How to Calculate a Pediatric Dose

One of the doctor's roles is to prescribe an appropriate dosage of medication for his or her patients. This can be more challenging for pediatric patients because, unlike for adults, the correct dosage varies based on age, size and stage of development. In most cases, dosages are determined by body weight. However, dosages for chemotherapy and critical-care drugs are based on body surface area (BSA), while dosages for drugs such as syrup of ipecac and Kaopectate are based on age.

Things You'll Need

• Pen and paper
• Calculator (optional)
• Pediatric drug reference guide
• Pediatric scale

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Instructions

Calculating dosages

• 1

  Consult a pediatric drug reference guide for information on dosage (see References for a source providing a list of guides). Note whether the drug is one where dosage is calculated based on weight, BSA or age.

• 2

  Determine the patient's age, weight in kilograms (children) or grams (infants), and height in centimeters. If your scale only provides weight in pounds, convert to kilograms by dividing the weight in pounds by 2.2. For children under 1, convert to grams by dividing the kilogram value by 1,000. If your scale only provides height in inches, convert to centimeters by multiplying the height in inches by 2.54.

• 3

  If the drug is one where dosage is calculated by weight, multiply the patient's weight by the dosage listed in the drug guide. For example, for a child weighing 10 kg taking a drug with a recommended dosage of 20 mg/kg/day, the daily dosage would be 10 kg x 20 mg/kg/day = 200 mg/day.

• 4

  If the drug is one where dosage is calculated by BSA, multiply the patient's height by his or her weight. Divide the result by 3,600. Take the square root of the result. Finally, multiply the result by the dosage listed in the drug guide. For example, for a child whose weight is 20 kg and whose height is 100 cm taking a medication with a recommended dosage of 2 mg/m2/day, the daily dosage would be: 20 x 100 = 2,000; 2,000/3,600 = 0.55; √0.55 = 0.74; 0.74 m2 x 2 mg/m2/day = 1.48 mg/day.

• 5

  If the drug is one for which dosage is calculated by age, multiply the patient's age by the dosage listed in the drug guide. Age is measured in years, months or days, as determined by the recommended dosage. For example, for a 6-year-old child taking a medication with a recommended dosage of 1 mg/years of age/day, the daily dosage would be 6 mg/day.

• 6

  Divide the daily dosage by the number of doses to be given per day. For example, if a medication with a recommended dosage of 4 mg/day is to be given twice per day, the dose for each administration of the drug would be 2 mg.

• 7

  Divide the dose by its concentration to calculate the dose in milliliters. Pediatric drugs are usually dispensed at a predetermined concentration. For example, for a drug administered in 2-mg doses and prepared in a 1-mg/mL concentration, each dose would be 1 mL.