Drug Calculation Formulas Every UK Nurse Needs

A working reference of the drug calculation formulas UK nurses use in routine practice. This chapter is a summary; for any specific patient calculation, cross-check against the BNF, the prescription, and the trust’s IV drug monograph.

Formula 1: Tablet or capsule calculation

Number of tablets = Dose required ÷ Dose per tablet

Example: Prescribed 1 g paracetamol. Stock 500 mg tablets.

1,000 mg ÷ 500 mg = 2 tablets.

Formula 2: Liquid oral medication

Volume required = (Dose required ÷ Dose per unit volume) × unit volume

Or more practically:

Volume = (Want ÷ Got) × Volume it's in

Example: Prescribed 200 mg of an antibiotic syrup. Stock 250 mg in 5 mL.

(200 ÷ 250) × 5 = 4 mL.

Formula 3: IV drip rate (drops per minute)

For gravity-fed IV.

Drops/min = (Volume in mL × Drip set factor) ÷ Time in minutes

Drip set factors:

• Standard clear-fluid set: 20 drops/mL.
• Blood set: 15 drops/mL.
• Micro/paediatric set: 60 drops/mL.

Example: 1,000 mL saline over 6 hours using standard set.

Time in minutes: 6 × 60 = 360.

Drops/min: (1,000 × 20) ÷ 360 = 55.6 ≈ 56 drops/min.

Formula 4: IV infusion rate (pump, mL per hour)

mL per hour = Total volume ÷ Time in hours

Example: 500 mL over 4 hours.

500 ÷ 4 = 125 mL/hour.

Formula 5: Continuous infusion (dose-based)

When a drug is prescribed at a dose per hour rather than a total volume.

mL/hour = (Dose required × Volume of solution) ÷ Amount of drug in solution

Example: Heparin 1,000 units/hour. Bag: 25,000 units in 500 mL.

(1,000 × 500) ÷ 25,000 = 20 mL/hour.

Formula 6: Weight-based dosing

Dose = Dose per kg × Patient weight in kg

Example: Gentamicin 5 mg/kg for an 80 kg adult.

5 × 80 = 400 mg.

Formula 7: Body Mass Index

BMI = Weight in kg ÷ (Height in metres)²

Example: 75 kg, 1.70 m.

1.70² = 2.89.

75 ÷ 2.89 = 26 BMI.

Formula 8: Fluid balance

Balance = Total intake − Total output

Including insensible losses (often estimated at 500-800 mL/day for an adult, more if febrile or tachypnoeic).

Unit conversions

The conversions used most:

• 1 g = 1,000 mg.
• 1 mg = 1,000 mcg.
• 1 mcg = 1,000 nanograms.
• 1 L = 1,000 mL.

The safety habits

Three habits that prevent calculation errors:

1. Convert units before calculating. Mismatched units are the most common error pattern. If the prescription is in g and the stock is in mg, convert one to the other first.

2. Sanity-check the answer. Does it look reasonable for the drug and patient? A paracetamol dose of 50 tablets or an insulin dose of 100 units for an adult on metformin alone are implausible. Recheck.

3. Double-check with a colleague. For high-risk drugs (insulin, opioids, anticoagulants, paediatric doses), a second nurse independently verifies the calculation. Many trusts mandate this.

The BNF and the prescription

The BNF (British National Formulary) is the UK reference for drug doses, indications, interactions and side effects. It’s free online at bnf.nice.org.uk and most NHS staff have institutional access.

For any calculation, the prescription is the binding source. The BNF tells you the standard dose range; the prescription tells you what’s actually prescribed for this patient. If the prescription is outside the BNF’s standard range, check with the prescriber before administering.

When to refuse to administer

A calculation that produces a result that’s clearly wrong, or a prescription that doesn’t make clinical sense, is a Section 18 trigger to refuse and escalate. Section 16 also applies, because raising concerns is a professional duty.

Don’t administer a drug you’re uncertain about. The Code is explicit that a nurse who administers despite uncertainty is responsible for the outcome. Pause, recheck, escalate.

The next chapter covers the Five Rights of medication administration, the framework that wraps around every drug calculation.