Weight-based dosing

Pup, the Pharm Pup mascot, thinking

Weight-based problems add two steps to a normal dose problem: change pounds to kilograms, and decide whether the order is per dose or per day.

The steps

  1. Weight in kg: kg = lb ÷ 2.2. Do not round yet.
  2. Dose: mg/kg × kg.
  3. Per day or per dose? If the order says mg/kg/day divided every 12 hours, divide the daily dose by 2 (every 8 hours: by 3; every 6 hours: by 4).
  4. Volume: dose ÷ concentration (D/H × Q).
  5. Safe range check: compare the ordered dose with the reference range for that weight.

Worked examples

Practice problem: acetaminophen 15 mg/kg, child 44 lb, supply 160 mg/5 mL
  1. 44 lb ÷ 2.2 = 20 kg
  2. 15 mg/kg × 20 kg = 300 mg
  3. 300 mg ÷ 160 mg × 5 mL = 9.375 mL

= 9.4 mL

Practice problem: amoxicillin 45 mg/kg/day divided every 12 hr, child 44 lb, supply 400 mg/5 mL
  1. 44 lb ÷ 2.2 = 20 kg
  2. 45 mg/kg/day × 20 kg = 900 mg/day
  3. 900 mg ÷ 2 doses = 450 mg per dose
  4. 450 mg ÷ 400 mg × 5 mL = 5.625 mL

= 5.6 mL per dose

Safe dose range check

Many questions give a reference range and ask whether the order is safe. Work out the range for the child's weight, then compare.

Practice problem: acetaminophen 240 mg ordered, child 33 lb, safe range 10 to 15 mg/kg/dose
  1. 33 lb ÷ 2.2 = 15 kg
  2. Low end: 10 mg/kg × 15 kg = 150 mg
  3. High end: 15 mg/kg × 15 kg = 225 mg
  4. 240 mg is above 225 mg

Not safe: hold and contact the prescriber

Body surface area (BSA)

Some drugs (often chemotherapy) are dosed per m² of body surface area. The Mosteller formula is BSA (m²) = √(height in cm × weight in kg ÷ 3,600).

Practice problem: 170 cm, 70 kg
  1. 170 × 70 = 11,900
  2. 11,900 ÷ 3,600 = 3.31
  3. √3.31

= 1.82 m²

Common mistakes

  • Using 44 lb as 44 kg: the dose comes out 2.2 times too large (20.6 mL instead of 9.4 mL in the first example).
  • Giving the whole daily dose at once (11.3 mL instead of 5.6 mL in the second example).
  • Rounding the weight to a whole kg before multiplying.

Practise: weight-based practice problems.

Questions, answered.

How do you calculate mg/kg?

Change the weight to kilograms (lb ÷ 2.2), then multiply by the ordered mg/kg. If the order is per day, divide by the number of doses per day.

Do you divide or multiply by 2.2?

To go from pounds to kilograms, divide by 2.2. To go from kilograms to pounds, multiply by 2.2.

How do I check if a pediatric dose is safe?

Multiply the low and high ends of the reference range by the weight in kg, and check that the ordered dose falls between them. If it does not, hold the dose and contact the prescriber.

Practise it in the app

Pharm Pup app screen: The replay: lb, kg and mg knocked out until mL stands
The replay: lb, kg and mg knocked out until mL stands.
Pharm Pup app screen: An IV drip round: 31 gtt/min by gravity, 125 mL/h on a pump
An IV drip round: 31 gtt/min by gravity, 125 mL/h on a pump.
Pharm Pup app screen: A timed mock round in your school format
A timed mock round in your school format.

Sources

Practice problems for study only, not clinical advice. Follow your school's rounding policy, your facility's policy and the prescriber's order. Reviewed 2026-10-30.

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