Solid dose forms — drug calculations
Dosage calculator: work out
the amount of drug
Dosage calculation is the foundation of every topic in drug calculations, from tablets to injections. Here you get the formula, a clear explanation and a calculator you can use straight away, whether you are practising or just need to check a calculation quickly.
The dosage formula
The amount of drug to give is worked out by comparing the prescribed dose with the available strength.
Amount = (500 ÷ 250) × 1 = 2 tablets
Work out a dosage
Fill in the values below to work out how much drug should be given.
Work out a dosage
The calculator is a study tool for practising drug calculations and should not be used as the basis for clinical dosing.
Remember the units
The most common source of error in dosage calculations is not the formula but the prescribed dose and the available strength being given in different units.
Check the units first
Is the dose in mg but the strength in micrograms (or the other way round)? Convert to the same unit before you put the numbers into the formula.
Amount per unit is rarely just 1
If the strength is given per 5 ml, for example 250 mg/5 ml, the amount per unit is 5 ml, not 1. Otherwise the answer will be wrong.
Does the drug need diluting before it is given? Work out the dilution →
Is the strength stated as a percentage? Work out a percentage solution →
Is it given as an infusion? Work out drops per minute →
See all the formulas, including unit conversions →
Frequently asked questions about dosage calculations
How do you calculate a drug dose?+
A dose is calculated with the formula Amount = (Prescribed dose ÷ Available strength) × Amount per unit. For example, 500 mg prescribed and 250 mg tablets available gives (500 ÷ 250) × 1 = 2 tablets.
What does available strength mean?+
The available strength is how much drug is in each unit you have to hand, for example mg per tablet or mg per ml in a vial of injection.
What do you do if the prescribed dose and the available strength use different units?+
Always convert to the same unit before using the dosage formula, for example by converting micrograms to mg (divide by 1000) or mg to micrograms (multiply by 1000).
Does the dosage formula work for tablets, injections and oral liquids?+
Yes. The same basic formula is used whatever the form of the drug. What differs is what the amount per unit refers to, for example one tablet, 1 ml of liquid or 1 ml of injection.
Practise dosage calculations in Dos
Dosage calculation is the foundation of every topic. 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.
