mg/kg dose calculator

A weight-based dose is the dose in mg/kg multiplied by body weight in kilograms, then divided by the number of doses per day if the total is a daily figure. The error that matters is misreading a total daily dose as a single dose, which gives a threefold or fourfold overdose.

When to use the weight-based dose calculator

Use for any weight-based order, which in practice means most paediatric prescribing and a substantial part of antimicrobial dosing. Always confirm whether the figure quoted is per dose or per day, because the same number means very different things.

The weight-based dose formula

Total dose = dose in mg/kg × weight in kg. If the order is a daily dose in divided doses, divide the total by the number of doses per day.

Worked example

An order reads 15 mg/kg per day in three divided doses for an 18 kg child. Total = 15 × 18 = 270 mg per day, so 90 mg three times daily.

The mistake that costs marks

The single most dangerous error in weight-based dosing is treating a daily dose as a single dose, or the reverse, which produces a threefold error before any arithmetic goes wrong. The second is a misplaced decimal point, which produces a tenfold one. Check the units and the frequency before you check the maths.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• Body surface area calculator (Mosteller and Du Bois) — Body surface area
• Paediatric dose calculator (Clark and Young) — Paediatric dose
• Ideal and adjusted body weight calculator — Ideal body weight

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with Paediatric dosing and Clark rule. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.