About the Benadryl Dosage Calculator
Benadryl is a brand of diphenhydramine, a first-generation antihistamine. This tool estimates a single dose from body weight using the standard weight-based rule found in clinical dosing references: about 1.25 mg/kg per dose, which works out to roughly 5 mg/kg per day when a dose is given every 6 hours. It then converts that dose into the volume of children's liquid (labeled 12.5 mg per 5 mL) and shows the maximum for a 24-hour period.
The formula
Weight in pounds is converted to kilograms by dividing by 2.2046. The single dose is weight (kg) × dose per kg, capped at 50 mg. The liquid volume is dose (mg) ÷ concentration (mg/mL), where children's Benadryl is 2.5 mg/mL. The maximum daily amount is the per-dose figure times four (every-6-hour dosing), capped at 300 mg — the usual adult daily ceiling.
How to get accurate results
- Weigh the person rather than estimating; the dose scales directly with weight, so a rough guess carries straight into the result.
- Match the liquid strength field to your actual product. Standard children's Benadryl is 12.5 mg per 5 mL, but chewables and adult tablets are different strengths.
- Measure the millilitres with a proper dosing syringe or the cup supplied with the medicine, not a kitchen spoon.
Putting the result in context
The numbers here follow the standard population formula and do not account for age, other medicines, kidney or liver function, or medical history. OTC diphenhydramine is generally not recommended for children under 6 without a doctor's guidance, and not at all for children under 2.
When to consult a professional
Use this estimate as a starting point, not a prescription. Always confirm the dose against the product label and check with a pharmacist or doctor before giving Benadryl, especially for young children, during pregnancy, or alongside other sedating medicines.