Lessons · Weight-based (mg/kg)
Dosage calculation
Weight-based dosage calculation (mg/kg) Many drugs - especially in pediatrics - are dosed by body weight in mg/kg. Convert the weight to kilograms first (1 kg = 2.2 lb), multiply by the ordered mg/kg, and, when the order is a daily dose divided across the day, divide by the number of doses.
The formula Dose = Weight (kg) × ordered mg/kg (÷ doses per day if divided)
How to solve it Convert pounds to kilograms if needed: lb × (1 kg ÷ 2.2 lb). Multiply the weight in kg by the ordered mg/kg. If the order is mg/kg/day divided into several doses, divide the daily total by the number of doses. Compare against the drug's safe range for the child's weight before giving. Common mistakes Skipping the lb-to-kg conversion and dosing on pounds. Confusing mg/kg/dose with mg/kg/day. Not checking the result against the published safe maximum. Safety check: Always verify a pediatric result against the drug's safe mg/kg range for that child - a plausible-looking number can still be unsafe.
Worked examples
See it solved A patient weighs 88 lb. The prescriber orders clindamycin 30 mg/kg/day divided into 4 equal doses.
Weight 88 lb
Ordered 30 mg/kg/day
Safe max 40 mg/kg/day
Frequency 4 doses/day
How many mg is each dose?
Answer: 300 mg per dose
Step-by-step solution 1. Convert the patient's weight to kilograms
88 lb × ( 1 kg ÷ 2.2 lb )
= 40 kg
2. Multiply weight by the ordered mg/kg/day
40 kg × ( 30 mg ÷ 1 kg )
= 1200 mg/day
3. Divide the daily dose into 4 equal doses
1200 mg/day ÷ 4 doses
= 300 mg per dose
A patient weighs 66 lb. The prescriber orders acetaminophen 10 mg/kg/dose.
Weight 66 lb
Ordered 10 mg/kg/dose
Safe max 15 mg/kg/dose
How many mg is the dose?
Answer: 300 mg
Step-by-step solution 1. Convert the patient's weight to kilograms
66 lb × ( 1 kg ÷ 2.2 lb )
= 30 kg
2. Multiply weight by the ordered mg/kg
30 kg × ( 10 mg ÷ 1 kg )
= 300 mg
A patient weighs 60 kg. The prescriber orders gentamicin 1.5 mg/kg/dose.
Weight 60 kg
Ordered 1.5 mg/kg/dose
Safe max 2.5 mg/kg/dose
How many mg is the dose?
Answer: 90 mg
Step-by-step solution 1. Multiply weight by the ordered mg/kg
60 kg × ( 1.5 mg ÷ 1 kg )
= 90 mg
Safe number notation Every number on DoseDrill is written the safe way - and here is why it matters:
Always use a leading zero for amounts under 1: write 0.5 mg , never .5 mg . A missing zero is read as 5 - a ten-fold overdose.Never use a trailing zero after a decimal: write 5 mg , never 5.0 mg . A stray zero is read as 50 .Read the full safe-notation guide →
Keep going More worked weight-based (mg/kg) examples:
Related formulas:
Practice only - never for real medication. DoseDrill generates problems to help nursing students learn dosage calculation. It must never be used to calculate, prepare, or administer medication to an actual patient. Real administration requires an independent double-check, current drug references, and your facility's policy. A wrong dose can harm or kill - always verify with a licensed professional.