Dosage Practice

mg/kg Dosage Calculation: Weight-Based Doses for Children and Adults

Multiply the dose per kilogram by the patient's weight in kilograms. If the weight is in pounds, divide by 2.2 and round to the nearest tenth of a kilogram first. If the order is per day in divided doses, divide the daily total by the number of doses.

The Formula

dose = mg/kg × weight in kg (kg = lb ÷ 2.2)

Per-day orders: divide the daily dose by how many times a day it is given.

How to work it out

  1. 1 Get the weight into kilograms. Pounds divided by 2.2, rounded to the nearest tenth.
  2. 2 Multiply the ordered mg/kg by the weight.
  3. 3 Read the order carefully: mg/kg/dose is a single dose; mg/kg/day is the whole day, to be divided by the frequency.
  4. 4 Then, if asked, carry that dose into a tablets or milliliters calculation.

Worked Examples

Example 1

Ordered: 15 mg/kg per dose. The child weighs 20 kg. How many mg per dose?

  1. 15 mg/kg × 20 kg = 300 mg.

Answer: 300 mg

Example 2

Ordered: 10 mg/kg per dose. The child weighs 44 lb. How many mg per dose?

  1. 44 lb ÷ 2.2 = 20 kg.
  2. 10 mg/kg × 20 kg = 200 mg.

Answer: 200 mg

Try five free problems

One exact answer, rounded as asked, no partial credit. No sign-up needed.

Problem 1 of 3No sign-up needed

Ordered: 10 mg/kg per dose. The child weighs 25 kg. How many mg per dose?

Round to the nearest whole number

mg

Common Mistakes

  • Using pounds as if they were kilograms

    A 44 lb child weighs 20 kg. Multiplying the mg/kg by 44 more than doubles the dose.

  • Giving the daily dose as one dose

    40 mg/kg/day divided every 8 hours is three doses. The single dose is the daily total divided by three.

  • Rounding the weight late

    Round the weight to the nearest tenth of a kilogram before multiplying. Rounding only at the end can shift the answer.

Questions Students Ask

What does q8h mean for the number of doses?

Every 8 hours is three doses in 24 hours. q6h is four, q12h is two, and BID, TID and QID are two, three and four.

Do adults get mg/kg doses too?

Yes, especially for IV antibiotics, anticoagulants and critical care drips. The arithmetic is identical.

Keep practicing in lesson 7: mg/kg Dosing

Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.

Open lesson 7