Medication Dosage by Weight Calculator
Calculate the correct medication dosage based on body weight using mg/kg dosing.
Supports adults and children in metric and imperial units.
How Medication Dosage Calculations Work
Medication dosage calculations determine how much of a drug to administer based on prescribed dose, patient weight, and available concentration. These calculations are critical for patient safety — errors are a leading cause of preventable harm.
Basic dose calculation:
Volume to administer = Prescribed dose ÷ Available concentration
Worked example — liquid antibiotic:
- Prescribed: amoxicillin 375 mg
- Available: 250 mg/5 mL suspension
Volume = 375 ÷ (250/5) = 375 ÷ 50 = 7.5 mL
Weight-based dose calculation (most common in pediatrics and IV medications):
Total dose = Weight (kg) × Dose per kg
Volume = Total dose ÷ Concentration
Example — IV gentamicin:
- Patient weight: 68 kg
- Order: 5 mg/kg IV once daily
- Available: 40 mg/mL
Total dose = 68 × 5 = 340 mg Volume = 340 ÷ 40 = 8.5 mL
Drip rate calculation for IV infusions:
Drip rate (mL/hr) = Volume (mL) ÷ Time (hr)
If 500 mL must infuse over 4 hours:
Drip rate = 500 ÷ 4 = 125 mL/hr
Drop rate (manual drip sets):
Drops per minute = (Volume mL × Drop factor) ÷ (Time in minutes)
Standard drop factor: 10, 15, or 20 drops/mL depending on the IV set.
The Five Rights of medication administration:
Right patient · Right drug · Right dose · Right route · Right time
Always double-check by having a second clinician independently verify calculations for high-alert medications (insulin, heparin, chemotherapy). A tenfold dosing error is more common than most people realize.