Short answer: Most dose calculations use one idea: desired dose divided by dose on hand, multiplied by the quantity it comes in. Infusion rates are volume divided by time, drip rates add the drop factor, and dilutions use C₁V₁ = C₂V₂. Convert units first and sense-check every answer.
The basic dose formula
Desired dose ÷ dose on hand × quantity on hand = amount to give.
Example: 750 mg is ordered. Tablets contain 250 mg. 750 ÷ 250 = 3 tablets.
Example: 60 mg is ordered. The stock is 20 mg per 5 mL. 60 ÷ 20 × 5 = 15 mL.
Infusion and drip rates
Pump rate (mL/hour) = volume ÷ time in hours. Example: 500 mL over 4 hours = 125 mL/hour.
Drip rate (drops/min) = volume × drop factor ÷ time in minutes. Example: 1,000 mL over 8 hours with a 20 drops/mL set = 1,000 × 20 ÷ 480 ≈ 42 drops/min.
Weight-based doses
Multiply the dose per kilogram by the weight. Example: 15 mg/kg for a 20 kg child = 300 mg. If the suspension is 120 mg per 5 mL, 300 ÷ 120 × 5 = 12.5 mL.
Dilutions
C₁V₁ = C₂V₂. Example: to make 200 mL of a 2% solution from a 10% stock, V₁ = 2 × 200 ÷ 10 = 40 mL of stock, made up to 200 mL.
Common traps
Most errors come from habits rather than maths.
- Mixing units, such as mg and mcg or g and mg.
- Using the wrong drop factor.
- Forgetting to convert minutes to hours.
- Not checking whether the answer is a plausible dose.
How to practise
Do five calculations every few days rather than fifty at once. Write the formula each time. Aim to finish each one in under a minute.
Related guide
For a step-by-step method for nurses with five problem types, see drug calculations for nurses.
Frequently asked questions
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