Pediatric Dose Calculator: Weight-Based Dosing for Any Drug

Medically reviewed by Dr. Taimoor Asghar, physician.

In short: Universal paediatric dose calculator: enter any drug's mg/kg dose, the child's weight in kg or lb, an optional maximum dose cap and concentration, and get the dose per administration and volume. Free, no sign-up. Use the calculator above, then read the guide below to interpret your result and its limitations.

Enter the mg/kg dose from the prescription or formulary, the child's weight in kilograms or pounds, and this calculator works out the dose per administration, applies an optional maximum dose cap with a warning, and converts to millilitres when you give the concentration.

How is the dose prescribed? Dose per kilogram (mg/kg)

Dosing interval (per-day mode)

Child's weight Weight unit Maximum dose per administration (mg) optional, from the product literature Concentration (mg/mL) optional, for the volume in mL

Dose per administration.

Why paediatric doses are calculated from weight

Children are not small adults, and a fixed dose that suits a teenager would overdose a toddler. Almost every paediatric medicine is therefore prescribed in proportion to body weight, in milligrams per kilogram (mg/kg), sometimes with an adjustment for age, body surface area or renal function for specific drugs. Weight-based prescribing only works if the arithmetic is done carefully, and medication errors in children are disproportionately arithmetic errors: a misplaced decimal point produces a tenfold overdose, and a pounds-to-kilograms mix-up produces a dose more than twice too large. A dedicated calculator removes the mental-arithmetic step, shows each stage of the working, and forces the two checks that prevent the commonest mistakes: which basis the prescription uses (per dose or per day), and whether a maximum dose applies.

This tool is deliberately drug-agnostic. It holds no dosing database and recommends nothing; the prescriber or the product literature supplies the mg/kg figure for the specific drug, indication and age group, and the calculator performs the multiplication, the division into doses, the optional maximum-dose cap and the optional conversion from milligrams to millilitres. That separation is a safety feature: the person using it must actively look up the correct per-kilogram dose for the drug in front of them, rather than trusting a generic table.

The two prescription bases: per dose and per day

Paediatric references write doses in two ways, and mixing them up is dangerous. In the per-dose basis, the reference states the amount for each administration, for example "10 mg/kg per dose every 8 hours". The calculation is a single multiplication: dose per kilogram times weight. The interval tells you how often to give it, but it does not change the size of each dose. In the per-day basis, the reference states the total for 24 hours, for example "40 mg/kg/day in 4 divided doses". Now there are two steps: multiply by weight to get the total daily amount, then divide by the number of doses per day to get each administration. A prescription of 40 mg/kg/day given every 6 hours for a 20 kg child is 800 mg across the day and 200 mg per dose, not 800 mg per dose. Giving the daily total at every administration would be a fourfold overdose, which is exactly the error the per-day mode's explicit two-step display is designed to prevent.

The interval and the number of doses per day are the same information in two forms: once daily (q24h) is 1 dose per day, every 12 hours (q12h) is 2, every 8 hours (q8h) is 3, every 6 hours (q6h) is 4, every 4 hours (q4h) is 6. The calculator's interval selector covers these standard schedules, with a custom option for less common ones such as five doses per day.

Maximum doses: why the cap matters

Weight-based dosing has an upper boundary. Most paediatric references state a maximum single dose, and frequently a maximum daily dose, beyond which the dose should not rise however heavy the child is. The usual purpose is to keep a large child or adolescent at or below the established adult dose, and for some drugs the maximum reflects a toxicity threshold rather than the adult dose. If you enter the maximum from the product literature, this calculator compares the calculated dose against it. When the weight-based result exceeds the cap, the final dose is the cap, and a red warning states plainly that capping occurred and shows the uncapped figure, because a silent adjustment is how discrepancies between a prescription and an administration go unnoticed. Note that the cap in this tool applies per administration; where a reference also states a maximum daily amount, check the total daily figure the calculator displays against it as well, especially in per-day mode with an unusual interval.

From milligrams to millilitres

Many paediatric medicines are liquids, and the dose has to be measured as a volume. The conversion is a division: millilitres equal milligrams divided by the concentration in mg/mL printed on the bottle. Concentrations are a notorious trap, because the same drug is often sold in more than one strength, and a volume calculated for one strength is wrong, sometimes dangerously wrong, for another. Always read the concentration from the actual bottle being used, not from memory, and re-check whenever a new bottle or a different brand is started. When a maximum dose cap has been applied, the calculator converts the capped dose, so the volume shown always matches the dose shown. Volumes are given to two decimal places; in practice, round to what an oral syringe can measure accurately, and use a proper oral syringe rather than a household spoon, whose volume varies widely.

Worked examples

Prescription	Weight	Working	Result
10 mg/kg per dose	15 kg	10 x 15	150 mg per dose
40 mg/kg/day, every 6 hours	20 kg	40 x 20 = 800 mg/day; 800 / 4 doses	200 mg per dose
15 mg/kg per dose, concentration 25 mg/mL	12 kg	15 x 12 = 180 mg; 180 / 25	180 mg = 7.2 mL per dose
50 mg/kg/day, every 12 hours, maximum 1000 mg per dose	45 kg	50 x 45 = 2250 mg/day; 2250 / 2 = 1125 mg, above the cap	1000 mg per dose, cap warning shown

Each of these can be reproduced in the calculator above, and each illustrates one of its functions: the simple multiplication, the per-day division, the volume conversion and the maximum dose cap.

Safety checks before any dose is given

A calculator is one layer of safety, not the whole system. Before administering any weight-based dose, confirm the weight is current and was measured, not estimated or copied from an old record, because children grow and an out-of-date weight silently corrupts every calculation based on it. Confirm the units at every step: weight in kilograms for the reference dose, concentration in mg/mL for the volume. Read the prescription basis twice, per dose or per day, and sanity-check the answer against the child's previous doses and against the adult dose, which a paediatric dose should not exceed unless the reference explicitly allows it. For high-alert medicines and for any dose that looks unusual, an independent second check by another clinician is standard practice in hospital settings and is worth copying anywhere. If any figure cannot be verified against the prescription and the product information, stop and clarify rather than estimating.

Drug-specific calculators

When you need the per-kilogram dose, indication-specific ranges, age restrictions and maximum doses for a particular medicine, use the drug-specific paediatric calculators on this site. Each one is built around that drug's own dosing references:

Adults and older adolescents dosed by weight outside paediatrics can also use the general drug dose calculator, which applies the same arithmetic with a maximum dose cap.

Key takeaways

Multiply the dose per kilogram by the child's weight in kilograms.
mg/kg/dose states how much drug is given in each single administration, and the interval (for example every 8 hours) is prescribed separately.
Most paediatric medicines have a maximum single dose (and often a maximum daily dose) that applies no matter how heavy the child is, so that a large child does not receive more than the adult dose.
Divide the dose in milligrams by the concentration of the liquid in milligrams per millilitre, which is printed on the bottle or in the product information.

Frequently asked questions

How do I calculate a child's dose from mg/kg?

Multiply the dose per kilogram by the child's weight in kilograms. For example, 10 mg/kg for a 15 kg child is 10 x 15 = 150 mg. If the prescription is written per day, such as 40 mg/kg/day, first multiply by the weight to get the total daily amount (40 x 20 kg = 800 mg/day), then divide by the number of doses per day: given every 6 hours there are 4 doses per day, so each dose is 800 / 4 = 200 mg.

What is the difference between mg/kg/dose and mg/kg/day?

mg/kg/dose states how much drug is given in each single administration, and the interval (for example every 8 hours) is prescribed separately. mg/kg/day states the total amount for the whole day, which must be divided into equal doses according to the interval: every 24 hours is 1 dose per day, every 12 hours is 2, every 8 hours is 3, every 6 hours is 4 and every 4 hours is 6. Confusing the two is a recognised source of dosing errors, so always check which basis the prescription uses.

Why does the calculator warn about a maximum dose?

Most paediatric medicines have a maximum single dose (and often a maximum daily dose) that applies no matter how heavy the child is, so that a large child does not receive more than the adult dose. If you enter the maximum from the product literature and the calculated weight-based dose exceeds it, the calculator caps the dose at the maximum and shows a clear warning instead of silently using the higher number.

How do I convert a dose in mg to a volume in mL?

Divide the dose in milligrams by the concentration of the liquid in milligrams per millilitre, which is printed on the bottle or in the product information. For example, a 150 mg dose of a suspension containing 25 mg/mL is 150 / 25 = 6 mL. Enter the concentration in the calculator and it performs this step for you, using the capped dose whenever a maximum dose was applied.

Can I enter the weight in pounds?

Yes. Select lb and the calculator converts to kilograms using the exact conversion 1 lb = 0.45359237 kg before calculating. Paediatric dosing references are written in kilograms, so if the weight was measured in pounds it must always be converted first; dosing directly from pounds without converting is a classic cause of a more than twofold overdose.

Does this calculator tell me how much of a specific drug to give?

No. This is a universal arithmetic tool: you supply the mg/kg dose from the prescription, formulary or product literature for the specific drug and indication, and the calculator does the multiplication, division, capping and unit conversion accurately. It holds no drug database and makes no recommendation. For drug-specific dosing, use the per-drug calculators linked on this page and always confirm against the current product information or your local formulary before administering.

References and further reading

Medical disclaimer: This calculator performs arithmetic only and does not recommend any dose for any drug. The per-kilogram dose, maximum dose and concentration must come from the prescription, the current product information or an approved formulary, and the result must be checked before any medicine is given to a child. If you are a parent or carer and anything about a dose is unclear, ask the prescribing clinician or pharmacist rather than relying on any online calculator.