Weight-Based Dosage Calculations (Pediatric and Adult)
How to calculate medication doses based on patient weight, with the mg/kg formula and worked examples for both adult and pediatric patients.
Many medications aren't dosed as a flat amount — they're dosed based on how much the patient weighs. This matters even more in pediatrics, where a dose that's safe for a 70 kg adult could be dangerously high for an 8 kg infant.
The basic weight-based formula
Total dose = Dose ordered per kg × Patient weight in kg
The "kg" in the ordered dose cancels the "kg" in the weight, leaving you with the drug unit (mg, mcg, or units).
Step 1: Always confirm the weight is in kilograms
A huge number of errors in this calculation category come from mixing up pounds and kilograms. If a patient's weight is documented in pounds, convert first:
Weight in kg = Weight in lbs ÷ 2.2
Sanity check: the number in kg should always be smaller than the number in pounds.
Worked example (adult)
Order: Administer a medication at 5 mg/kg. Patient weighs 154 lbs.
Step 1: Convert weight — 154 ÷ 2.2 = 70 kg Step 2: Apply the formula — 5 mg/kg × 70 kg = 350 mg
Worked example (pediatric)
Order: Administer a medication at 10 mg/kg/day, divided into 2 doses. Patient weighs 15 kg (33 lb).
Step 1: Calculate total daily dose — 10 mg/kg × 15 kg = 150 mg/day Step 2: Divide into the ordered number of doses — 150 mg ÷ 2 = 75 mg per dose
Why pediatric weight-based dosing carries extra risk
Pediatric patients have much less margin for a calculation error than adults — a mistake that would be a minor overdose in an adult can be clinically significant in an infant. This is part of why pediatric medication errors are a consistent focus area in nursing education and on the NCLEX.
Some high-precision pediatric medications use body surface area instead of weight alone — see pediatric dosing by body surface area.
Common mistakes to avoid
- Forgetting to convert pounds to kilograms before calculating
- Confusing a total daily dose with a per-dose amount when a medication is divided into multiple doses
- Using an outdated or incorrectly recorded weight, especially in a patient whose weight has changed recently
Exam habit
Underline the time unit in every order: "per dose," "per day," or "per hour." It decides whether you divide at the end.
Weight-based dosing is just one piece of the full calculation picture nursing students need to master. The Complete Dosage Calculation Practice Workbook covers this topic in depth alongside pediatric body-surface-area dosing, IV calculations, and more — all with full worked solutions.