Drug Calculation Methods:
There are four main ways in which drugs may be prescribed, these ways are:
In this guide we will describe how to calculate all via these methods and offer examples.
1) TABLET or CAPSULE FORM
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.
2) LIQUID MEDICINES
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  which is 6mL.
3) BODY SURFACE AREA:
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/m2 and the patient has an overall surface area of 1.36m2. To calculate the dosage we simply take 1.36m2 and multiply by the dosage of 600mg/m2 which equals 816 mg
4) BODY WEIGHT
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