Weight-Based Dose Calculator (mg/kg)
Turn a per-kilogram order (mg/kg/day or mg/kg/dose) into the amount per dose and per day, with optional maximum-dose caps, the liquid volume to measure, tablets per dose and the total for a course.
How it works
Per kg per day: daily dose = weight(kg) × dose/kg/day, dose = daily dose ÷ doses per day · Per kg per dose: dose = weight(kg) × dose/kg/dose, daily dose = dose × doses per day · dose = min(dose, maximum single dose, maximum daily dose ÷ doses per day) · volume (mL) = dose ÷ concentration (per mL) · tablets = dose ÷ tablet strength
Many medicines, especially for children, are ordered per kilogram of body weight so that each person gets a dose matched to their size. The order is written either per day (for example 30 mg/kg/day, the total for 24 hours, divided into several doses) or per dose (for example 10 mg/kg/dose, given each time). Choose which one your order uses, enter the weight in kilograms or pounds and pick the dose unit (mg, mcg, g, units, mEq or mmol). The calculator gives the amount per dose and per day, the dosing interval and the equivalent rate in the other form so you can cross-check. Most labels also set a maximum single or daily dose that applies however heavy the patient is; enter it under More options and the dose is capped. For a liquid, enter the concentration as written on the bottle, such as 250 per 5 mL, to get the volume to measure, optionally rounded to what your syringe can measure. You can also count tablets per dose and total the amount for a course. The tool is generic: it does not know any drug’s limits, so always check the result against the drug label or a pharmacist.
Worked example
A 70 kg patient is ordered 10 mg/kg/day in 3 divided doses: the daily dose is 70 × 10 = 700 mg, so each dose is 700 ÷ 3 ≈ 233.3 mg, every 8 hours (3.33 mg/kg/dose). With a suspension of 250 mg per 5 mL (50 mg/mL), one dose is 233.3 ÷ 50 ≈ 4.67 mL. If instead a 20 kg child is ordered 15 mg/kg/dose 4 times a day with a maximum single dose of 250 mg, the weight-based dose of 300 mg is capped at 250 mg, giving 1,000 mg a day.
Sources: NIH/NLM DailyMed — drug labeling and dosing · ISMP — List of error-prone abbreviations, symbols and dose designations
Frequently asked questions
Is the rate per day or per dose?
Read the order carefully. “mg/kg/day” is the total for the whole day and is split across the doses; “mg/kg/dose” is given at each dose. Choose the matching option at the top of the calculator. The results also show the same order written the other way, which is a quick way to spot a mix-up.
How do I enter a concentration like 250 mg/5 mL?
Type 250 and choose “per 5 mL” next to the concentration field. The calculator divides by 5 to get 50 mg per mL. Use the same unit as the dose: for a microgram order, enter micrograms per mL.
What does the maximum dose cap do?
Many drugs have a maximum single or daily dose that applies even if the per-kg calculation comes out higher, which often happens in larger children and adults. Enter the label’s maximum under More options and the dose is capped. The weight-based dose before the cap is still shown.
Should I round the dose?
Doses are usually rounded to a measurable volume or a practical tablet size, and rounding rules vary by drug and setting. You can round the volume to 0.1, 0.2, 0.5 or 1 mL and see the dose that volume actually delivers. The calculator warns you when rounding changes the dose by more than 10%.
Which weight should I use?
Use the weight your prescribing reference specifies for that medicine. Most drugs use actual weight, but some are dosed on ideal, adjusted or lean body weight, especially in obesity. Weigh children on the day if you can, because their weight changes quickly.
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Educational estimate only — not medical advice, and not a substitute for a clinician, pharmacist or product label. Dosing, fluid and kidney decisions must be confirmed by a qualified professional using the actual drug, patient and situation. Do not use this tool to self-treat.