How to Calculate a Dose by Weight (mg/kg)

Many medicines, especially for children, are ordered by weight: so many milligrams for each kilogram the patient weighs (mg/kg). The math takes two steps: get the weight in kilograms, then multiply.

For learning only, not for patient care. With a real patient, always follow the order, the drug’s label and your facility’s policies, and double-check every dose with a pharmacist or a second nurse.

Try it with your own problem ↓

The steps

Get the weight in kilograms. If it is in pounds, divide by 2.2 (1 kg = 2.2 lb), and round the kilograms to a tenth.
Multiply: dose = mg/kg × kg. The kilograms cancel and leave milligrams.
If the order is per day (mg/kg/day) and divided, divide the daily dose by the number of doses: q8h is 3 a day, q6h is 4, q12h is 2.
Compare with any maximum dose or safe range, and question an order outside it.
If the drug is a liquid, turn the milligrams into milliliters with D/H × Q.

A worked example

The order is 20 mg/kg for a child who weighs 55 lb. How many milligrams is the dose?

Pounds to kilograms: 55 ÷ 2.2 = 25 kg.
Multiply: 20 mg/kg × 25 kg = 500 mg.
Check the size: 25 is less than 55, as it should be, because a kilogram is heavier than a pound.

The dose is 500 mg (for 25 kg).

Why it works

A mg/kg order is a rate, like a price per pound at the store: the bigger the patient, the more medicine it takes, so the dose grows with the weight. Writing it as 20 mg/1 kg × 25 kg shows the kilograms cancelling and leaving milligrams, which is a good check that the problem is set up right.

Try it with your own problem

Open The Dose Calculator (free) with this example already filled in. It shows every step, and you can type over it with your own numbers.

Work it out: 20 mg/kg for a 55 lb childOpens The Dose Calculator with this example

Or try these

More nursing math, every step written out: open The Dose Calculator.

Common mistakes

Multiplying the pounds by 2.2. That makes the weight, and the dose, almost 5 times too big. Pounds ÷ 2.2 = kilograms.
Using the pounds as if they were kilograms. 20 × 55 = 1,100 mg, more than twice the right dose.
Missing “per day”. For 40 mg/kg/day divided q8h, each dose is a third of the daily amount, not the whole amount.
Skipping the safe-range check. Always compare with the reference range or the maximum dose.

Questions people ask

How do I work out a mg/kg/day dose divided q8h?

For 40 mg/kg/day and 15 kg: 40 × 15 = 600 mg a day. q8h means every 8 hours, 3 doses a day, so each dose is 600 ÷ 3 = 200 mg.

Is 1 kg exactly 2.2 lb?

1 kg is about 2.2046 lb. Nursing math uses 2.2 unless your school or facility says otherwise.

How do I check a safe dose range?

Work out the low and high ends from the reference (for example 25–50 mg/kg/day), then see whether the ordered dose falls between them.

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