Safety first: this calculator checks arithmetic for a dose that a prescriber or product label has already given you. It contains no drug information and cannot tell you whether a dose is right for a person, a medicine or a condition. If anything here doesn't match the label or what your prescriber told you, stop and ask your pharmacist.
How to Calculate a Dose in mg/kg
Dose (mg) = Weight (kg) × Prescribed dose (mg/kg)
Weight-based doses are common for children and for some medications in adults. The prescription gives a rate, such as "15 mg/kg per dose", and the actual amount depends on body weight.
| Weight | Prescribed | Calculation | Dose |
|---|---|---|---|
| 20 kg | 15 mg/kg per dose | 20 × 15 | 300 mg per dose |
| 44 lb (19.96 kg) | 10 mg/kg per dose | 19.96 × 10 | 199.6 mg per dose |
| 30 kg | 40 mg/kg per day, in 2 doses | 30 × 40 ÷ 2 | 600 mg per dose (1,200 mg/day) |
These numbers show the method only. They are not doses for any particular medicine.
Per Dose or Per Day? Read the Prescription Carefully
This is the most important detail. "10 mg/kg per dose" and "10 mg/kg per day" give very different amounts:
- Per dose: multiply weight by the rate to get each dose.
- Per day: multiply weight by the rate to get the daily total, then divide by the number of doses. "30 mg/kg/day divided every 8 hours" means three doses a day, each one third of the daily total.
If the prescription doesn't say which, ask before calculating.
Pounds to Kilograms
kg = lb ÷ 2.2046 (or lb × 0.4536)
Mixing up pounds and kilograms is one of the most common dosing errors, and it more than doubles (or halves) the dose. Choose the unit carefully. The calculator warns you if a weight looks like it was entered in the wrong unit. Use a recent weight; children's weights change quickly.
Converting mg to mL for Liquid Medicine
Volume (mL) = Dose (mg) ÷ Concentration (mg per mL)
Liquid strength is printed on the bottle as an amount per volume, such as "160 mg/5 mL". That's 32 mg in each mL. A 240 mg dose is 240 ÷ 32 = 7.5 mL.
- The same medicine can come in more than one strength, for example infant and children's versions. Always use the strength on the bottle you're holding.
- Measure with an oral syringe or the dosing cup that came with the medicine. Kitchen spoons are not accurate.
- The calculator rounds mL to one decimal place, matching common syringe markings, and also shows the exact amount.
Maximum Doses
Many weight-based prescriptions include a maximum, so a heavier person doesn't receive more than a set amount per dose or per day. If your prescription or label lists one, enter it. The calculator uses whichever is lower, the calculated dose or the maximum, and tells you when the maximum was applied.
Common Mistakes This Calculator Helps Catch
- Weight entered in pounds when the unit is kilograms, or the reverse
- A per-day rate treated as per-dose, giving several times too much
- Confusing mg with mL when measuring a liquid
- Using the concentration of a different product or strength
- Ignoring a maximum dose on the label
For how long a bottle of liquid will last at the dose you calculated, use the liquid medication days supply calculator. For tablets, see the tablets and capsules calculator.
When to Call Instead of Calculating
Call your pharmacist or prescriber if the calculated dose doesn't match the label, if you're unsure of the weight or the units, or if the dose is for an infant. If someone may have taken too much medicine, call Poison Control at 1-800-222-1222 right away, or 911 for an emergency.