Paediatric Dose Calculator

This is a general-purpose weight-based dose calculator for children. You enter the reference dose from a trusted source (in mg per kg), and it works out the dose per administration, the total for the day, and, if you add the liquid strength, the volume to measure. It can also cap the result at a maximum single dose and maximum daily dose.

It deliberately does not contain a drug list. It can’t know the right dose for a particular drug, age or condition, so the reference dose has to come from the BNFc, your local formulary or the product information.

How the calculation works

• Per-dose basis: dose = reference dose (mg/kg) × weight (kg). The daily total is that dose × the number of doses per day.
• Per-day basis: daily total = reference dose (mg/kg/day) × weight (kg), then divided by the number of doses to get each dose.
• Caps: if you enter a maximum single dose, no dose will exceed it. If you enter a maximum daily dose, each dose is reduced so the day’s total stays within it.
• Volume: mL = dose (mg) ÷ concentration (mg per mL). A label of “120 mg/5 mL” is 24 mg/mL.

Common sources of paediatric dosing errors

• Decimal-point errors that give a tenfold overdose or underdose.
• Mixing up mg and micrograms, or mg and mL.
• Confusing per-dose with per-day doses, so the daily total is split (or multiplied) incorrectly.
• Estimated rather than measured weight, or the wrong weight (for example the birth weight).
• Different concentrations of the same liquid, so the same volume delivers a different dose.

An independent second check of the calculation, ideally by another person, is standard safe practice.

Weight, obesity and large children

Weight-based dosing assumes typical body composition. For children who are obese, many drugs are dosed on an adjusted or ideal weight instead, and above roughly 40 kg the adult dose often becomes the ceiling. Check the guidance for the specific drug.

Frequently asked questions

Which drugs does this calculator cover?

It works with any drug that is dosed in mg/kg. You supply the reference dose, so it isn’t tied to a particular medicine — and it can’t tell you whether that dose is right.

What’s the difference between “per dose” and “per day” doses?

A per-dose figure is what each administration should contain. A per-day figure is the total for 24 hours that has to be divided across the doses. Using the wrong one is a classic source of error.

Why should I enter maximum doses?

Weight-based doses can exceed a safe adult or single-dose ceiling in larger children. Entering the maximums lets the calculator cap the result and tell you when it has done so.

How do I get the mL from a label like “250 mg/5 mL”?

Choose the mg/5 mL option and enter 250. The calculator converts it to 50 mg/mL and divides the dose by that.

Is it safe to rely on this calculator alone?

No. It is an aid for the arithmetic, not a prescribing tool. Always verify against the BNFc or local protocol and, where possible, have the calculation independently checked.

Medical disclaimer

Sources & references

• NICE. British National Formulary for Children (BNFc). bnfc.nice.org.uk
• NICE. Medicines optimisation: the safe and effective use of medicines to enable the best possible outcomes (NG5). nice.org.uk/guidance/ng5