Skip to content
Rebuilding civilization

Health

Medicine dose by body weight

Milligrams per dose and per day, millilitres of syrup or number of tablets from a dose prescribed in mg/kg.

kg

Actual weight, measured on scales if possible. Any weight error carries straight into the dose.

mg/kg

From the leaflet or the prescription, in milligrams per kilogram. Make sure it is not in micrograms (µg) or grams.

For example, every 8 hours = 3 doses; every 6 hours = 4 doses.

mg/mL

Milligrams per millilitre. If the bottle says “250 mg/5 mL”, that is 50 mg/mL.

mg/d

The leaflet's maximum for this patient. If the calculated dose exceeds it, the dose capped at the maximum is also shown.

Result

Dose per administration
— mg
Volume per dose
— mL
Tablets per dose
—
Daily dose
— mg/d
Daily dose capped at maximum
— mg/d
Dose per administration capped at maximum
— mg
Volume per dose capped at maximum
— mL
Tablets per dose capped at maximum
—

How it is calculated

Dose per administration (mg) = weight (kg) × dose (mg/kg). If the prescribed dose is per day, divide it: dose per administration = weight × dose (mg/kg/day) ÷ doses per day. Daily dose = dose per administration × doses per day.

Volume per dose (mL) = dose per administration (mg) ÷ concentration (mg/mL). Tablets per dose = dose per administration (mg) ÷ mg per tablet.

If there is a maximum daily dose and the daily dose exceeds it, the capped dose per administration = maximum daily dose ÷ doses per day.

Worked example: child of 20 kg, 10 mg/kg per dose, 3 doses a day, syrup at 20 mg/mL. Dose per administration = 20 × 10 = 200 mg; daily dose = 200 × 3 = 600 mg; volume per dose = 200 ÷ 20 = 10 mL.

Example with a maximum: adult of 70 kg, 15 mg/kg/day in 3 doses, maximum 1000 mg/day. Daily dose = 70 × 15 = 1050 mg, which exceeds 1000 mg; the capped dose is 1000 ÷ 3 = 333.3 mg per dose.

Keep in mind

  • Support calculation: it does not replace the package leaflet or the judgement of a doctor or pharmacist. Always double-check the dose before giving it.
  • The most serious error is units: 1 g = 1000 mg and 1 mg = 1000 µg (micrograms). Also check whether the concentration is per mL or per 5 mL.
  • Measure liquids with an oral syringe or dosing cup, never with kitchen spoons.
  • Only split scored tablets, and at most in half. Capsules and modified-release or enteric-coated tablets must not be split.
  • In children the per-kilogram dose must not exceed the adult dose. In newborns, the elderly, pregnancy, obesity and kidney or liver failure doses are often different: check the leaflet or ask a professional.

See also

More tools in “Health”