Numeracy and Medication Calculation
Working accurately
Medication calculation rewards a fixed method. Know the conversions: 1 g = 1000 mg, 1 mg = 1000 micrograms, 1 L = 1000 mL, and 1% solution = 1 g per 100 mL = 10 mg/mL. For liquids, use the standard formula: volume required = (dose required ÷ stock strength) × stock volume. For tablets, use the same ratio logic, and question any prescription that demands splitting a tablet that is not scored or is modified-release. For infusions, mL per hour = total volume ÷ time in hours; drops per minute = (mL per hour × drop factor) ÷ 60. Weight-based doses in mg/kg apply particularly to children, where a decimal error is amplified.
Good practice behaviours
Convert everything to a single unit before calculating. Estimate the answer first, then compute, then check that the result is clinically plausible. Follow the rights of administration, check calculations independently with a second nurse where policy requires, and never transcribe doses from memory or from a similar-sounding drug name.
Traps
- Mixing micrograms and milligrams - the commonest source of tenfold errors.
- Decimal point slips, especially in paediatric and syringe-driver calculations.
- Forgetting that infusion times under one hour change the arithmetic.
- Rounding drops per minute up or down without stating the rounding.
Rule of thumb
If your answer looks surprising - a volume larger than the vial, or a rate faster than the pump allows - stop. Re-check the units before checking the arithmetic.
Worked example
Prescribed 0.25 mg of a drug; stock is 500 micrograms in 2 mL. Convert: 0.25 mg = 250 micrograms. Volume = (250 ÷ 500) × 2 = 1 mL. Cross-check: 500 micrograms in 2 mL is 250 micrograms per mL, so 1 mL delivers the prescribed dose.