How to Calculate Drug Dosage – The Easy Way

One of the first skills medical professionals learn is how to calculate drug dosage. Doing the math wrong can lead to ineffective patient treatments or cause a toxic drug overdose.  In most cases, the dosage information presents as mg/kg, or a ratio of milligrams of the drug to kilograms of body weight. The first step in determining a proper dosage is to figure out the patient’s weight in the right measurement.

Pounds to Kilograms

At the center of the process is determining the body weight of the patient in kilograms – the International System of Units base meter for mass. One kilogram equals 2.2 pounds. To calculate body weight in kilograms, you must divide the weight in pounds by that number. A person who weighs 165 pounds measures in at 75 kilograms.

What is Mg/Kg?

Drugs come with a general instruction that relies on milligrams vs. kilograms to create a base formula for the calculation of the drug dosage. What this means is the dosage is a certain number of milligrams for every kilogram of body weight. If the general formula list 25 mg/kg, the calculations is 25 milligrams for every kilogram the patient weight.

Doing the Calculation

The true heart of how to calculate drug dosage lies in figuring the mg/kg ratio. This is done by multiplying the base milligram dosage by the patient’s body weight in kilograms. In this way, drug manufacturers give medical personnel a base formula that allows them to individualize each dosage by the patient. A person weighing in at 125 pounds requires less medication that a patient who weighs 225 pounds.

To make the calculation, simply multiply the number of milligrams by the person’s weight in kilograms. If the drug list 25 mg/kg, the patient who weighs 125 pounds get 1421 mg. of medication.

125/2.2 = 56.82 kg.
56.82 * 25 = 1421 mg.

Maximum Dose

Understanding the concept of maximum is a critical step in learning how to calculate drug dosage. Some drugs list a maximum dosage of a medication. If the calculated dose exceeds that number, you just give the maximum listed. If the maximum dose is 650 mg., then the person in the example above would get 650 mg., not 1421 because it exceeds the maximum dose.

Pediatric vs. Adult

When it comes to how to calculate drug dosage, weight is part of determining the dose. For drugs that offer a pediatric vs. adult maximum, weight is a factor in figuring out who is an adult. Calculate the minimum adult weight by dividing the maximum dosage by the general dosage. If the maximum dose is 650 mg., the minimum weight for an 25 mg/kg adult dose is 26 kg or 57.2 pounds.

Determining the mg/kg ratio is the hard part when you are learning how to calculate drug dosage. From there, you simply look at the milligrams provided in the drug to figure out how much to give. For example, if you determine a patient needs 650 mg. and the drug comes in 325 mg. tablets, the patient gets two to meet the right dosage.

It is vital the person dispensing medication learn the proper formula for dosage and master the math. From figuring kilograms of body weight to factoring in maximum dosage, the calculations are not complex, just basic elementary school math.

About this Angie’s List Expert: Cary Byrd is the president and founder of eDrugSearch.com. Based in San Antonio, eDrugSearch.com is a free cost comparison engine that helps consumers get safe access to affordable medications and advocates licensed online pharmacies as a widely accepted alternative.