These are the setups taught in Open RN’s Nursing Skills textbook, which works every problem by dimensional analysis. Each formula below has one worked example from that book. Always check your answer against your facility’s policy and a second person when your workplace requires it.
Common conversions
| Unit | Equals |
|---|---|
| 1 kilogram (kg) | 2.2 pounds (lb) |
| 1 gram (g) | 1,000 milligrams (mg) |
| 1 milligram (mg) | 1,000 micrograms (mcg) |
| 1 liter (L) | 1,000 milliliters (mL) |
| 1 teaspoon (tsp) | 5 mL |
| 1 tablespoon (Tbsp) | 15 mL |
| 1 ounce (oz) | 30 mL |
Tablets and liquids: desired over have
Dose to give = (dose ordered ÷ dose on hand) × quantity on hand
This is often written as D/H × Q. Open RN sets the same problem up by dimensional analysis, and the arithmetic is identical.
Example. The order is 25 mg. Each tablet contains 6.25 mg. (25 mg ÷ 6.25 mg) × 1 tablet = 4 tablets
Weight-based doses (mg/kg)
Dose in mg = ordered mg/kg × weight in kg. Convert pounds to kilograms first, then use the concentration to find the volume.
Example. A child weighs 22 lb. The order is 15 mg/kg, and the medication comes as 160 mg in 5 mL.
- 22 lb ÷ 2.2 = 10 kg
- 15 mg/kg × 10 kg = 150 mg
- 150 mg × (5 mL ÷ 160 mg) = 4.6875 mL, which rounds to 4.69 mL (Open RN rounds children’s doses to the hundredth)
To check a safe dose range, multiply the weight by the lowest and highest mg/kg. For a 7 kg infant and a range of 10 to 15 mg/kg per dose, the safe range is 70 to 105 mg.
IV flow rate (mL/hr) for a pump
mL/hr = total volume in mL ÷ time in hours. If the time is in minutes, multiply by 60 ÷ minutes.
Examples.
- 200 mL over 2 hours = 100 mL/hr
- 100 mL over 30 minutes = 100 × 60 ÷ 30 = 200 mL/hr
IV drip rate (gtt/min) by gravity
Drops per minute = (volume in mL × drop factor) ÷ time in minutes. The drop factor is the number of drops in 1 mL for that tubing, and it’s printed on the package. Macrodrip tubing is 10, 12, 15 or 20 gtt/mL. Microdrip tubing is 60 gtt/mL.
Examples.
- 500 mL over 4 hours with 60 gtt/mL tubing: (500 × 60) ÷ 240 = 125 gtt/min
- 250 mL over 2 hours with 10 gtt/mL tubing: (250 × 10) ÷ 120 = 20.83, which rounds to 21 gtt/min (round drops to the nearest whole number)
Sources
- Open RN (WisTech Open), CC BY 4.0, Nursing Skills 2e, chapter 5, Math calculations. Every formula setup, worked example and conversion on this page. Checked October 15, 2026.