Nursing school maths
Dosage Calculations Made Reliable
Pick one method and use it every time. Dimensional analysis, ratio and proportion, and the desired-over-have formula all reach the same answer. Students who lose marks are almost never students who chose the wrong method; they are students who switch between methods under pressure.
Last updated August 2026
- Three methods compared
- Worked examples
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Why this one has to be automatic
Dosage calculation appears in your maths modules, in pharmacology, in clinical assessments and on exit exams, and most programmes set a higher pass mark for it than for anything else, often requiring a perfect or near-perfect score. That is not arbitrary: it is the one calculation where being slightly wrong harms someone.
The conversions to memorise
Metric
- 1 kg = 1000 g
- 1 g = 1000 mg
- 1 mg = 1000 mcg
- 1 L = 1000 mL
Household and imperial
- 1 kg = 2.2 lb
- 1 tsp = 5 mL
- 1 tbsp = 15 mL
- 1 oz ≈ 30 mL
- 1 cup ≈ 240 mL
Learn these in both directions. A question that gives you pounds and asks for a dose per kilogram is testing the conversion at least as much as the arithmetic.
Three methods that all work
| Method | The setup | Suits you if |
|---|---|---|
| Desired over have | (Desired ÷ Have) × Quantity | You want the fewest steps and the units are already consistent |
| Ratio and proportion | have : quantity = desired : x, then cross-multiply | You think in relationships and want to see the proportion |
| Dimensional analysis | Multiply by conversion factors until only the wanted unit remains | You want the units themselves to catch your errors |
Dimensional analysis is worth the extra setup on anything multi-step. If the units do not cancel to what the question asked for, you have made an error before you have done any arithmetic.
Worked examples
Tablets
Order: 0.25 mg. Available: 0.125 mg tablets.
(0.25 ÷ 0.125) × 1 tablet = 2 tablets.
Liquid, with a conversion
Order: 500 mg. Available: 250 mg in 5 mL.
(500 ÷ 250) × 5 mL = 10 mL.
Units that do not match
Order: 1 g. Available: 250 mg per capsule. Convert first: 1 g = 1000 mg.
(1000 ÷ 250) × 1 capsule = 4 capsules.
Weight-based and paediatric doses
Weight-based questions add two steps, and both are where errors appear: converting pounds to kilograms, and deciding whether the ordered dose is per day or per dose.
A child weighs 44 lb. The order is 15 mg/kg/day divided into three doses.
44 ÷ 2.2 = 20 kg. 20 × 15 = 300 mg per day. 300 ÷ 3 = 100 mg per dose.
Always check the answer against a safe range if one is given. A calculation can be arithmetically perfect and still produce a dose you should question.
Infusion rates
- mL per hour = total volume ÷ total hours.
- Drops per minute = (volume × drop factor) ÷ time in minutes.
- Dose per hour from a rate = concentration × mL per hour.
Round drops per minute to a whole number, because you cannot deliver a fraction of a drop. Round millilitres per hour to whatever your pump accepts, which your programme will specify.
Where marks are actually lost
- Not converting first. Mixed units are the most common single cause of a wrong answer.
- Per day versus per dose. Read the frequency before you calculate, not after.
- Rounding too early. Round once, at the end.
- Ignoring the sanity check. Eleven tablets is almost never right. If the answer looks absurd, it is.
- Switching methods mid-question. Choose one, and finish in it.
Start practising for your exam
Reading about the exam only goes so far. Sit a timed set and find out where you actually stand.
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