Pediatric Medication Dosage Calculator

Understanding Weight-Based Pediatric Medication Dosing

Children are not small adults. Their bodies metabolize medications differently, their organs are still maturing, and their proportions differ substantially from adults. A dose appropriate for a 70 kg adult would be dangerous—or ineffective—if given unmodified to a 20 kg child. Instead, pediatric medication dosing relies on weight-based calculations, where the dose per kilogram of body weight is specified based on clinical trials, pharmacokinetic studies, and decades of safety data. This calculator helps parents, students, healthcare workers, and medical professionals quickly determine appropriate weight-based medication doses for children, serving as an educational tool and a reference aid when prescribing information is not immediately at hand.

Dosing by weight addresses the fact that drug absorption, distribution, metabolism, and excretion are all influenced by body size and composition. A larger child has more tissues in which a drug can distribute, a proportionally different blood volume, and different organ function than a smaller child. By anchoring dosing to kilograms, healthcare providers ensure that each child receives a quantity appropriate to their physiology. This calculator presents standard pediatric doses for common medications as a reference tool. Always verify doses with official prescribing information, consult with pharmacists, and never adjust doses without professional guidance.

The Mathematics of Weight-Based Dosing

The fundamental pediatric dosing formula is remarkably simple:

For medications given multiple times daily, the total daily dose is then divided by the frequency. If a medication is prescribed at 10 mg/kg and a child weighs 25 kg, the single dose is 10 × 25 = 250 mg. If given twice daily, each individual dose is 250 mg. If the medication is available as a liquid suspension with a specific concentration (e.g., 20 mg/mL), the volume administered would be 250 ÷ 20 = 12.5 mL per dose.

For some medications, the prescribing information specifies a total daily dose in mg/kg rather than a per-dose amount. In those cases, multiply the dose per kg by weight to get the total daily dose, then divide by the frequency to find the individual dose. The relationship is:

This calculator automates both scenarios. For weight-based doses (per single dose), it multiplies weight by the specified dose per kg. For daily doses, it first multiplies weight by the daily rate, then divides by frequency. The result is the dose in milligrams per single administration.

Worked Example: Calculating Amoxicillin Dose for a Child

A 28 kg child presents with bacterial sinusitis. The pediatrician prescribes amoxicillin at 25–45 mg/kg/day in divided doses (twice daily). Using the calculator:

Step 1: Enter weight: 28 kg

Step 2: Select amoxicillin: 25-45 mg/kg/day (using 45 mg/kg for acute infection)

Step 3: Set frequency: 2 times daily

Step 4: Calculate: (45 mg/kg/day × 28 kg) ÷ 2 = 1260 ÷ 2 = 630 mg per dose

Result: The child should receive 630 mg (or approximately 625 mg) twice daily. If amoxicillin suspension is dispensed at 250 mg/5 mL, this corresponds to 12.5 mL (2.5 teaspoons) per dose. The prescriber or pharmacist would round this to a practical volume, such as 12 mL or 13 mL, depending on the available formulation. This demonstrates how weight-based dosing translates to practical medication administration.

Common Pediatric Medication Dosing Reference Table

The following table provides standard weight-based doses for frequently prescribed pediatric medications. These are reference values based on commonly used pediatric dosing guidelines (AAP, AMA, etc.) and should always be verified against current prescribing information and clinical judgment:

This table emphasizes that pediatric dosing guidelines vary by drug class and clinical scenario. Some medications use weight-based dosing (mg/kg), while others have fixed starting doses (such as fluoxetine, which is typically 10 mg/day regardless of weight). Always consult the prescribing information and discuss with the prescriber or pharmacist about the appropriate dose and frequency for the individual child.

Why Pediatric Dosing Differs from Adult Dosing

Several physiological factors justify distinct pediatric dosing regimens. First, children have higher metabolic rates than adults (per kg of body weight), meaning their bodies process many drugs faster, requiring more frequent doses or higher per-dose amounts. Second, their livers and kidneys are still developing, affecting drug clearance—some medications are metabolized more slowly in young children, while others are cleared faster. Third, body composition differs: infants have proportionally more water and less fat than adults, changing how lipophilic drugs distribute. Fourth, protein binding and blood-brain barrier permeability vary with age and development. These factors mean that a simple "one-tenth of the adult dose" rule is unsafe and that careful weight-based calculations are essential.

Age-Adjusted Dosing and BSA Dosing

While weight-based dosing (mg/kg) is most common in general pediatrics, some medications—particularly chemotherapy agents and immunosuppressants—are dosed by body surface area (BSA), measured in meters squared (m²). BSA-based dosing is thought to more accurately account for organ function and metabolic rate in cancer treatment. The Mosteller formula for BSA is:

This calculator focuses on weight-based dosing, which is appropriate for the vast majority of pediatric medications. BSA-based dosing requires height in addition to weight and is beyond this tool's scope; consult specialized oncology or transplant guidelines for those medications.

Special Considerations: Renal and Hepatic Impairment

Children with compromised kidney or liver function may require dose adjustments even when weight-based calculations are otherwise correct. A child with renal disease cannot clear renally-excreted drugs as efficiently, risking accumulation and toxicity. Conversely, some drugs are metabolized by the liver, and hepatic impairment slows clearance. Premature infants have especially immature livers and kidneys and require age-appropriate adjustments. This calculator provides standard doses for children with normal organ function. Any child with documented renal, hepatic, or metabolic disease should have doses verified by their healthcare provider or clinical pharmacist.

Using the Calculator

Enter the child's weight in kilograms (if weight is given in pounds, divide by 2.205 to convert). Select the medication from the dropdown list. If your medication is not listed, select "Custom" and enter the dose per kg from the prescribing information. Set the frequency (how many times per day the child will receive a dose). The calculator will compute the single-dose amount in milligrams. If you have the medication's concentration (mg/mL), enter it to see the approximate volume that would need to be administered. The result is an educational reference to help understand pediatric dosing; always verify against prescribing information and clinical judgment.

Important Disclaimers and Limitations

Medical Disclaimer: This calculator is provided for educational and reference purposes only and does not constitute medical advice. Medication dosing must always be determined by a qualified healthcare provider (physician, nurse practitioner, or physician assistant) based on the individual child's diagnosis, age, weight, renal and hepatic function, allergies, and other medications. Never administer any medication without explicit instructions from a healthcare provider. Incorrect dosing can cause serious harm or death. If you have questions about a child's medication dose, contact the prescriber, pharmacist, or poison control center immediately.

The doses presented are based on standard pediatric guidelines current as of 2024 but are not exhaustive and may not apply to all populations, ages, or clinical scenarios. Individual manufacturers' prescribing information may differ slightly. Preterm infants, children with organ impairment, and those on multiple medications require individualized dosing. Some medications have age restrictions (e.g., certain antibiotics are not recommended under a specific age) that this calculator does not enforce. Always consult authoritative sources such as the FDA-approved package insert, UpToDate, or official pediatric dosing handbooks before administering any medication.

This tool is not a substitute for professional medical judgment and should not be used as the sole basis for medication administration in clinical practice. Healthcare students should use this as a learning aid alongside comprehensive pharmacology and pediatrics courses. Parents and caregivers should always administer medications exactly as prescribed by their child's healthcare provider.