The Straight-Talking Nurse

Contents

1. Introduction
2. Part 1: Foundations
   1. 1Cells, Genetics and Microorganisms
   2. 2The Skeleton and Muscles
   3. 3Joints, Nerves and Blood Vessels in Clinical Practice
   4. 4Speaking the Language: Medical Terminology and Normal Values
   5. 5Assessing the Patient: A Systematic Approach and Classic Signs
3. Part 2: Medicines
   1. 6Pharmacology Essentials and Safe Administration
   2. 7Insulin, Drug Monitoring and Antidotes
   3. 8Drug Calculations Without the Panic (in this sample)
4. Part 3: Body Systems
   1. 9The Heart and Circulation
   2. 10Breathing: Pneumonia, Asthma, COPD and Tuberculosis
   3. 11Infection: Sepsis, Meningitis and Travel-Related Fever
   4. 12Sexual, Reproductive and Men's Health
   5. 13The Digestive System
   6. 14The Liver and Gallbladder
   7. 15Kidneys and the Urinary Tract
   8. 16Hormones: Thyroid, Diabetes and Gynaecomastia
   9. 17The Skin
   10. 18Ear, Nose, Throat and Mouth
   11. 19The Eye
   12. 20The Brain and Nervous System
   13. 21Bones, Joints and Back Pain
   14. 22Blood: Anaemia, White Cells and Sickle Cell Disease
   15. 23Mental Health
5. Part 4: Special Groups
   1. 24Pregnancy, Birth and the Newborn
   2. 25Early Pregnancy Loss and Gynaecology
   3. 26Children and Their Medicines
   4. 27Older People and Care Home Residents
6. Part 5: Emergency and Critical Care
   1. 28Emergencies: Resuscitation, Shock, Fluids, Burns, Bites and Stings
   2. 29The Deteriorating Patient: Ward Scenarios
7. Part 6: Professional Practice and Public Health
   1. 30The Professional Nurse: Standards, Safety and Core Skills
   2. 31Consent, Mental Capacity and Best Interests
   3. 32Revalidation and Reflection: A Practical Guide
   4. 33Public Health: Infection Prevention, Immunisation and Nutrition
8. Appendices
   1. ANormal Values in UK Units
   2. BWhere the Source Material Differed from UK Practice
   3. CReferences
   4. DPicture Credits
   5. EGlossary of Abbreviations
   6. FQuiz Answers

Chapter 8: Drug Calculations Without the Panic

Plenty of good nurses go cold at the words “drug calculation”. You don't need to be a maths whizz. You need one formula, a firm grip on units, a few habits that catch mistakes, and practice. We'll work through all four: converting units, working out tablets, liquids and injections, setting infusion pumps and gravity drips, weight-based doses for children, and percentage and ratio strengths, with worked examples and practice questions at the end.

Why calculations go wrong

Most calculation errors aren't about hard maths. They come from a few predictable slips:

• mixing up units, especially milligrams and micrograms, or milligrams and millilitres;
• putting the decimal point in the wrong place, which makes a dose ten times too big or too small;
• confusing a dose per kilogram with a total dose, or a daily dose with a single dose;
• using the wrong strength of a liquid or injection;
• programming a pump with the right numbers in the wrong boxes;
• working it out in a hurry, with interruptions, and not checking.

Chapter 6 covers the wider rules for giving medicines safely, and Chapter 26 explains the extra care children's doses need. This one is about the arithmetic itself.

Three habits before you calculate

Estimate first. Before you work anything out, ask roughly what the answer should be. If the prescription is 500 mg and the tablets are 250 mg, the answer is obviously around two. If your sum then says 20, you'll spot it straight away.

Write the units at every step. Don't write “500 ÷ 250”. Write “500 mg ÷ 250 mg”. Units that don't cancel properly are the quickest warning that you've set the sum up wrong.

Ask whether the answer makes sense. An answer of more than a couple of tablets, a fraction of a tablet that can't be measured, or an unusually large volume for an injection should make you stop and recheck. It may still be right, but it needs a second look.

Units and conversions

The units you'll use most are the metric units of weight and volume. Each step up or down the ladder is a factor of 1,000, so converting means moving the decimal point three places.

Going the other way, divide by 1,000. So 0.125 mg is 125 micrograms, 1.5 g is 1,500 mg and 250 ml is 0.25 litres. Some medicines, such as insulin and heparin, are measured in units instead. Units can't be converted into milligrams, so the prescription and the product must both be in units.

The one formula you need

Almost every calculation for tablets, liquids and injections uses the same formula:

You might also see it written as D ÷ H × V (desired dose, what you have, volume). It's the same thing.

Tablets and capsules

Now the example from the On Shift box. 0.125 mg is prescribed and the tablets contain 62.5 micrograms.

• Convert first: 0.125 mg × 1,000 = 125 micrograms.
• 125 micrograms ÷ 62.5 micrograms × 1 tablet = 2 tablets.

Only split tablets that are scored and that the product information or your pharmacist says can be split. Capsules should never be opened unless a pharmacist has advised it. If a dose would need a tablet cut into anything smaller than a half, ask whether a different strength or a liquid is available.

Oral liquids

Measure liquid doses with an oral or enteral syringe, never a spoon or a medicine pot for small volumes. Liquids often come in more than one strength, so always read the strength on the bottle in your hand, not the one you remember.

Injections

Some injections come as a powder that you reconstitute. The final volume after reconstitution can be slightly more than the volume of liquid you added, because the powder takes up space (this is called displacement value). The product information or the Injectable Medicines Guide tells you what concentration you end up with, so use that figure in the formula.

Percentage and ratio strengths

Some medicines and fluids are labelled with a percentage or a ratio rather than mg per ml. These look scarier than they are.

Percentage strength

For solutions, a percentage strength (written % w/v, weight in volume) means grams per 100 ml.

• 1% means 1 g in 100 ml, which is 1,000 mg in 100 ml, or 10 mg per ml.
• 2% means 2 g in 100 ml, or 20 mg per ml.
• Sodium chloride 0.9% contains 0.9 g of sodium chloride in every 100 ml, which is 9 g per litre.
• Glucose 5% contains 5 g of glucose in every 100 ml, which is 50 g per litre.

Ratio strength

A ratio strength such as “1 in 1,000” means 1 g in 1,000 ml.

• 1 in 1,000 = 1 g in 1,000 ml = 1,000 mg in 1,000 ml = 1 mg per ml.
• 1 in 10,000 = 1 g in 10,000 ml = 0.1 mg per ml, so 10 ml contains 1 mg.

You'll mainly meet ratio strengths with adrenaline, which is exactly why they matter. Chapter 28 explains the two adrenaline strengths used in emergencies and why the one used in cardiac arrest must never be confused with the one used for anaphylaxis. Many organisations now label adrenaline in mg per ml as well, and that's the safer way to think about it.

Infusion rates for pumps (ml per hour)

Infusion pumps and syringe drivers are set in millilitres per hour (ml/hour). You'll also see this written as mL/h. There are two common kinds of sum.

Volume over time

When you know the total volume and how long it should run:

Dose per hour

Some infusions are prescribed as a dose per hour rather than a volume. First work out the concentration of the infusion (how much drug is in each ml), then divide the dose per hour by the concentration.

A fixed-rate intravenous insulin infusion uses the same thinking. In adults with diabetic ketoacidosis, it's usually prescribed at 0.1 units/kg/hour (Chapter 16). For an 80 kg adult, that's 0.1 × 80 = 8 units/hour. With a syringe of 50 units of insulin in 50 ml (1 unit/ml), the rate is 8 units/hour ÷ 1 unit/ml = 8 ml/hour. Your local protocol sets the concentration and the checks, so that's the one to follow.

Doses per kilogram per minute

In critical care, emergency departments and theatres, some infusions are prescribed in micrograms per kilogram per minute. It's the same sum with two extra steps.

Drip rates for gravity infusions (drops per minute)

Where an infusion runs by gravity, without a pump, you control it by counting drops in the drip chamber. The giving set's packaging tells you its drop factor: how many drops make 1 ml. Standard sets for clear fluids are commonly 20 drops per ml. Blood administration sets often have a different drop factor, so always read the packet.

Gravity drips are less precise than pumps and can speed up or slow down when the patient moves. Check them often. If a drip has fallen behind, don't speed it up to “catch up” unless the prescriber agrees; ask instead. Many fluids and most medicines given by continuous infusion should go through a pump, and children's infusions should always be given by pump.

Weight-based doses and maximum dose checks

Lots of children's doses, and some adult ones, are prescribed per kilogram of body weight. Chapter 26 sets out the four principles behind safe children's doses. The arithmetic is the same every time.

Now the same medicine for a 72 kg teenager: 15 mg/kg × 72 kg = 1,080 mg. That's over the 1 g maximum, so the dose is capped at 1 g. A weight-based sum in a big child or a heavy adult can easily produce a dose larger than the maximum, which is why the cap matters.

Daily doses work slightly differently. Medicine G is prescribed at 30 mg/kg/day in three divided doses for a 12 kg child.

1. Total per day: 30 mg/kg × 12 kg = 360 mg a day.
2. Per dose: 360 mg ÷ 3 = 120 mg three times a day.

If you'd treated “30 mg/kg” as a single dose, you'd have given three times too much.

Rounding and measuring

Do the whole calculation first and round only at the end. Rounding halfway through can push the answer off by more than you'd think. Then round to something you can actually measure, and no further. A 1 ml oral syringe can measure much smaller steps than a 10 ml one, so pick a syringe that suits the volume. If the dose can't be measured accurately with what you've got, don't guess; ask the pharmacist.

Keeping calculations safe

In practice:

• Check the dose in the BNF or BNFc. If a prescribed dose looks unusual, it might be wrong. Query it before you give it, not afterwards.
• Use the right resource for injections. The Injectable Medicines Guide (or your local equivalent) tells you how to reconstitute, dilute and give each injectable medicine.
• Get an independent second check where your policy requires one. Many organisations require two registered practitioners to check controlled drugs, intravenous medicines, insulin infusions and children's medicines. An independent check means the second person works it out for themselves from the prescription and the product, rather than just agreeing with your answer.
• Use a calculator if you like, but estimate first. Calculators don't make mistakes; fingers do.
• Protect yourself from interruptions. Many wards use “do not disturb” tabards or a quiet area for preparing medicines. Use them.
• Speak up. If a calculation doesn't make sense, or you're not confident, stop and ask. That's not a weakness. It's what a safe nurse does.

Practice questions

Have a go at these before you look at the answers. Every medicine and number here is an example for practising the maths only — always check the BNF/BNFc and local policy.

1. Medicine H 1 g is prescribed. The tablets contain 500 mg. How many tablets do you give?
2. Medicine J 0.25 mg is prescribed. The tablets contain 125 micrograms. How many tablets do you give?
3. Medicine K 400 mg is prescribed. The oral suspension contains 250 mg in 5 ml. What volume do you give?
4. Medicine L 40 mg is prescribed. Each ampoule contains 80 mg in 2 ml. What volume do you draw up?
5. 1,000 ml of fluid is prescribed over 10 hours. What rate do you set on the pump?
6. 250 ml is to be given over 30 minutes by pump. What rate do you set?
7. 1,000 ml is to run by gravity over 6 hours through a giving set with a drop factor of 20 drops/ml. How many drops per minute?
8. A child weighing 16 kg is prescribed Medicine M at 7.5 mg/kg per dose. The suspension contains 125 mg in 5 ml. What is the dose in mg, and what volume do you give?
9. Medicine N is prescribed at 10 mg/kg per dose, maximum 500 mg per dose. The patient weighs 58 kg. What dose do you give?
10. How many milligrams of drug are in 10 ml of a 2% solution?
11. A solution has a strength of 1 in 1,000. What volume contains 0.5 mg?
12. Medicine P is prescribed at 4 mg/hour. The syringe contains 50 mg in 50 ml. What rate do you set, and how long will a full syringe last?

Answers to the practice questions

1. 1 g = 1,000 mg. 1,000 mg ÷ 500 mg × 1 tablet = 2 tablets.
2. 0.25 mg = 250 micrograms. 250 micrograms ÷ 125 micrograms × 1 tablet = 2 tablets.
3. 400 mg ÷ 250 mg × 5 ml = 8 ml.
4. 40 mg ÷ 80 mg × 2 ml = 1 ml.
5. 1,000 ml ÷ 10 hours = 100 ml/hour.
6. 250 ml ÷ 30 minutes × 60 = 500 ml/hour.
7. 6 hours = 360 minutes. 1,000 × 20 ÷ 360 = 55.6, so about 56 drops per minute.
8. Dose: 7.5 mg/kg × 16 kg = 120 mg. Volume: 120 mg ÷ 125 mg × 5 ml = 4.8 ml.
9. 10 mg/kg × 58 kg = 580 mg, which is over the 500 mg maximum, so you give 500 mg.
10. 2% = 2 g in 100 ml = 20 mg/ml. 10 ml × 20 mg/ml = 200 mg.
11. 1 in 1,000 = 1 mg/ml. 0.5 mg ÷ 1 mg/ml = 0.5 ml.
12. Concentration: 50 mg ÷ 50 ml = 1 mg/ml. Rate: 4 mg/hour ÷ 1 mg/ml = 4 ml/hour. A full 50 ml syringe lasts 50 ml ÷ 4 ml/hour = 12.5 hours.

Quick quiz

Test yourself on this chapter. The answers are in Appendix F of the full book.

1. Multiple choice. 1 milligram equals: (a) 10 micrograms (b) 100 micrograms (c) 1,000 micrograms (d) 0.001 micrograms
2. Short answer. Example for practising the maths only — always check the BNF/BNFc and local policy. 600 mg is prescribed and the suspension contains 200 mg in 5 ml. What volume do you give?
3. Short answer. Example for practising the maths only — always check the BNF/BNFc and local policy. 500 ml is to run by gravity over 4 hours through a set with a drop factor of 20 drops/ml. How many drops per minute?
4. True or false? A 1% solution contains 10 mg per ml.
5. True or false? It's acceptable to write “mcg” on a chart as long as your handwriting is clear.

References and further reading

• Joint Formulary Committee (2026) British National Formulary (online). London: BMJ Group and Pharmaceutical Press. Available at: https://bnf.nice.org.uk/ (Accessed: 18 October 2026).
• Joint Formulary Committee (2026) Prescription writing (BNF medicines guidance). Available at: https://bnf.nice.org.uk/medicines-guidance/prescription-writing/ (Accessed: 18 October 2026).
• Nursing and Midwifery Council (2018) Future nurse: standards of proficiency for registered nurses. Available at: https://www.nmc.org.uk/standards/standards-for-nurses/standards-of-proficiency-for-registered-nurses/ (Accessed: 18 October 2026).
• Nursing and Midwifery Council (2018) The Code: professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: https://www.nmc.org.uk/standards/code/ (Accessed: 18 October 2026).
• Paediatric Formulary Committee (2026) BNF for children (online). London: BMJ Group, Pharmaceutical Press and RCPCH Publications. Available at: https://bnfc.nice.org.uk/ (Accessed: 18 October 2026).
• Royal Pharmaceutical Society and Royal College of Nursing (2019) Professional guidance on the administration of medicines in healthcare settings. Available at: https://www.rcn.org.uk/workingwithus/-/media/royal-college-of-nursing/documents/working-with-us/endorsements/professional-guidance-on-the-administration-of-medicines-in-healthcare-settings.pdf (Accessed: 18 October 2026).