When a nurse or doctor calculates a medication dose based on a person's height and weight, he first determines the person's body surface area (BSA). This gives him a figure (expressed in meters squared) that is then multiplied by a drug-specific number to give the desired medication dose. This process allows the health care provider to give enough of the drug to ensure it's effective, but not so much that the person will have unwanted side effects. This level of dosing accuracy is critical when administering potential medications like chemotherapy drugs, which are extremely toxic in higher doses.

Access a BSA calculator. You can find these on the Internet at a site like Halls MD (see the Resources section). Many health care facilities also have their preferred formula embedded in a BSA calculator on their Intranet site.

Identify the formula for BSA that's used in your health care setting. There are several recognised formulas for determining how much medication to give a person of a certain height and weight. These formulas include Mosteller, DuBois, Haycock, Boyd, and Gehan and George. They give slightly different results, but the Mosteller formula is becoming the preferred one.

Determine the patient's height. Body surface area calculators will accept this information in inches, a combination of feet and inches, or in centimetres. It's best to obtain a standing height, but not every patient is willing or able to stand upright.

Measure the individual's weight. Use the most accurate scale available, and then enter this data in pounds or kilograms into the BSA calculator.

Click on the calculate button. The BSA calculator will automatically determine the person's BSA in meters squared. For example, the BSA for a man who is 61 inches tall and weighs 90.7 Kilogram, using the Mosteller formula, is 1.98 meters squared.

Identify the recommended dosing formula for a particular medication. Manufacturers publish the recommended dosing formula in terms of dose/meters squared. You can use an online calculator, or your own hand-held calculator, to calculate how much of a drug to give the person. If her BSA is 1.5 meters squared and the recommended drug dose is 200 mg per meter squared, the medication dose is then 300 mg.

#### Tip

Some websites, like Halls MD, have a safety check calculator that provides a way to ensure the data you entered for the BSA determination fit the person you're describing. For example, you may be asked to enter the person's age, gender, and body description and then recalculate the BSA. If the data are incompatible (e.g. height doesn't match age), the safety check calculator will give you an error message.

#### Warning

It's easy to make an error, especially in critical situations or on busy days. Having a colleague double-check your calculations provides an extra level of safety.