Weight-Based Dosing (mg/kg): Definition and Worked Example

Weight-based dosing: Ordering a drug per kilogram of body weight (mg/kg/dose or mg/kg/day), so the dose scales with the client's size.

In weight-based dosing the order gives an amount per kilogram, so the dose depends on the client’s weight. It is the rule in pediatrics and common for adult drugs such as heparin and many antibiotics.

1. Convert the weight to kilograms (divide pounds by 2.2).
2. Multiply by the ordered amount per kg.
3. For a per-day order, divide by the number of doses a day (“divided every 12 hours” is 2 doses).
4. Check the result against the safe dose range.

Watch the wording: mg/kg/dose and mg/kg/day differ by the number of doses, and mixing them up is a common exam trap.

Order: amoxicillin 45 mg/kg/day PO divided every 12 hours. The client is a 3-month-old infant who weighs 15 lb (1 kg = 2.2 lb).

Solve for

Dose from mg/kg

Weight

15 lb

Doses per day

2

Ordered dose per kg

45 mg/kg/day

Formula Dose = mg/kg × weight in kg.

1. kg = 15 ÷ 2.2 = 6.82 kg; round the weight to the nearest tenth: 6.8 kg.
2. Daily dose = 45 mg/kg/day × 6.8 kg = 306 mg/day.
3. Divided every 12 hours is 2 doses a day: 306 ÷ 2 = 153 mg per dose.
4. 153 mg is exact, so no rounding is needed.

Answer 153 mg

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