Oral and injectable doses
| What you need | Formula | Example |
|---|---|---|
| Tablets or capsules | Desired ÷ have × 1 tablet | 650 mg ÷ 325 mg × 1 = 2 tablets |
| Liquids and injections (mL) | Desired ÷ have × volume | 375 mg ÷ 250 mg × 5 mL = 7.5 mL |
| Reconstituted powders | Desired ÷ concentration after mixing | 500 mg ÷ 250 mg/mL = 2 mL |
| Weight-based dose | Dose per kg × weight (kg) | 15 mg/kg × 20 kg = 300 mg |
| Daily dose divided | Daily dose ÷ doses per day | 900 mg/day ÷ 3 = 300 mg every 8 hours |
| Body surface area (Mosteller) | √(cm × kg ÷ 3,600) | √(165 × 70 ÷ 3,600) = 1.79 m² |
| Dose per m² | Dose per m² × BSA | 100 mg/m² × 1.79 m² = 179 mg |
IV infusions
| What you need | Formula | Example |
|---|---|---|
| Pump rate | mL/hr = volume (mL) ÷ time (hr) | 1,000 mL ÷ 8 hr = 125 mL/hr |
| Gravity drip rate | gtt/min = mL × drop factor ÷ minutes | 1,000 × 15 ÷ 480 = 31 gtt/min |
| Infusion time | Hours = volume ÷ mL/hr | 1,000 mL ÷ 125 mL/hr = 8 hr |
| Dose-per-hour drips | mL/hr = (units or mg)/hr ÷ (units or mg)/mL | 1,440 units/hr ÷ 100 units/mL = 14.4 mL/hr |
| mcg/kg/min drips | mL/hr = mcg/kg/min × kg × 60 ÷ mcg/mL | 5 × 80 × 60 ÷ 1,600 = 15 mL/hr |
| Dose infusing | mcg/kg/min = mL/hr × mcg/mL ÷ 60 ÷ kg | 15 × 1,600 ÷ 60 ÷ 80 = 5 mcg/kg/min |
Conversions to know by heart
- 1 g = 1,000 mg; 1 mg = 1,000 mcg; 1 L = 1,000 mL
- 1 kg = 2.2 lb; 1 inch = 2.54 cm
- 1 tsp = 5 mL; 1 tbsp = 15 mL; 1 fl oz = 30 mL; 1 cup = 240 mL
- °C = (°F − 32) × 5 ÷ 9; °F = °C × 9 ÷ 5 + 32
- X% = X g per 100 mL; 1:1,000 = 1 g per 1,000 mL = 1 mg/mL
Rounding in one look
- Do not round until the final answer.
- Tablets: whole, or half if scored.
- mL: tenths at 1 mL or more, hundredths under 1 mL.
- gtt/min: whole drops.
- mL/hr: tenths (or whole numbers if your program says so).
- Weight in kg: tenths. BSA: hundredths.
Frequently asked questions
What is the basic formula for dosage calculation?
Desired over have times quantity: D ÷ H × Q. It works for tablets (Q = 1 tablet) and for liquids (Q = the volume that contains the dose on hand).
Which formula is used for IV drip rates?
For a gravity infusion, gtt/min = volume in mL × drop factor ÷ time in minutes. For a pump, mL/hr = volume ÷ hours.