Calculator
Weight-based dose calculator (mg/kg)
Enter the dose your doctor or pharmacist prescribed in mg/kg, your child’s weight, and the strength printed on the bottle. The calculator returns the amount to give per dose in mL or tablets, the daily total, and a mg/kg/day cross-check so a ten-fold error is easy to spot. It doesn’t decide the dose for you: for over-the-counter medicines, the medicine pages do that from published tables.
Calculate a dose
What the frequency abbreviations mean
| On the prescription | Means | Doses in 24 hours |
|---|---|---|
| qD, q24h | Once a day | 1 |
| BID, q12h | Twice a day | 2 |
| TID, q8h | Three times a day | 3 |
| QID, q6h | Four times a day | 4 |
| q4h | Every 4 hours | 6 |
| q3h | Every 3 hours | 8 |
| q2h | Every 2 hours | 12 |
| q48h, QOD | Every other day | 0.5 |
| PRN | As needed, up to the stated limit | - |
Worked example
A doctor prescribes amoxicillin at 45 mg/kg/day, twice a day for a child who weighs 15 kg, and the pharmacy gives you the 250 mg/5 mL suspension.
- Daily total: 45 mg/kg/day × 15 kg = 675 mg a day.
- Per dose: 675 mg ÷ 2 doses = 337.5 mg.
- Strength per mL: 250 mg ÷ 5 mL = 50 mg in each mL.
- Volume: 337.5 mg ÷ 50 mg/mL = 6.75 mL, so about 6.8 mL twice a day.
The calculator does these four steps and shows them under the result, so you can check the arithmetic against the pharmacy label.
The error this catches
The most common weight-based mistake is treating a mg/kg/day figure as if it were mg/kg/dose, or the reverse. With three doses a day, that’s a three-fold error. The calculator always prints the mg/kg/day the result works out to, so you can compare it against the range your prescriber intended.
It also warns when the volume is bigger than a standard oral syringe holds, or too small to measure accurately, which is what a ten-fold strength mix-up usually looks like.
What this calculator will not do
- It doesn’t know your child’s medicine, condition, allergies, or kidney and liver function, and it has no maximum dose built in.
- It won’t choose a dose. For over-the-counter medicines, use the dosage charts, which come from published tables.
- It isn’t a substitute for the label on your child’s prescription. If the two disagree, call your pharmacist.
Common questions
How do you calculate a dose in mg/kg?
Multiply the mg/kg figure by your child’s weight in kg. If the prescription is written per day (mg/kg/day), divide that daily total by the number of doses per day to get one dose. Then divide the dose in mg by the liquid’s strength in mg per mL to get the mL to give.
What is the difference between mg/kg/day and mg/kg/dose?
mg/kg/day is the total for the whole day, which you then split between doses. mg/kg/dose is the amount for each single dose. Mixing them up is the most common weight-based dosing error, which is why this calculator asks which one you have and shows the daily total either way.
How many doses is q8h?
3 doses in 24 hours. See the abbreviation table above: BID is 2, TID is 3, QID is 4, and q6h is also 4.
Does this calculator tell me the right dose for my child?
No. It converts a dose someone has already prescribed into mL or tablets. For over-the-counter medicines where a published dose table exists, use the medicine pages instead, which look the dose up for you.
Sources
- Metric Units and the Preferred Dosing of Orally Administered Liquid Medications. American Academy of Pediatrics Committee on Drugs, Pediatrics 2015;135(4):784-787. Checked 2026-09-18.