Dosage Calculation Formulas Nurses Need

Almost every dosage calculation question uses one of these five formulas. Learn them, show your work, and double-check your units.

1. Desired over have

Example: The order is amoxicillin 500 mg. You have 250 mg per 5 mL. (500 ÷ 250) × 5 mL = 10 mL.

2. Weight-based dose

Example: 10 mg/kg for a 44 lb child. 44 ÷ 2.2 = 20 kg. 10 × 20 = 200 mg.

3. IV rate in mL per hour (pump)

Example: 1,000 mL over 8 hours = 125 mL/hr.

4. Drip rate in drops per minute (gravity)

Example: 1,000 mL over 8 hours with tubing of 15 gtt/mL. (1,000 × 15) ÷ 480 = 31.25, so 31 gtt/min.

5. Weight-based drips (mcg/kg/min)

Example: Dopamine 5 mcg/kg/min for an 80 kg patient. The bag is 400 mg in 250 mL, which is 1,600 mcg/mL. (5 × 80 × 60) ÷ 1,600 = 15 mL/hr.

Tips that save points

• Convert units first (mg to mcg, lb to kg).
• Ask yourself if the answer makes sense. Ten tablets for one dose is almost always wrong.
• Follow your program's rounding rules.

Quick answers

For learning only. Lab ranges and policies vary. Always follow your nursing program, a current drug reference, and your facility's policies.