The Revision Guide for Student Nurses (Part I)

Calculation of Drug Doses (Key Notes)


    This module gives examples of the calculations required in the dispensing of medicines.
    Many drugs are supplied in a concentrated form, and you may be required to either reconstitute a powdered drug with water to give the desired concentration, or to dilute a liquid concentrate. Some injectable drugs are supplied at a given concentration with a dose rate in mg/kg. Remember that:
    • 1kg = 1000g
    • 1g = 1000mg
    • 1mg = 1000mcg
    • 1L = 1000mls
    • To convert the concentration of a solution expressed in percentages into mg/ml simply multiply the percentage figure by 10.
    • % solution = weight in grams x100 divided by the volume of solution in mls.
    • Weight in grams = volume of solution in mls x % solution divided by 100.
    Example - Stella the Staffie is admitted. She weighs 20kg and requires a daily injection of a 7.5% drug in solution. The dose is 10mg/kg. Calculate the volume of the dose to be administered.
    • A 7.5% solution = 75mg in 1ml.
    • Dose rate = 10mg/kg.
    • Dose rate x body weight (10 x 20) = 200mg.
    • Mg required divided by mg in solution (200 divided by 75) = 2.67mls/day.
    Tablets and capsules are often supplied in various strengths. They may be classified in micrograms (mcg), milligrams (mg) or grams (g). Drug doses may be calculated by either body weight (commonly) or by body surface area.

    Excellent examples are described in "Practical Veterinary Nursing" (see recommended reading) and Multiple Choice Questions in Veterinary Nursing (Volume 1) edited by the College of Animal Welfare, Butterworth Heinemann.