Formulae for calculating drug dosages

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Formulae for calculating drug dosages

Solid drugs

A whole tablet is always preferable to a broken tablet as the latter will have no definite and predictable dose. Always check with the pharmacy department whether a drug in tablet form can be broken to achieve the prescribed dose.

dose required=strength required/stock strength×volume or number of stock tabletsExample:

A patient is prescribed 30 mg amitriptyline. Amitriptyline is available in 10-mg tablets.


Number of tablets required=30/10×1=3

Liquid drugs

volume required=strength required/stock strength×volume of stock solution


A child is prescribed 240 mg paracetamol. Paracetamol suspension is available as a solution of 120 mg/5 ml.


volume of paracetamol required=240/120 ×5=10 ml

Drugs measured in units

Some medications may be available in different strengths and it is essential that the planning of administration includes the selection of a suitable combination of strengths to provide the prescribed dose in the minimum amount of tablets or liquid. amount required=strength required/stock strength×volume of solution

Examples with calculations:

1. A patient is prescribed 100 000 units of nystatin. The stock solution or unit measurement is 100 000 units in 1 ml of solution.

volume of solution required=100 000 units/100 000 units=1 ml

2. A patient is prescribed 12 000 units of heparin (for subcutaneous injection). Stock ampoules contain 25 000 units in 5 ml of solution.

volume required=12 000/25 000×5=60 000/25 000=2.4 ml

Preparation of solutions

volume required=strength required/stock strength×volume of stock solutionExample with calculation:

A vial of amoxicillin (500 mg) is mixed with 4.6 ml water for injections to give a concentration of 100 mg/ml. What volume of the solution is required to give an injection of 125 mg?125/100×1=1.25 ml

Intravenous infusions

There are two main types of giving set in use and they deliver fluid at the rate of 20 or 60 drops per millilitre.

time for infusion (hours)=volume (ml)/rate (ml/hour)

Example with calculation:If a patient is to receive 500 ml normal saline at the rate of 100 ml per hour, how long will it take to deliver the fluid?500/100=5 hours

Rate of drops per minute

rate=volume (ml)/time (hours)×60

(a) If the giving set has a flow rate of 20 drops per millilitre:rate=500×20 (drops/ml)/5×60=33 drops per minute

(b) If the giving set has a flow rate of 60 drops per millilitre:

rate=500×60 (drops/ml)/5×6=100 drops per minute