NHS Drug Calculations
This independent UK medicines calculation practice tool helps healthcare learners check basic dose, volume and unit-conversion arithmetic. It does not prescribe a medicine, choose a dose, or replace the prescription, medicine label, BNF, local policy or an independent clinical check.
Dose-to-volume calculator
Safety: the required dose and stock dose must be expressed in the same unit before using this calculation. Always check the actual product concentration and prescription.
Formula
Volume to administer = required dose ÷ dose provided × volume containing the provided dose.
This is the standard “what you want ÷ what you have × what it is in” relationship used in NHS medicines-calculation training and Trust guidance.
Worked example
If the prescribed dose is 20 mg and the available liquid contains 50 mg in 5 mL, the arithmetic is 20 ÷ 50 × 5 = 2 mL. This example demonstrates maths only; it is not a recommendation to administer any particular medicine.
Unit conversions
Drug calculations often require units to be made consistent first. Moving from grams to milligrams or milligrams to micrograms multiplies by 1,000; moving in the opposite direction divides by 1,000. Decimal and unit errors can cause serious medication errors, so never rely on a calculator result without checking the units.
What this calculator does not do
It does not calculate a patient’s prescribed dose, select a medicine, account for renal or hepatic adjustment, maximum doses, age-specific restrictions, interactions, contraindications, reconstitution displacement, infusion-device settings or local administration rules. Weight-based, paediatric, neonatal and infusion calculations require the relevant prescription and clinical guidance.
Safe medicines calculations
NHS medicines-calculation training emphasises accurate calculations, awareness of personal limitations and safety checks. Clinical staff should follow their organisation’s medicines policy and use authoritative medicine information. Calculator-assisted arithmetic should not replace an independent manual check where local policy requires one.
Authoritative sources
- NHS Learning Hub: Introduction to Medicines Calculations
- NHS Learning Hub: Calculating Drug Doses
- NHS: Medicines information
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FAQs
What is the basic drug-volume calculation?
When the units match, divide the required dose by the dose provided and multiply by the volume containing the provided dose.
Can this calculator tell me what dose to prescribe?
No. It only performs arithmetic using values you enter. It must not be used to choose or prescribe a dose.
Do mg and micrograms need converting first?
Yes. Required and stock-dose units must be consistent before applying the formula. One milligram equals 1,000 micrograms.
Can I use this for paediatric or neonatal medicines?
This basic tool is not a substitute for paediatric or neonatal prescribing and administration guidance. Those calculations may involve weight, body surface area, maximum doses, reconstitution and other safeguards.
Safety disclaimer: Educational arithmetic only. Never use this page to decide a medicine, dose, route or frequency. Verify the prescription, medicine strength, units and local policy with an appropriately qualified healthcare professional.