How do you calculate an mg/kg dose?
Multiply the dose per kilogram by the weight, then divide by the concentration. A weight-based order gives the dose for each kilogram of body weight, and two short steps turn it into something you can measure.
Dose (mg) = Ordered dose (mg/kg) × Weight (kg)
Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)
When the order is written per day (mg/kg/day), add one step in the middle: divide the daily dose by the number of doses per day. That gives the amount for each administration. The second step is the same sum as the mg to mL calculator.
How to use the mg/kg dose calculator
- Under The order is, choose mg/kg per dose or mg/kg per day, divided, exactly as the prescription is written.
- Enter the Ordered dose in mg/kg and the Weight in kg.
- For a daily order, enter the Doses per day (for example 3 for every 8 hours).
- If a ceiling applies, enter the Maximum single dose (optional) in mg.
- Enter the Liquid concentration in mg/mL from the label. Read the Volume per dose and the Dose per administration.
Worked examples
Example 1 – per dose. The order is 15 mg/kg, the child weighs 12 kg and the liquid is 24 mg/mL. Dose: 15 × 12 = 180 mg. Volume: 180 ÷ 24 = 7.5 mL.
Example 2 – per day, divided. The order is 30 mg/kg/day in 3 divided doses, the child weighs 18 kg and the liquid is 50 mg/mL. Daily dose: 30 × 18 = 540 mg. Single dose: 540 ÷ 3 = 180 mg. Volume: 180 ÷ 50 = 3.6 mL.
Example 3 – the maximum applies. The order is 20 mg/kg with a maximum single dose of 500 mg, the patient weighs 45 kg and the liquid is 50 mg/mL. Dose: 20 × 45 = 900 mg. That is above 500 mg, so the calculator uses 500 mg and shows a warning. Volume: 500 ÷ 50 = 10 mL.
Quick reference
Dose and volume per dose with a 25 mg/mL liquid.
| Weight | 10 mg/kg | Volume | 15 mg/kg | Volume |
|---|---|---|---|---|
| 5 kg | 50 mg | 2 mL | 75 mg | 3 mL |
| 10 kg | 100 mg | 4 mL | 150 mg | 6 mL |
| 15 kg | 150 mg | 6 mL | 225 mg | 9 mL |
| 20 kg | 200 mg | 8 mL | 300 mg | 12 mL |
| 30 kg | 300 mg | 12 mL | 450 mg | 18 mL |
Common mistakes
- Per day read as per dose. A daily total given in one go is two, three or four times too much.
- Pounds instead of kilograms. 1 kg is about 2.2 lb, so using pounds more than doubles the result.
- An old weight. Children grow quickly. A weight from months ago can make the dose noticeably wrong.
- The wrong weight for the patient. In obesity, it’s worth checking which weight your protocol asks for. The guide to actual, ideal and adjusted body weight explains the difference, and the adjusted body weight calculator works it out.
- The wrong product strength. The same medicine may come as several liquids of different strength. Enter the concentration of the bottle in your hand.
- Ignoring the maximum. In heavier children and adults, mg/kg × weight can exceed the usual maximum dose.
A quick sense check
It’s worth asking whether the result fits the size of the patient. A weight-based dose for a child should rarely be larger than the usual adult dose of the same medicine. If it is, we recommend checking the order, the weight and the units before going further.
Frequently asked questions
How do I calculate a dose in mg/kg?
Multiply the dose per kilogram by the weight in kilograms. 15 mg/kg for a 12 kg child is 15 × 12 = 180 mg. Then divide by the concentration to get the volume: 180 mg of a 24 mg/mL liquid is 7.5 mL. The calculator does both steps and shows each one.
What is the difference between mg/kg per dose and mg/kg per day?
A per-dose order is the amount for each administration; a per-day order is the 24-hour total, which you divide by the number of doses. 30 mg/kg/day in 3 doses is 10 mg/kg each time. Reading a daily total as a single dose multiplies the dose by the number of doses per day.
What does the maximum single dose field do?
It caps the dose. If the dose for one administration comes out above the maximum you enter, the calculator uses the maximum, shows a warning and works out the volume for that amount. With a per-day order the cap applies to each divided dose, not to the daily total. Leave the field empty when no ceiling applies. It’s best to take the maximum from the prescription or the product information, not from memory.
Can I enter the weight in pounds?
No, the calculator expects kilograms. To convert, divide pounds by 2.2: 44 lb ÷ 2.2 = 20 kg. Entering pounds as if they were kilograms more than doubles the dose, one of the classic weight-based errors. If the weight is recorded in pounds, it helps to convert it and write down the kg value first.
Which weight should I use?
A recent, measured weight in kilograms is advisable. For patients with obesity, some protocols use ideal or adjusted body weight instead of actual weight, so it’s best to follow the one your local guidance names. An estimated or months-old weight, especially in a growing child, can shift the dose noticeably.
How is the result rounded?
The calculator shows the dose and the volume to two decimal places. Round only at the end, and only to what your oral or injection syringe can actually measure. If you round the dose in mg first and then work out the volume, small errors add up.