Dosing FAQ
Quick, plain-language answers to the questions parents ask most about how Perfect Dose calculates a dose — and when to skip the calculator and call a doctor.
Contents
1. Why a per-drug multiplier instead of a single dose range?
Each medication is processed by your child's body differently. Acetaminophen (Tylenol) is dosed at 15 mg/kg, ibuprofen (Motrin/Advil) at 10 mg/kg, and diphenhydramine (Benadryl) at 1.25 mg/kg — three very different multipliers. Using one number for all three would mean a hefty overdose of one drug and an under-dose of another.
For more detail on each medication, see our Tylenol, Motrin, and Benadryl dosing pages, or open the calculator to enter your child's weight.
2. Why weight instead of age?
Drug labels group kids by age, but kids of the same age can differ in weight by a factor of two or more. A 2-year-old at the bottom of the "2–3 years" band may weigh half as much as a 3-year-old at the top — yet the label recommends a single dose for both. Weight follows your child's actual body size, so a weight-based dose is much more likely to land in the safe range. That is why emergency physicians dose by weight, not age.
Try it on the Perfect Dose calculator — enter your child's current weight and the medication in front of you.
3. Why does the weight input expire after a short window?
Children's weight changes faster than parents realize, especially during illness — dehydration, fluid retention, or a missed growth checkup can all shift the number. Stale weights are one of the most common causes of accidental over- or under-dosing, because the dose you'd give a child at 35 lb is meaningfully different from the dose you'd give at 50 lb.
Perfect Dose re-prompts you for a fresh weight before each calculation so the result reflects your child right now — not what they weighed at their last well-child visit. Open the calculator each time you dose.
4. How does the max single-dose cap work?
Every medication has a per-dose ceiling — acetaminophen at 650 mg, ibuprofen at 400 mg, diphenhydramine at 50 mg. Even if a child's weight × the mg/kg multiplier would compute a higher number, the result is capped so a heavier child never receives an unsafe dose in one sitting.
The calculator surfaces the cap clearly when it applies, so you can see exactly why the dose stops where it does. Open the calculator to see the cap in action.
5. When should I call a doctor instead of using the calculator?
The calculator is an educational reference for routine over-the-counter dosing. It is not a substitute for clinical judgment. Seek immediate medical attention if your child:
- Has difficulty breathing
- Appears unresponsive or unusually drowsy
- Has a fever above 104°F (40°C)
- Shows signs of severe dehydration
- Has a seizure
- Appears seriously ill for any reason
When in doubt, contact your pediatrician or go to the emergency room. See the full medical disclaimer, or open the calculator for routine doses.