# Pediatric Dosage Calculations

*Caring for children who are ill challenges every nurse to function at the highest level of professional competence. Giving medication is one of the most important nursing responsibilities. Commercial unit-dose packaging sometimes does not include dosages for children, so the nurse must calculate the correct dosage to provide safe and effective care.*

- Body Weight Method
- Converting Pounds to Kilograms
- Calculating Dosage Using Body Weight Method

- Body Surface Area Method
- Responsibilities After Dosage Calculation

Two methods of computing doses are used to determine accurate pediatric medication dosages: the body weight method and the body surface area method.

### Body Weight Method

The first method of computing dosages uses the child’s weight.

- Weight in kilograms. The child’s weight in kilograms is used to calculate a safe dose range for the child.

#### Converting Pounds to Kilograms

To use the body weight method of dosage calculation, a child’s weight recorded in pounds has to be converted into kilograms.

- Set up a proportion using the number of pound in a kilogram in one fraction and the unknown weight in kilograms in the other fraction.
- For example, for a child weighing 42 pounds, the conversion is set up as follows: = The fractions are then cross-multiplied: 2.2 lb x Xkg= 1 kg x 42 lb; the problem is solved for X.
- Divide each side by 2.2 and cancel the units that are in both the numerator and the denominator; = X= X= 19 kg.
- The child who weighs 42 pounds weighs 19 kilograms.

#### Calculating Dosage Using Body Weight Method

After converting the child’s weight into kilograms, a safe dose range for that child is calculated.

- For example, if a dosage range of 10 t0 30mg/kg of body weight is a safe dosage range and the child weighs 20 kg, calculate the low safe dose using the following: = Cross multiply the fractions: 10mg x 20kg = 1kg x Xmg.
- Solve for X by dividing each side of the equation by 1 (canceling the units that are both the numerator and the denominator): = 200 x 1 =1X200 = X.
- The low safe dose range of this medication for the child who weighs 20kg is 200mg.
- To calculate the high safe dose for this child, use the following:= Cross multiply the fractions: 30mg x 20kg x Xmg.
- Solve for X by dividing each side of the equation by 1 (canceling the units that are in both the numerator and the denominator): 30mg x 20kg =600×1= 1X600=X.
- The high safe dose range of this medication for the child who weighs 20 kg is 600mg.

### Body Surface Area Method

The second formula used to calculate dosages is the body surface area (BSA) method.

- West nomogram. The West nomogram, commonly used to calculate BSA, is a graph with several scales arranged so that when two values are known, the third can be plotted by drawing a line with a straight edge.
- The child’s weight is marked on the right scale, the height on the left scale.
- Use a straight edge to draw a line between the two marks.
- The point where the lines cross the column labeled SA (surface area) is the BSA expressed in square meters (m2).
- The average adult BSA is 1.7 m2; thus the formula to calculate the appropriate dosage for a child is estimated child’s dose.
- For example, a child is 37 inches (95cm) tall and weighs 34 lb (15.5 kg).
- The usual adult doe of the medication is 500mg.
- Place and hold one end of a straight edge on the first column at 37 inches and move it so that it lines up with 34 lb in the far right column.
- On the SA column, the straight edge falls across 0.64 (m)2.
- You are ready to do the calculation. =0.38; you know now that the child’s BSA is 0.38 that of the average adult.
- You are ready to calculate the child’s dose by multiplying 0.38 times 500mg: 0.38×500= 190.
- The child’s dose is 190mg.

### Responsibilities After Dosage Calculation

After computing the dosage, the nurse should:

- Double-check the computation. The nurse should always have the computation checked by another staff person qualified to give medication or someone in the department who is delegated for this purpose.
- Compute separately. Errors are easy to make and easy to overlook; a second person should do the computation separately; then both results should be compared.