in your hand.
Safety tips
- Use the dosing syringe or cup that comes with the medicine. Kitchen teaspoons hold anywhere from 3 to 7 mL.
- Weigh the child rather than guessing; doses based on age bands are less precise.
- Check the maximum: many medicines also have a maximum per dose and per 24 hours, whatever the weight. For heavier children, the adult dose is usually the ceiling.
- Count every product: cold and flu remedies often contain the same active ingredient as a pain reliever.
- Write down the time and amount of each dose when several people care for the child.
In case of an overdose, or if you are unsure, call Poison Control at 1-800-222-1222 (US), available 24/7.