Pediatric Dose Calculator

What This Calculator Does

This page applies the two classical scaling rules, Clark’s Rule and Young’s Rule. Both answer the same question: given a known adult dose, what fraction of it suits a child? They are estimates, and it is worth being clear about that up front, because they are not how a modern prescription is written.

Clark’s Rule scales by weight:

Child dose = (Child weight in lb ÷ 150) × Adult dose

The 150 is a notional average adult weight in pounds. A 50 lb child gets a third of the adult dose.

Young’s Rule scales by age:

Child dose = (Age ÷ (Age + 12)) × Adult dose

An 8-year-old gets 8/20, so 40% of the adult dose. Young’s Rule takes no account of size at all, which is its whole weakness.

Worked example:

A 60 lb, 8-year-old child, adult dose 500 mg. Clark’s: (60 ÷ 150) × 500 = 200 mg Young’s: (8 ÷ 20) × 500 = 200 mg

They agree here because a 60 lb 8-year-old is close to average. Push either input and they separate fast. A heavy 8-year-old at 90 lb gets 300 mg by Clark’s and still 200 mg by Young’s, a 50% gap on the same child.

When these rules are the wrong tool

Honestly? Most of the time. Clark’s and Young’s date from an era before per-kilogram dosing tables existed, and modern practice dose almost everything as mg per kg of body weight from a drug-specific table, because the right dose depends on the drug’s own pharmacology and not on a fixed fraction of an adult’s. Neither rule knows anything about the medicine you are giving.

Where they still earn their keep is as a sanity check. If a label or a calculation hands you a child dose that is wildly off what Clark’s Rule suggests, something deserves a second look before anyone swallows anything.

If you have a proper mg/kg figure from a prescription or a label, use the Pediatric Weight-Based Dose Calculator instead. It handles the maximum-dose cap and the liquid concentration, which these two rules cannot.

Common pediatric doses (verify with your pharmacist):

• Acetaminophen (Tylenol): 10–15 mg/kg every 4–6 hours, max 5 doses per 24h
• Ibuprofen (Advil/Motrin): 5–10 mg/kg every 6–8 hours (not for infants under 6 months)
• Amoxicillin: 25–45 mg/kg/day divided every 8–12 hours (prescription)

Weight conversion reminder:

kg = lb ÷ 2.2046

A 33 lb child weighs just under 15 kg. The rounder ÷ 2.2 you see everywhere is close enough for a Clark’s Rule estimate, but it runs about 0.2% high, so do not carry it into a real mg/kg calculation where a decimal place matters.

Critical safety rules:

• Always use the oral syringe that came with the medication. A kitchen teaspoon holds anywhere from 2.5 to 7 mL depending on which drawer it came out of.
• Never dose by age alone. A small 4-year-old and a large 4-year-old are not the same patient, and that is precisely what Young’s Rule cannot see.
• Double-check the concentration. Infant drops and children’s liquid have been sold at very different strengths, and old bottles outlive the reformulations.
• Do not give aspirin to anyone under 16 (risk of Reye’s syndrome)
• When in doubt, call your pediatrician or pharmacist before dosing