Dose by weight from a mg/kg rate and body weight, capped at the single and daily maximum, with the liquid volume in mL for a given strength.
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The calculated dose is 300 mg (9.38 mL) every 6 hours, 4 times a day, 1,200 mg in total.
No limit reached.
Measure with an oral syringe or the device supplied, not a kitchen spoon.
A weight-based dose is body weight in kilograms times the prescribed rate in mg/kg, given either per dose or per day and divided into doses. The result is capped at the maximum single dose and at the daily maximum divided by the number of doses, whichever is lower. For a liquid it is converted to millilitres with the product strength: dose × mL ÷ mg.
It is used to check a calculated dose against a prescription or product label. With the defaults, 20 kg at 15 mg/kg gives 300 mg, which is 9.38 mL of a 160 mg per 5 mL liquid, taken 4 times a day for 1,200 mg in total.
The rate, the maximums and the strength must come from the prescription or the label; the calculator holds no dosing information for any medicine. The BNF for Children notes that a calculated child's dose should not exceed the adult maximum, which is why the caps apply.
Multiply body weight in kilograms by the dose rate in mg/kg. At 15 mg/kg, a 20 kg child's dose is 20 × 15 = 300 mg. If the rate is per day, divide by the number of doses, then cap the result at the maximum single and daily dose. The rate, maximums and strength must come from the prescription or label.
These steps check arithmetic only and hold no dosing information for any medicine. If your result differs from the prescription or label, ask a pharmacist or doctor before giving the dose.
Prescriptions write the rate as mg/kg/dose or mg/kg/day. A per-dose rate gives the amount for each dose; a per-day rate gives the 24-hour total, which you divide by the number of doses.
For a 12 kg child at 40 mg/kg/day in 3 doses, the day's total is 12 × 40 = 480 mg, or 160 mg per dose. Reading the same 40 mg/kg as per dose gives 480 mg every time, three times the intended amount.
Rates are per kilogram, so multiply a weight in pounds by 0.45359237. A 44 lb child weighs 19.96 kg, and at 15 mg/kg the dose is 299.4 mg.
Multiplying the pound figure by the rate instead gives 44 × 15 = 660 mg, 2.2 times too much.
The dose is the smallest of three numbers: weight × rate, the maximum single dose, and the maximum daily dose divided by the number of doses.
Most labels give an adult maximum that a weight-based dose must not exceed. Leaving a maximum blank skips that check; it does not mean the medicine has no limit.
Divide the dose by the liquid's strength: mL = dose in mg × mL in the strength ÷ mg in the strength. A 300 mg dose of a 160 mg per 5 mL liquid is 300 × 5 ÷ 160 = 9.375 mL, about 9.4 mL.
A stronger liquid needs less volume for the same dose: at 320 mg per 5 mL, 300 mg is 4.69 mL. Check the strength on the bottle every time the product changes.
Measure with an oral syringe or the device that came with the medicine, not a kitchen spoon. The FDA's guidance on dosing devices asks for a device marked in the same units as the label directions.
If the volume falls between the device's markings, ask the pharmacist how to round it.
Dividing 24 hours by the number of doses gives an evenly spaced schedule: 4 doses a day is every 6 hours, 3 doses every 8 hours. Where a label says "up to" a number of doses, "as needed" or gives set times, follow the label.
These are hypothetical numbers for checking the arithmetic, not doses for any medicine.
| Scenario | Calculated dose (mg) | Liquid volume (mL) | Total per day (mg) |
|---|---|---|---|
| Default | 300 | 9.38 | 1200 |
| Daily cap | 750 | 23.44 | 3000 |
| Per-day example | 160 | 3.20 | 480 |
The body surface area calculator estimates BSA, the body-size measure behind doses written in mg per m². The BMI calculator relates weight to height.
Checked against: 20 × 15 = 300 mg; 300 × 5 / 160 = 9.375 mL; 300 × 4 = 1200 mg; 24 h / 4 = 21600 s
Checked against: min(80 × 15, 1000) = 1000 mg; 1000 × 1 / 100 = 10 mL
Checked against: min(50 × 20, 1000, 3000 / 4) = 750 mg; 750 × 5 / 160 = 23.4375 mL
Checked against: 12 × 40 / 3 = 160 mg; 160 × 5 / 250 = 3.2 mL; 24 h / 3 = 28800 s
Multiply body weight in kilograms by the dose rate in mg/kg. A 20 kg child at 15 mg/kg gets 20 × 15 = 300 mg per dose. If the rate is per day, divide by the number of doses: 12 kg at 40 mg/kg/day in 3 doses is 480 mg a day, or 160 mg per dose. Convert pounds to kilograms first; 44 lb is 19.96 kg.
Divide the dose in mg by the strength in mg per mL. A 300 mg dose of a 160 mg per 5 mL liquid is 300 × 5 ÷ 160 = 9.375 mL, about 9.4 mL. The FDA's 2011 guidance for over-the-counter liquid medicines asks for a measuring device marked in the same units as the label directions, which is why an oral syringe or the supplied cup is used rather than a kitchen spoon.
Because a heavier child's mg/kg dose can exceed the adult dose. The BNF for Children advises that calculated doses should not exceed the adult maximum: at 8 mg/kg with a 300 mg maximum, a 10 kg child receives 80 mg, but a 40 kg child receives 300 mg rather than 320 mg. This calculator applies both a single-dose and a daily maximum and shows which limit, if any, reduced the dose.
A per-dose rate gives the amount for each dose; a per-day rate gives the total for 24 hours, which is then divided by the number of doses. At 40 mg/kg, a 12 kg child would get 480 mg per dose on a per-dose reading but 160 mg per dose if the 40 mg/kg is per day in three doses, a threefold difference. Prescriptions state which applies, often as mg/kg/dose or mg/kg/day.
For information only, not medical advice. These formulas describe populations, not you; talk to a clinician before acting on a result.
This calculator includes 7 worked examples with independently sourced answers. They run in the test suite, and you can run them here too.