Drug Calculation Methods:

There are four main ways in which drugs may be prescribed, these ways are:

  1. In tablet or capsule form
  2. Liquid medicines
  3. Prescribed based on body surface area
  4. Prescribed based on body weight

In this guide we will describe how to calculate all via these methods and offer examples.



When giving via tablet or capsule form be sure to check the dose and the prescribed dose to make sure they are in the same unit. Change the large unit to the smaller unit to avoid decimal point confusion.

Formula =   Amount of Drug Prescribed / Amount of Drug in Each Tablet

Example =   The patient is prescribed 250mg of Aspirin and you have tablets that are 500mg each. Using the formula above you know 250mg /500mg  = 0.5 or half of one tablet.



Calculating dosages for liquids is more complicated than for tablets or capsules. The reason for this is there may be different concentrations of drug in medicines and as a result it will take a different volume of medicine to achieve the required amount.

Formula  = [ amount of drug prescribed / amount of drug in each unit ] multiplied by [ volume drug is in (i.e. 1mL) / 1]

Example  =  The patient is prescribed amoxicillin 150 mg. The bottle label states that there is 250mg in 10mL.  [150mg/250ml] multiplied by [10ml / 1] = [0.60] multiplied by [10] which is 6mL.



Calculating drug dosage according to body surface area can be tricky.  This guide will go through both the formula and calculation steps for calculating dosage according to body surface area and will also include a body surface area calculator.

Formula  = Surface Area ( m2)  multiplied by  dose per m2  = dose in grams or milligrams

Example  =  The patient has been charted a drug which has the dosage of 600mg/mand the patient has an overall surface area of 1.36m2. To calculate the dosage we simply take 1.36mand multiply by the dosage of 600mg/m2 which equals 816 mg


Dosage based on body weight is one of the easier way to understand as you are more than likely to have come across similar calculations before.

Formula = [ Body Weight (lb or kg) ] multiplied by [dose per kg or lb]

Example = Calculating the dosage given to a patient weighing 165lbs at 5mg/lb is as follows.  165 multiplied by 5 = 825mg



  • drugs that are prescribed using surface area or weight may be given as a calculated dose several multiplied by per day or the dose calculated for the whole day. Be sure to check the specifications with the prescription and that if the drug is for the whole day it is split correctly into the right doses over the day.