What this calculator does
The Cat Benadryl Dosage Calculator estimates how much diphenhydramine (the active ingredient in plain Benadryl) a cat may receive based on its body weight. Veterinarians commonly use a guideline of about 1 mg of diphenhydramine per kilogram of body weight. This tool converts your cat's weight into an estimated dose in milligrams, and also shows the equivalent volume of children's liquid Benadryl and the fraction of a 25 mg tablet.
How to use it
Enter your cat's weight and choose pounds or kilograms. The default dose is 1 mg/kg, which is a typical starting point, but you can adjust it within the safe range if your vet recommends a different amount. The calculator instantly displays the total dose in milligrams, the liquid volume (assuming a 2.5 mg/mL children's syrup), and how many standard 25 mg tablets that equals.
The formula explained
The core equation is $$\text{Dose (mg)} = \text{Weight (kg)} \times \text{dose per kg}$$ Pounds are converted to kilograms first (\(1\,\text{lb} = 0.45359237\,\text{kg}\)). Liquid volume is the dose divided by the concentration: most children's Benadryl liquid is 12.5 mg per 5 mL, i.e. \(2.5\,\text{mg/mL}\).
Worked example
A cat weighs 10 lb. Converting: \(10 \times 0.45359237 = 4.54\,\text{kg}\). At 1 mg/kg the dose is about 4.54 mg. In children's liquid that is \(4.54 \div 2.5 \approx 1.81\,\text{mL}\), or roughly 0.18 of a 25 mg tablet.
FAQ
Is Benadryl safe for cats? Diphenhydramine can be used in cats under veterinary guidance, but cats are sensitive to many medications, so never give it without first consulting your vet.
Which Benadryl product should I use? Only plain diphenhydramine — avoid combination products containing decongestants, acetaminophen, or alcohol, which can be toxic to cats.
How often can it be given? Dosing frequency (often every 8–12 hours) and total daily limits should be confirmed by your veterinarian based on your cat's health.
