Dosage Calculator — Dose by Weight in mg/kg or mg/lb

Enter the patient's body weight and the prescribed dose rate to calculate the exact medication dose in milligrams instantly. This weight-based dosing calculator supports both mg/kg and mg/lb units, making it suitable for pediatric, adult, and veterinary dose calculations.

Created by Ashar PervaizMedically reviewed by Dr. Syeda Khadija AkbarLast reviewed 14 October 2026

One Prescription, Three Different Numbers

A weight-based prescription contains more numbers than it appears to. Read the line "amoxicillin 40 mg/kg/day in three divided doses" and there are three separate quantities in play: the rate (40 mg/kg/day), the total daily dose in milligrams, and the amount that actually goes into the spoon or syringe each time. Confusing the second for the third gives a patient three times their intended dose and is one of the most reproducible errors in medication maths.

This page works through the three questions that sit between a prescription and an administered dose, in the order they have to be answered: which weight to use, what the total comes to, and how to split and measure it.

Which Weight Do You Dose On?

The instinctive answer is "the one on the scale", and for most patients it is right. But actual body weight is not the dosing weight for every drug or every patient, and a calculator will happily multiply whatever you type in. The distinction matters most at the extremes of body composition.

Ideal body weight is calculated from height using the Devine equations, which take a baseline at 5 feet and add a fixed increment per additional inch:

Men: IBW (kg) = 50 + 2.3 × (height in inches over 60)
Women: IBW (kg) = 45.5 + 2.3 × (height in inches over 60)

A woman of 5 feet 6 inches has an IBW of 45.5 + (2.3 × 6) = 59.3 kg. If she weighs 95 kg, adjusted body weight at the commonly used 0.4 factor is 59.3 + 0.4 × (95 − 59.3) = 73.6 kg. Dosing the same 5 mg/kg order on those three weights gives 475 mg, 297 mg, or 368 mg — a spread of more than 60% from a single prescription. Which one is correct is a drug-specific question answered by the prescribing reference, not by the calculator.

For children the question is usually settled the other way: paediatric dosing is on actual weight, measured today, in kilograms, because growth makes any recorded weight stale quickly.

From mg/kg to the Amount in the Syringe

Once the weight is settled, the total dose is a multiplication:

Dose (mg) = Weight (kg) × Prescribed rate (mg/kg)

A 24 kg child prescribed 15 mg/kg of paracetamol needs 360 mg. That is the answer to the arithmetic, but nobody administers milligrams. The second step converts it into a measurable volume using the strength on the bottle:

Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)

Paediatric suspensions are labelled per 5 mL rather than per mL, which is where the conversion trips. A bottle marked 250 mg/5 mL is 50 mg/mL, so 360 mg is 360 ÷ 50 = 7.2 mL. Reading the label as 250 mg/mL and dividing gives 1.44 mL — a fifth of the intended dose, and an error that looks entirely reasonable in a syringe.

Always divide the labelled strength by 5 before using it. The dose stock calculator handles the per-5-mL form directly if you would rather not convert by hand.

"Three Times a Day": Splitting a Daily Dose

A large share of dosing errors live in this one step. Prescriptions are frequently written as a daily total to be divided, and the number that gets calculated is the daily total, not the individual dose.

Single dose = (Weight × Daily rate in mg/kg/day) ÷ Number of doses per day

An 18 kg child on 40 mg/kg/day of amoxicillin in three divided doses: 18 × 40 = 720 mg per day, divided by 3 = 240 mg per dose. At 250 mg/5 mL that is 240 ÷ 50 = 4.8 mL, three times daily. Giving 720 mg at each administration would deliver 2,160 mg in a day against an intended 720.

Prescription abbreviations carry the frequency, and they are worth reading precisely because several look alike:

The last two rows are not interchangeable with the rows above them even though the dose count matches. "Three times daily" is usually taken with meals across waking hours; "every eight hours" means a dose overnight. For antibiotics where the time spent above a minimum concentration drives the effect, that difference is clinically real rather than pedantic.

When the Weight-Based Answer Is Too High

Weight-based dosing assumes a linear relationship between size and dose, and that assumption fails at the top end. Many paediatric drugs carry a maximum single dose or a maximum daily dose that applies regardless of what the multiplication produces, usually pegged to the standard adult dose.

A 60 kg adolescent on a 15 mg/kg paracetamol order calculates to 900 mg, above the usual 1 g single dose only marginally — but the same child on a drug capped at 500 mg would calculate to 900 mg and need capping. Any weight-based result should be compared against the reference maximum before it is given, and a heavier child should never receive more than an adult would.

The same ceiling logic applies in reverse for renal or hepatic impairment, where the correct dose can be lower than weight alone suggests. That adjustment is driven by clearance rather than mass — our pharmacokinetics calculator covers the relationship between clearance, half-life and maintenance dosing, and our creatinine clearance calculator estimates the kidney side of it from an age, a weight and a blood result.

Pounds, and Veterinary Orders

Orders written in mg/lb appear in veterinary practice and in some consumer product labelling. The conversion is exact: 1 lb = 0.45359237 kg, so a rate in mg/lb is roughly 2.2 times smaller in magnitude than the same clinical intent expressed in mg/kg.

Weight in kg = Weight in lb × 0.4536

A 44 lb dog is 19.96 kg. Prescribed at 2 mg/lb, the dose is 88 mg; converting the rate instead and applying 2 mg/kg to 19.96 kg gives 39.9 mg. Both calculations are internally consistent and one of them is half the intended dose, so keep the units of the order and the units of the weight aligned rather than converting mid-problem. Species-specific references matter here too: doses for cats, dogs and horses diverge sharply, and human references do not transfer.

One Prescription, Start to Finish

Putting the steps together on a single realistic order. A 27 kg child is prescribed cefalexin 25 mg/kg/day in two divided doses; stock is 125 mg/5 mL.

Daily dose = 27 × 25 = 675 mg
Per dose = 675 ÷ 2 = 337.5 mg
Concentration = 125 ÷ 5 = 25 mg/mL
Volume per dose = 337.5 ÷ 25 = 13.5 mL, twice daily

Four operations, each one a place an error can enter: the wrong weight, the wrong divisor, an unconverted per-5-mL strength, or a decimal slip in the final division. Working them in a fixed order and writing each intermediate result down is what makes the check possible; a single answer with no working cannot be verified by anyone else.

Sense-Checking the Result

• Is the volume measurable? Oral syringes read to 0.1 or 0.2 mL. A result of 0.03 mL cannot be given accurately and usually signals a concentration error.
• Is the volume plausible for the patient? More than about 10 mL in a single dose for an infant, or a tablet count above two, is worth re-deriving before it is given.
• Does the daily total stay under the reference maximum once every dose is added up, including any of the same drug in a combination product?
• Was the weight measured, in kilograms, and recorded today for a child?
• Does the frequency in your calculation match the frequency written on the prescription, not the one you expected to see?

For intravenous orders, where a rate and a duration join the arithmetic, continue with the IV calculator. For a longer walkthrough with additional examples, see our guide to medication dose calculation.

One class of order will not fit the calculator above, and it is worth knowing why rather than forcing it. Immunoglobulin is dosed in grams per kilogram instead of milligrams, and the rate it is given at is ordered in mg/kg/min, so reaching a pump setting needs the product concentration as well as the weight. The IVIG calculator handles that sequence, including the step-up schedule a single rate cannot describe.

Dosing Questions People Actually Get Wrong

Educational reference only. Confirm any clinical calculation independently before acting on it. How this page is reviewed