Drug Dosage by Weight

A child of 18 kilos on 15 mg/kg a day split into four doses needs 67.5 mg each time, which with a syrup of 100 mg per 5 ml is 3.4 ml. Do the arithmetic without misplacing a decimal point.

Each dose

—

Each dose—

Which as syrup is—

Per day—

One dose every—

Syrup per day—

Against the maximum—

How this was worked out

Indicative result. This does not replace a prescription. The milligrams per kilo and the ceilings come from the leaflet or from whoever prescribes; here only the arithmetic is done, on the figures you type.

The formula

mg per dose = weight × dose per kilo ÷ doses per day

Where it comes from

Dosing by weight is the norm in paediatrics and for many adult drugs, because what determines the blood concentration is body size and not age. The calculation has two steps that get mixed up easily: first the total dose for the day, and only then the split between doses. Plenty of errors come from applying the daily dose to each individual dose, which multiplies what is given by four.

How to work it out by hand

Multiply the weight by the milligrams per kilo the leaflet gives
That is the dose for the whole day, not for one administration
Divide by the number of doses
To convert to syrup, apply the concentration: mg per dose × ml of the bottle ÷ mg of the bottle

What is worth knowing

This page carries no drug table and never will: the milligrams per kilo depend on the drug, the indication, the route and the country, and they are set by whoever has the leaflet in front of them. What the calculator does is the arithmetic, which is exactly where factor-of-ten errors happen. Always check the daily maximum separately, because going over in one dose and accumulating too much across the day are different problems.

Frequently asked questions

How do I calculate a dose by weight?

Multiply the weight in kilos by the milligrams per kilo from the leaflet, then split the result between the day's doses.

Are mg/kg per dose or per day?

Almost always per day, but some regimens are expressed per dose. Check the leaflet before calculating: the difference is several-fold.

How do I go from milligrams to millilitres?

With the concentration on the bottle. If it is 100 mg in 5 ml, every millilitre carries 20 mg.

Does it work for adults?

Yes. Many drugs are dosed by weight in adults too, especially in hospital.