Child Medicine Dosage Calculator

Child medication dosing by weight uses the child’s weight (not age) to calculate the correct dose of common over-the-counter medicines. Weight-based dosing is more accurate because children of the same age can vary significantly in body mass.

Core dosing formula: Dose (mg) = Child Weight (kg) × Dose per kg (mg/kg) Volume to give (mL) = Dose (mg) ÷ Concentration (mg/mL)

What each variable means:

• Child Weight: always measure in kilograms for medical dosing. To convert: Weight (kg) = Weight (lbs) × 0.4536, which is what this calculator uses. Dividing by 2.205 is the same conversion written the other way round.
• Dose per kg (mg/kg): the recommended therapeutic dose per kilogram of body weight. This comes from clinical studies that established effective and safe amounts.
• Concentration (mg/mL): the strength of the liquid medicine. Always check the bottle, concentrations vary between brands and formulations.
• Volume: the actual amount of liquid (mL) you measure and give. Use the measuring syringe included with the medicine, not a household teaspoon.

Acetaminophen (Children’s Tylenol) dosing: The accepted range is 10–15 mg/kg per dose, every 4–6 hours, maximum 5 doses a day. This calculator uses 12.5 mg/kg, the middle of that range, rather than the top of it. Standard liquid concentration: 160 mg/5 mL, which is 32 mg/mL.

Ibuprofen (Children’s Advil) dosing: The accepted range is 5–10 mg/kg per dose, every 6–8 hours, and it is not for children under 6 months. This calculator uses 7.5 mg/kg, again the middle. Standard liquid concentration: 100 mg/5 mL, which is 20 mg/mL. Note that this is not the same strength as the acetaminophen liquid, and mixing the two up is one of the easier mistakes to make at 3am.

Why the middle and not the top? Because the top of the range is a ceiling, not a target, and there is no benefit to starting there. If the middle dose is not controlling a fever after a full interval, that is a conversation with your pediatrician rather than a reason to creep upward on your own.

Worked example: Child weighs 18 kg (about 40 lbs). Needs acetaminophen.

Dose = 18 kg × 12.5 mg/kg = 225 mg Concentration = 160 mg/5 mL Volume = 225 ÷ 160 × 5 = 7.0 mL

For ibuprofen: Dose = 18 kg × 7.5 mg/kg = 135 mg Volume = 135 ÷ 100 × 5 = 6.8 mL

Those two volumes are close to each other, which is exactly why the concentration is worth checking rather than assuming.

Maximum dose caps:

• Acetaminophen: never exceed 1,000 mg per dose or 4,000 mg/day (the adult limits, which a child never passes)
• Ibuprofen: never exceed 400 mg per dose or 1,200 mg/day
• The calculator applies all four of these and tells you when one has taken effect
• Children under 2: always check with a physician before giving any OTC medication

Chewables are not a small-child form. A 160 mg chewable is a coarse unit, and half of one is the smallest piece anyone can break cleanly. For a baby whose whole dose is 40 mg that is four times too much, so this calculator will not quote a tablet count below about 10 kg (22 lbs) and sends you to the liquid instead. It also checks the rounded tablet dose against the 15 mg/kg per-dose ceiling and says so if the rounding pushed past it. Above that weight the tablet is fine, because one whole tablet is close enough to a real dose that the rounding no longer matters.

Safety reminders: Never give aspirin to children (Reye’s syndrome risk). Never give ibuprofen to infants under 6 months. Use only the measuring device provided in the package, since kitchen spoons vary by more than a third between drawers.