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Pediatric Dose Calculator

Calculate safe pediatric medication doses using weight-based dosing, Clark's Rule, Young's Rule, Fried's Rule, or the BSA method. Fast and free.

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How the Pediatric Dose Calculator Works

Calculating medication doses for children requires a fundamentally different approach than adult dosing. Children metabolize drugs differently based on weight, age, body surface area, and developmental stage. This pediatric dose calculator supports five evidence-based methods recognized by clinical pharmacology guidelines, enabling healthcare providers and caregivers to estimate safe, effective doses tailored to each child.

The Five Pediatric Dosing Methods

1. Weight-Based Dosing (Recommended First Choice)

Weight-based dosing is the gold standard for pediatric pharmacotherapy. The formula is: Child's Dose = Weight (kg) x Dose per kg (mg/kg). For example, amoxicillin for otitis media is commonly prescribed at 40-45 mg/kg/day. For a 20 kg child: 20 x 40 = 800 mg/day, typically divided into two or three doses. The FDA Guidance for Industry on Pediatric Dosing recommends weight-based dosing as the primary method whenever mg/kg data is available for the specific medication.

2. Clark's Rule

Clark's Rule proportions the adult dose using the child's weight relative to a standard adult weight of 68 kg (150 lbs): Child's Dose = (Weight kg / 68) x Adult Dose. Example: a 34 kg child requiring acetaminophen, with an adult dose of 1,000 mg, receives (34 / 68) x 1,000 = 500 mg. According to StatPearls on NCBI Bookshelf, Clark's Rule remains a widely taught approximation method in pharmacology and is most reliable for children weighing more than 10 kg when dose-per-kg reference data is not available.

3. Young's Rule (Ages 2-12 Years)

Young's Rule estimates dose using the child's age in years: Child's Dose = [Age (years) / (Age + 12)] x Adult Dose. For an 8-year-old child requiring ibuprofen (adult dose 400 mg): (8 / 20) x 400 = 160 mg. This method is appropriate for children aged 2 to 12 years and provides a reasonable estimate when only the patient's age is known, as documented by the University of Washington Pharmacy Calculations resource (Lesson 4).

4. Fried's Rule (Infants Under 2 Years)

Fried's Rule is designed for infants and toddlers, using age in months: Child's Dose = [Age (months) / 150] x Adult Dose. For an 18-month-old infant with an adult reference dose of 500 mg: (18 / 150) x 500 = 60 mg. The denominator 150 anchors infant doses proportionally across the first two years of life, making this rule appropriate for children up to 24 months. Beyond that age, Young's Rule or weight-based methods offer better accuracy.

5. Body Surface Area (BSA) Method

The BSA method offers the highest precision for high-risk medications such as chemotherapeutic agents and growth hormones: Child's Dose = (BSA m2 / 1.73) x Adult Dose. The denominator 1.73 m2 represents average adult BSA. BSA is commonly derived using the Mosteller formula: BSA = sqrt[(Height cm x Weight kg) / 3600]. For a child with a BSA of 0.87 m2 and an adult dose of 100 mg: (0.87 / 1.73) x 100 = 50.3 mg. The University of Iowa Pressbooks Pediatric Endocrinology chapter identifies BSA-based dosing as the preferred approach for growth hormone replacement and pediatric oncology protocols.

Choosing the Right Method

  • Weight-based dosing: First choice for most medications when mg/kg reference data exists.
  • BSA method: Preferred for chemotherapy, growth hormones, and narrow therapeutic index drugs.
  • Clark's Rule: Best backup when dose-per-kg data is unavailable for children over 10 kg.
  • Young's Rule: Suitable for ages 2-12 when only age is known.
  • Fried's Rule: Reserved for infants under 24 months when weight is unavailable.

Safety and Clinical Limitations

Every calculated dose must be verified against published maximum single-dose and total daily-dose limits for the specific medication. Renal impairment, hepatic dysfunction, prematurity, and pharmacogenomic variation can significantly alter drug metabolism in pediatric patients, requiring downward adjustments. This calculator provides an estimate for educational and reference purposes only. All final dosing decisions must be confirmed by a licensed healthcare professional or clinical pharmacist before administration to any patient.

Reference

Frequently asked questions

What is the most accurate method for calculating pediatric doses?
Weight-based dosing (mg/kg) is the most accurate method for most pediatric medications because it directly accounts for the child's body mass. The FDA recommends this approach whenever dose-per-kg data is available. For high-risk drugs such as chemotherapy agents and growth hormones, the body surface area (BSA) method provides even greater precision by incorporating both height and weight into the calculation.
How does Clark's Rule work for calculating a child's medication dose?
Clark's Rule calculates a child's dose by dividing the child's weight in kilograms by 68, which represents the assumed average adult weight in kg, then multiplying by the adult dose. For example, a 20 kg child needing a drug with a 500 mg adult dose would receive (20 / 68) x 500 = approximately 147 mg. Clark's Rule is most reliable for children weighing over 10 kg, as noted by StatPearls on NCBI Bookshelf.
What is the difference between Young's Rule and Fried's Rule for pediatric dosing?
Young's Rule uses the child's age in years and applies to children aged 2 to 12: Child Dose = [Age / (Age + 12)] x Adult Dose. Fried's Rule uses age in months and applies specifically to infants under 24 months: Child Dose = [Age in months / 150] x Adult Dose. Both are age-based approximation methods used when weight data is unavailable, but Fried's Rule is calibrated for the distinct physiology of the infant population.
When should the BSA method be used instead of weight-based dosing for children?
The BSA method is preferred over weight-based dosing for chemotherapeutic agents, growth hormone replacement therapy, and any medication with a narrow therapeutic index. Because BSA accounts for both height and weight, it captures physiological differences more precisely than weight alone. Pediatric oncology protocols and endocrinology guidelines, including those published by the University of Iowa, consistently mandate BSA-based dosing to minimize the risk of toxicity or underdosing.
Can the pediatric dose calculator be used for all types of medications?
The pediatric dose calculator provides dose estimates applicable to many common medications, but it is not appropriate for all drugs. Certain medications, including anticonvulsants, immunosuppressants, and aminoglycoside antibiotics, require individualized dosing guided by therapeutic drug monitoring, renal function, or pharmacogenomic testing. Always cross-reference any calculated result with current prescribing information or consult a licensed pharmacist before administering medication to a child.
How do you convert a child's age to months for Fried's Rule?
To apply Fried's Rule, multiply the child's age in complete years by 12 and add any additional months. For example, a child aged 1 year and 6 months equals 18 months total. The Fried's Rule formula then becomes: (18 / 150) x Adult Dose = 0.12 x Adult Dose. This calculation method is validated only for children up to 24 months of age; beyond that threshold, Young's Rule or weight-based dosing provides more accurate estimates.