For Pediatricians

1. Clinical references

Each medication in the calculator is grounded in standard pediatric references and the current manufacturer label. Below are the sources that inform the dose tables — AAP clinical guidance where applicable, and the labeled-product information (concentration, age ranges, single-dose ceiling) used as the upper safety bound.

Acetaminophen (Tylenol)

• American Academy of Pediatrics (AAP), Committee on Drugs. "Acetaminophen Toxicity in Children." Pediatrics, 2001.
• Tylenol (acetaminophen) Infants' Suspension 160 mg / 5 mL — McNeil Consumer Healthcare, current U.S. label.
• Tylenol Children's Suspension 160 mg / 5 mL — McNeil Consumer Healthcare, current U.S. label.
• Tylenol Children's Chewables 80 mg — McNeil Consumer Healthcare, current U.S. label.
• Tylenol Junior Strength 160 mg caplets — McNeil Consumer Healthcare, current U.S. label.

Ibuprofen (Motrin / Advil)

• American Academy of Pediatrics (AAP), Section on Clinical Pharmacology. Ibuprofen dosing guidance for febrile children.
• Motrin Infants' Drops 50 mg / 1.25 mL — Johnson & Johnson Consumer Inc., current U.S. label.
• Children's Motrin / Advil Suspension 100 mg / 5 mL — Johnson & Johnson Consumer Inc., current U.S. label.
• Motrin Junior Strength 100 mg chewables — Johnson & Johnson Consumer Inc., current U.S. label.

Diphenhydramine (Benadryl)

• Children's Benadryl Allergy Liquid 12.5 mg / 5 mL — Johnson & Johnson Consumer Inc., current U.S. label.
• Pediatric antihistamine references: standard peak effect at 1.25 mg/kg per single dose, with a 50 mg adult-OTC ceiling.

Cetirizine (Zyrtec)

• Children's Zyrtec Allergy Syrup 5 mg / 5 mL — Johnson & Johnson Consumer Inc., current U.S. label.
• Children's Zyrtec Chewables 10 mg — Johnson & Johnson Consumer Inc., current U.S. label.
• Allergic rhinitis guidelines — second-generation antihistamines, once-daily dosing, 12-month minimum age.

Loratadine (Claritin)

• Children's Claritin Allergy Syrup 5 mg / 5 mL — current U.S. label.
• Allergic rhinitis references — second-generation antihistamine, once-daily dosing, 24-month calculator minimum age.

2. Weight-based mg/kg calculation

The base formula is the same for every drug in the calculator: a per-kilogram multiplier pulled from the dosing_constants table, applied to the entered weight in kilograms, and capped against a per-dose ceiling:

mg = weight_kg × dose_per_kg, then mg = min(mg, max_single_dose_mg).

Per-drug multipliers used at the calculator:

• Acetaminophen (Tylenol): 15 mg/kg per single dose, single-dose ceiling 650 mg.
• Ibuprofen (Motrin / Advil, infant and children's): 10 mg/kg per single dose, single-dose ceiling 400 mg.
• Diphenhydramine (Benadryl): 1.25 mg/kg per single dose, single-dose ceiling 50 mg.
• Cetirizine (Zyrtec): 0.25 mg/kg per single dose, single-dose ceiling 10 mg.
• Loratadine (Claritin): 0.2 mg/kg per single dose, single-dose ceiling 10 mg.

The mg is then converted into the unit expected for the formulation in front of the parent (mL for liquids, whole-and-half-tablet counts for chewables). The multipliers above are stored in dosing_constants; this page quotes them as static facts of record so a clinician can audit the calculator without re-querying the database.

3. Age and minimum-age eligibility

Each medication has a min_age_months floor that gates the calculator. The calculator surfaces this gate before accepting a weight, so an under-age child cannot get a result without acknowledging the age exception. The current floors are:

• Acetaminophen: 2 months.
• Ibuprofen (infant and children's): 6 months.
• Diphenhydramine (Benadryl): 24 months.
• Cetirizine (Zyrtec): 12 months.
• Loratadine (Claritin): 24 months.

The min_age_months gate is the safety floor; the manufacturer's labeled age-band information lives on the per-medication SEO pages, where it is easier to quote alongside the product-relevant caveats:

• Acetaminophen — Tylenol
• Ibuprofen — Motrin / Advil
• Diphenhydramine — Benadryl
• Cetirizine — Zyrtec
• Loratadine — Claritin

4. Rounding rules

The raw mg result is converted into the formulation-specific unit, then rounded so a parent can actually measure it:

• Liquids (mL): the raw mL is computed from the mg result and the formulation's concentration, then capped against the formulation's max_form_ml, and finally rounded to the nearest 0.1 mL. Typical mid-range calculators (e.g. 12.5 kg × 0.46875 = 5.86 mL of the 160 mg/5 mL acetaminophen suspension) end up at one decimal place that is straightforward on the dosing syringe that ships in the box.
• Chews / chewable tablets: the raw tablet count is computed from the mg result and the per-tablet mg, then rounded to the nearest 0.5 tablet (half-tablet allowance for Junior Strength products), and finally capped against the formulation's max_tabs.
• Cap placement: the per-dose ceiling (max_single_dose_mg) is applied after rounding so the displayed number is the truncated, safe number when a heavy child approaches the ceiling — never a softened number that exceeds it.

The exact code path is in public/js/calculator.js: liquids go through Math.round(capped * 10) / 10, tablets go through a nearest-half-tablet helper (Math.round(n * 2) / 2).

5. Formulation-specific logic

Each formulation row in dosing_constants carries the four numbers that map the calculated mg onto the physical product in front of the parent:

• concentration_mg & concentration_ml — the labeled strength of the liquid suspension (e.g. 160 mg / 5 mL for children's acetaminophen; 100 mg / 5 mL for children's ibuprofen; 12.5 mg / 5 mL for children's Benadryl; 5 mg / 5 mL for children's Zyrtec and Claritin).
• form_multiplier — the mL/kg (or tabs/kg) factor that bakes in both the mg/kg dose and the concentration, so the calculator only does one multiplication per result.
• max_form_ml — the per-dose ceiling in mL for that specific bottle (e.g. 20.3 mL for acetaminophen liquid, 20 mL for children's ibuprofen liquid, 10 mL for infant ibuprofen drops, 20 mL for Benadryl, 10 mL for Zyrtec or Claritin).
• max_tabs — the per-dose ceiling in tablet count for each chewable form (4 tablets for acetaminophen 160 mg, 4 tablets for ibuprofen Junior Strength 100 mg).

In the medication picker on the calculator, these four fields drive the result the parent sees — so a clinically-trained reader can trace any given output back to a discrete, auditable row in dosing_constants.

6. Interval and 24-hour caps

Every dose has a interval_min_hours and interval_max_hours in dosing_constants — a floor and a ceiling on how many hours must pass before another dose is given. Stacked on top of that is max_doses_per_24h, the absolute ceiling on how many doses fit in one day for that drug. The current values:

• Acetaminophen: 4–6 hour interval, max 5 doses in 24 hours.
• Ibuprofen (infant and children's): 6–8 hour interval, max 4 doses in 24 hours.
• Diphenhydramine (Benadryl): 6–8 hour interval, max 4 doses in 24 hours.
• Cetirizine (Zyrtec): 24 hour interval (once daily), max 1 dose in 24 hours.
• Loratadine (Claritin): 24 hour interval (once daily), max 1 dose in 24 hours.

Perfect Dose is a reference, not a prescription. It surfaces the next-dose clock time and the running 24-hour count so a parent can pace dosing around those bounds, but it does not enforce a strict every-dose schedule. Clinical judgment — including the decision to hold a dose — always overrides the calculator. See the full medical disclaimer.

7. Physician attribution

The doses, mg/kg multipliers, and methodology on this page are reviewed by Kurt Hansen, MD, an Emergency Physician and the founder of Perfect Dose App. Perfect Dose is an educational reference and does not create a physician-patient relationship; the medical disclaimer governs use of the calculator.

For the parent-facing version of this material — plain-language answers to common dosing questions — see the Dosing FAQ.