How drug calculations are taught and tested in UK nursing: the “what you want over what you’ve got” formula, converting between grams, milligrams, micrograms and nanograms, tablets and liquids, and IV infusions in mL/hr and drops per minute with UK giving sets. Every worked example below is calculated from the same formulas the site’s calculators use, and each one links to a calculator so you can check it.
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Amount to give = what you want ÷ what you’ve got × what it’s in. For tablets “what it’s in” is 1 tablet; for liquids it is the volume on the label. Prescribed digoxin 125 micrograms, stock 62.5 microgram tablets: 125 ÷ 62.5 × 1 = 2 tablets. Get the units the same before you divide.
UK universities and NHS trusts teach it in those words, sometimes shortened to “want over got times volume”. It is the same calculation American textbooks call D/H × Q (desired over have times quantity), so anything you read about that method applies. The dosage calculation methods guide sets it beside ratio-proportion and dimensional analysis with the same problems solved each way.
Three habits keep it safe. Convert first, so both doses are in the same unit. Estimate before you calculate: if the prescription is double the tablet strength, the answer is two tablets, and a result of twenty means something has gone wrong. And round only at the end, following the rule for that medicine or your trust’s policy.
Each step down the scale is a factor of 1,000. Grams to milligrams, milligrams to micrograms, micrograms to nanograms: multiply by 1,000 each time, or divide by 1,000 to go back up.
| From | To | Do this | Example |
|---|---|---|---|
| grams (g) | milligrams (mg) | × 1,000 | 0.5 g = 500 mg |
| milligrams (mg) | micrograms | × 1,000 | 0.125 mg = 125 micrograms |
| micrograms | nanograms | × 1,000 | 0.25 micrograms = 250 nanograms |
| nanograms | micrograms | ÷ 1,000 | 500 nanograms = 0.5 micrograms |
| micrograms | milligrams (mg) | ÷ 1,000 | 62.5 micrograms = 0.0625 mg |
| litres (L) | millilitres (mL) | × 1,000 | 0.5 L = 500 mL |
UK prescription-writing guidance asks for micrograms, nanograms and units to be written in full. Handwritten “mcg” or “µg” can be misread as “mg”, “ng” as “mg”, and “U” as a zero, and each misreading is a thousandfold or tenfold error. You will still see “mcg” on some packaging and in American material, including the practice tools on this site, so read every label slowly.
The microgram and nanogram step catches people because the drugs involved are potent and the numbers look small. Alfacalcidol capsules come as 250 nanograms, which is 0.25 micrograms. Write “0.25 mg” by mistake and the dose is a thousand times too big. Two other conventions guard against the same kind of error: put a zero before a decimal point (0.5 mL, never .5 mL) and leave off trailing zeros (5 mg, never 5.0 mg).
The drug unit converter handles grams, milligrams, micrograms and nanograms if you want to check a conversion.
For tablets, “what it’s in” is one tablet, so the calculation is simply what you want divided by what you’ve got. The answer should almost always be a whole number or a half. Only scored tablets can be halved, and many trusts discourage splitting at all if a suitable strength exists. If you get 0.3 or 7.5 tablets, recheck the units before anything else.
| Medicine | Prescribed | Stock | Give | Working | Calculator |
|---|---|---|---|---|---|
| Furosemide | 40 mg | 20 mg tablets | 2 tablets | 40 ÷ 20 × 1 | Check |
| Digoxin | 125 micrograms | 62.5 micrograms tablets | 2 tablets | 125 ÷ 62.5 × 1 | Check |
| Levothyroxine | 75 micrograms | 25 micrograms tablets | 3 tablets | 75 ÷ 25 × 1 | Check |
| Prednisolone | 30 mg | 5 mg tablets | 6 tablets | 30 ÷ 5 × 1 | Check |
| Bisoprolol | 1.25 mg | 2.5 mg tablets | 0.5 tablets | 1.25 ÷ 2.5 × 1 | Check |
For liquids, “what it’s in” is the volume the strength refers to: 5 mL for most oral suspensions, 1 mL for most injections, and occasionally 2 mL or more. Read it off the label every time, because the same drug is often stocked in two strengths.
| Medicine | Prescribed | Stock | Give | Working | Calculator |
|---|---|---|---|---|---|
| Paracetamol oral suspension | 180 mg | 250 mg in 5 mL | 3.6 mL | 180 ÷ 250 × 5 | Check |
| Amoxicillin oral suspension | 250 mg | 125 mg in 5 mL | 10 mL | 250 ÷ 125 × 5 | Check |
| Morphine sulfate injection | 2.5 mg | 10 mg in 1 mL | 0.25 mL | 2.5 ÷ 10 × 1 | Check |
| Furosemide injection | 20 mg | 10 mg in 1 mL | 2 mL | 20 ÷ 10 × 1 | Check |
| Gentamicin injection | 120 mg | 40 mg in 1 mL | 3 mL | 120 ÷ 40 × 1 | Check |
Oral syringes are graduated in fixed steps, so check that the syringe you pick can actually measure the dose you calculated. Injection volumes under 1 mL need a 1 mL syringe. When the answer is under 1, write the leading zero: 0.25 mL.
Most UK infusions run through a volumetric pump set in mL/hr, which is the volume divided by the time in hours. Gravity infusions are counted in drops per minute, and they still appear on numeracy tests and on wards without enough pumps:
drops/min = volume (mL) × drop factor (drops/mL) ÷ time (minutes)
The drop factor is printed on the giving-set packet. In UK practice the usual values are:
| Giving set | Drop factor | Shortcut at the bedside |
|---|---|---|
| Standard (crystalloid, clear fluids) | 20 drops/mL | drops/min = mL/hr ÷ 3 |
| Blood administration set | 15 drops/mL | drops/min = mL/hr ÷ 4 |
| Paediatric, micro-drop or burette set | 60 drops/mL | drops/min = mL/hr |
A blood set has a filter and forms larger drops, so fewer drops make up each millilitre. Drops are counted as whole numbers, so round the final answer to the nearest drop, not at each step.
| Infusion | Volume and time | Drop factor | Pump rate | Gravity rate | Working | Calculator |
|---|---|---|---|---|---|---|
| Sodium chloride 0.9% | 1,000 mL over 8 h | 20 (standard) | 125 mL/hr | 42 drops/min | 1,000 × 20 ÷ 480 = 41.67 | Check |
| Glucose 5% | 500 mL over 4 h | 20 (standard) | 125 mL/hr | 42 drops/min | 500 × 20 ÷ 240 = 41.67 | Check |
| Compound sodium lactate (Hartmann’s) | 1,000 mL over 6 h | 20 (standard) | 166.7 mL/hr | 56 drops/min | 1,000 × 20 ÷ 360 = 55.56 | Check |
| Red cells (one unit) | 300 mL over 3 h | 15 (blood) | 100 mL/hr | 25 drops/min | 300 × 15 ÷ 180 = 25 | Check |
| Sodium chloride 0.45% via burette | 250 mL over 10 h | 60 (paediatric) | 25 mL/hr | 25 drops/min | 250 × 60 ÷ 600 = 25 | Check |
For infusion rates by pump, the IV flow rate calculator gives mL/hr; to find out when a bag will finish, use the IV duration calculator.
The Nursing and Midwifery Council’s standards for pre-registration nursing programmes (standard 4.6) require every programme to include a health numeracy assessment related to nursing proficiencies and the calculation of medicines, which must be passed with a score of 100 percent. Each university designs its own assessment, so the details differ, but most cover the same ground as this page: unit conversions, tablets, liquids and injections, infusion rates and drops per minute, and sometimes weight-based doses for children.
Formats vary. Some are online, some on paper; some allow a basic calculator and some do not; many allow more than one attempt across the programme. Your module handbook is the authority on your own test. NursingCalc is not affiliated with the NMC or any university, and nothing here is an official past paper. The NMC’s test of competence for nurses trained outside the UK also includes a numeracy section.
The practice hub generates unlimited problems of each type with instant marking and a worked solution. The tools are written for both UK and US students, so they use the abbreviation “mcg” and include 10 and 15 drops/mL sets alongside the UK’s 20 and 60. Set your rounding rules on the hub before you start.
What you want ÷ what you’ve got × what it’s in. “What it’s in” is 1 for tablets and the stock volume for liquids. It is the same calculation as the D/H × Q formula used in American textbooks.
A standard giving set for clear fluids delivers 20 drops/mL, a blood administration set 15 drops/mL, and a paediatric, micro-drop or burette set 60 drops/mL. The drop factor is printed on the packet, so check it before you calculate.
Multiply the volume in mL by the drop factor, then divide by the time in minutes. For 1,000 mL over 8 hours through a 20 drops/mL set: 1,000 × 20 ÷ 480 = 41.7, which rounds to 42 drops per minute.
The NMC standards for pre-registration nursing programmes require a health numeracy assessment, including the calculation of medicines, to be passed with a score of 100 percent. Each university sets its own test and number of attempts.
Abbreviations such as mcg, µg and ng are easily misread as mg in handwriting, which turns the dose into a thousandfold error. UK prescription-writing guidance asks for micrograms, nanograms and units to be written out in full.