mg/kg calculation for pediatric dosing
How to turn a weight based plan (mg/kg) into an amount (mg per dose). Educational only: this page does not recommend specific doses.
Clinical disclaimer
What mg/kg means
- mg is the amount of medication (mass).
- kg is the patient weight in kilograms.
- mg/kg is a weight based plan. It may be written per dose or per day (for example, mg/kg/day divided into multiple doses).
Core formulas
The calculation depends on whether the plan is written per dose or per day.
If the plan is mg/kg per dose
mg per dose = (dose in mg/kg per dose) * (weight in kg)
If the plan is mg/kg per day
mg per day = (dose in mg/kg/day) * (weight in kg) mg per dose = (mg per day) / (doses per day)
Worked example (hypothetical)
The numbers below are made up to demonstrate the math. They are not dosing recommendations.
Example A: mg/kg per dose
- Weight: 18 kg
- Plan: 9 mg/kg per dose
- mg per dose = 9 * 18 = 162 mg
Example B: mg/kg per day divided into 2 doses
- Weight: 18 kg
- Plan: 30 mg/kg/day divided twice daily
- mg per day = 30 * 18 = 540 mg/day
- mg per dose = 540 / 2 = 270 mg per dose
Common pitfalls
- Using pounds (lb) as if they were kilograms (kg).
- Mixing up mg/kg per dose with mg/kg per day.
- Rounding the weight too aggressively before multiplying.
- Rounding early (before you convert to volume and confirm the measuring device).
- Decimal point errors (for example, 1.5 vs 15).
- Skipping the maximum daily limit check. See maximum daily dose basics.
Safety checks before acting on the number
- Confirm the most recent reliable weight and units (kg).
- Confirm whether the plan is per dose or per day and how many doses per day are intended.
- Check whether there is a maximum daily limit or other guardrails in your reference.
- If the dose will be given as a liquid, convert to volume using the concentration. See mg/kg to mL conversion basics.
- Use clear rounding and teach-back when giving caregiver instructions. See rounding guidance.
Tools and related pages
- Pediatric dose calculator
- Antibiotic dosing calculator hub
- OTC dosing charts (age/weight based)
- Medication examples (open full guidance pages):
- Augmentin (amoxicillin-clavulanate)
- Azithromycin
- Prednisolone
- Ondansetron
Frequently asked questions
Use the weight in kilograms and the ordered plan. For mg/kg per dose: mg per dose = (mg/kg) * (kg). For mg/kg per day: mg per day = (mg/kg/day) * (kg), then divide by doses per day to get mg per dose.
No. mg/kg per dose and mg/kg per day are different instructions. Mixing them up can multiply or shrink the intended daily exposure. Confirm which one you are using before doing any math.
Convert to kilograms first (kg = lb / 2.2). Use the most recent reliable weight and keep enough precision to avoid rounding errors.
Pause if the result feels unexpectedly large or small, if a decimal point is involved, if the frequency is unclear, or if there is a maximum daily limit that could be exceeded. A second check with a calculator or colleague is reasonable.