Pediatric Dosage Calculations

Peds dosing has no margin for error. Doses depend on age, weight, and physiology, so a small miscalculation does real harm. Two methods give you a safe pediatric dose: the body weight method and the body surface area method.

Body Weight Method

This method uses the child's weight to calculate a safe dose range.

Converting Pounds to Kilograms

The body weight method needs the weight in kilograms, so a weight charted in pounds has to be converted first.

• Set up a proportion: the number of pounds in a kilogram in one fraction, the unknown weight in kilograms in the other.
• For a child weighing 42 pounds, cross-multiply: 2.2 lb x X kg = 1 kg x 42 lb, then solve for X.
• Divide each side by 2.2 and cancel the units that appear in both numerator and denominator: X = 19 kg.
• So a child who weighs 42 pounds weighs 19 kilograms.

Calculating Dosage Using Body Weight

Once you have the weight in kilograms, calculate the safe dose range.

• Say the safe range is 10 to 30 mg/kg and the child weighs 20 kg. For the low dose, cross-multiply: 10 mg x 20 kg = 1 kg x X mg.
• Divide each side by 1 and cancel the matching units: X = 200. The low safe dose for a 20 kg child is 200 mg.
• For the high dose: 30 mg x 20 kg = 1 kg x X mg, which solves to X = 600. The high safe dose for a 20 kg child is 600 mg.

Body Surface Area Method

The second method uses body surface area (BSA).

• West nomogram. The West nomogram is the usual tool for BSA. It is a graph with several scales: when two values are known, the third is found by drawing a straight line across them.
• Mark the child's weight on the right scale and the height on the left scale, then draw a line between the two marks.
• Where the line crosses the SA (surface area) column is the BSA in square meters (m2).
• The average adult BSA is 1.7 m2, so a child's dose is estimated from the ratio of the child's BSA to that adult average.
• Example: a child is 37 inches (95 cm) tall and weighs 34 lb (15.5 kg). The usual adult dose of the medication is 500 mg.
• Line up 37 inches with 34 lb on the nomogram. The SA column reads 0.64 m2.
• The child's BSA (0.64 m2) is 0.38 of the average adult BSA.
• Multiply 0.38 by the adult dose: 0.38 x 500 mg = 190 mg. The child's dose is 190 mg.

Nursing Responsibilities After Dosage Calculation

The math is not the end of the job. These steps keep the dose safe:

• Double-check the computation. Have another qualified staff member check your math.
• Compute independently. Errors are easy to make and easy to miss. A second person runs the calculation on their own, then you compare results.
• Verify the order against your calculation. Before you give anything, compare your calculated dose to the charted order. If they differ, clarify with the prescriber first.
• Match the drug to the child. Formulation and dose follow age, weight, and developmental stage. Pain relief for a 3-year-old, for example, means an age-appropriate formulation and dose.