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 conventional adult reference BSA is 1.73 m2, so a child's dose is estimated from the ratio of the child's BSA to that adult average: child dose = adult dose x (child BSA / 1.73) (Body Surface Area Estimation in Children).
- 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 about 0.37 of the 1.73 m2 adult reference (0.64 / 1.73 = 0.37).
- Multiply 0.37 by the adult dose: 0.37 x 500 mg = 185 mg. The child's dose is about 185 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. ISMP advises reserving independent double checks for the highest-risk drugs and doses, where they catch the most errors rather than becoming a rushed formality (ISMP).
- 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.
Frequently Asked Questions
Why are pediatric doses calculated by weight or body surface area? Children differ widely in size and physiology, so a flat adult dose is unsafe. Weight-based (mg/kg) and body surface area methods scale the dose to the individual child, and the two correlate closely when applied correctly (Body Surface Area Estimation in Children).
How do you convert pounds to kilograms for dosing? Divide the weight in pounds by 2.2. A 42-pound child weighs about 19 kg (42 / 2.2 = 19). Always dose from the kilogram weight, not pounds.
What adult body surface area value is used in the BSA method? The conventional adult reference is 1.73 m2. The child's dose equals the adult dose times the child's BSA divided by 1.73 (Body Surface Area Estimation in Children).
What is the West nomogram? It is a graph with scales for height, weight, and surface area. Mark height on the left and weight on the right, draw a straight line between them, and read the body surface area in square meters where the line crosses the SA column.
Should every pediatric dose get an independent double check? Not every dose. ISMP recommends targeting independent double checks at the highest-risk medications and calculations, such as concentrated opioids and chemotherapy, so the check stays meaningful instead of routine (ISMP).
What do you do if your calculated dose does not match the order? Stop and clarify with the prescriber before giving anything. A mismatch between your independent calculation and the charted order is a flag, not a rounding difference to ignore.