Guide — drug calculations
Drug calculations: guide, formulas and practice
Everything about drug calculations in one place: what they are, all the topics, the formulas, how to practise and how to prepare for exams.
What are drug calculations?
Working out the right amount of a medicine from a prescription.
Drug calculations are a core skill in nursing programmes and are also used in other healthcare courses. You work out how many tablets to give, how many mL, how much needs diluting or what rate an infusion should run at. The aim is patient safety: a miscalculated dose can cause harm. Read the full explanation in What are drug calculations?
All the topics in drug calculations
Content varies between universities. These are the areas you will meet most often, with theory and tools for each.
| Topic | Read more |
|---|---|
| Weights and measures | Unit conversion |
| Dosage calculations | Dosage calculations |
| Tablets and oral liquids | Tablets and oral liquids |
| Percentages and concentration | Percentage solutions, mmol, units and ratios |
| Dilution | Dilution |
| Drops and infusions | Drip rate, Infusion rate |
| Weight and body surface area | Dose per kg and m² |
| Time and daily dose | Daily dose and timing |
| Rounding | Rounding |
| All formulas | Drug calculation formulas |
Calculators: Body surface area calculator (Mosteller) for dose per m², Percent to mg/mL: converter and explanation, Infusion time calculator: how long does an infusion take?, Daily dose calculator: dose per kg and per administration, How to calculate mg per mL: formula and calculator, How many drops are in a mL? Drop factor explained.
The basic formulas
The three that carry most of the work.
All the formulas with examples are on the formulas page.
How to learn drug calculations
Understand the formulas
Start with units and dosage calculations. Every other topic builds on them.
Work it out yourself
Reading is not enough. Try the practice questions and the free quiz.
Follow a plan
Use the 14-day plan or the weekly revision plan.
Avoid careless errors
See the 12 tips.
Drug calculations exams
The format is set by your university or employer.
Exam format, pass marks and rounding rules vary between universities and NHS trusts, so follow your course handbook and local policy. Practising under timed conditions with a calculator is a good way to prepare, and the quiz is a quick way to see which topics need more work.
Thinking about an app? How to choose a drug calculation app.
Frequently asked questions
What are drug calculations?+
Working out the right amount of a medicine from a prescription, for example the number of tablets, the volume in mL or the infusion rate.
What topics do drug calculations cover?+
Commonly units, dosage calculations, percentages and concentration, dilution, drops, infusion rate, and doses by weight or body surface area. This varies between universities.
How do you calculate a dose?+
Divide the prescribed dose by the strength: Amount = Prescribed dose ÷ Strength. A prescription of 150 mg with 50 mg tablets gives 3 tablets.
How do you practise drug calculations?+
Work through questions yourself, one topic at a time, with a calculator and your working written down. Go through your mistakes and mix topics before an exam.
Is there an app for drug calculations?+
Yes. Dos is a practice app for drug calculations, with practice tests of ten questions, a built-in calculator next to each question and statistics per topic.
How do I pass a drug calculations exam?+
Practise regularly, check your units, do a sense check on every answer and practise under timed conditions. The format is set by your university, so read your course handbook.
Practise drug calculations in Dos
Dos is a practice app for drug calculations: practice tests of ten questions, a built-in calculator next to each question, and statistics per topic.
