Cat & Dog Dosage Calculator

By Manuel Villanueva, 2003, Updated 10/3/20

Use numbers like 25 or 25.5
Use numbers like 25 or 25.5
For example "mg"

Body Surface Area Formula For Cats and Dogs

K m × BW 0.67 100

Where Km for cats = 10.4 and Km for dogs = 10.1. Body weight (BW) is measured in kilograms. Body surface area (BSA) is expressed in m2. Why would you want to calculate your cat or dog's body surface area BSA ? BSA can be used to calculate your pet's energy and fluid requirements as well as drug dosages.

Convert Human Dose into Cat or Dog Dose Formula

BSA × HD 1.73 m 2
  • BSA = Pet BSA
  • HD = Human dose

1.73 m2 is the average BSA of an adult human.


Using the formulas above, what would the BSA of a American cocker spaniel weighing 23 pounds (lb)? Next if humans take 25 milligrams (mg) of diphenhydramine (Benadryl) for itching how much would this same cocker spaniel require? First let's convert the dog's weight in kilograms (kg)

  • 23 lb ÷ 2.2 = 10.45 kg
  • Next Km = 10.1 because this is a dog. Now plug your numbers into the formula:
  • 10.1 × 10.450.67 ÷ 100 = 0.486693815653 m2

So the dog's BSA = .49 m2 (rounded). The calculator on this page displays the result in one hundredths (easier on the eyes) but actually uses this long number to calculate drug dosages. Next calculate the dose of medication using the second formula:

  • 0.486693815653m2 ÷ 1.73 m2 × 25 mg = 7.03 mg

Using this formula you would give this cocker spaniel 7.03 milligrams of diphenhydramine.

Using the formulas on this page, calculate the BSA of a 7 kilogram cat. If a person takes 650 milligrams of buffered aspirin how much would this cat take? Please note this is just an example. Buffered aspirin is not safe on cats. The cat's weight is already in kilograms and Km = 10.4 because this is a cat just plug the numbers in the formula:

  • 10.4 × 70.67 ÷ 100 = 0.383044319428 m2

So the cat's BSA = .38 m2. Next calculate the dose of medication using the second formula:

  • 0.383044319428 m2 ÷ 1.73 m2 × 650 mg = 143.92 mg

Using this formula this cat would take 143.9 milligrams of buffered aspirin.


For educational purposes only. Do not use for actual dosing. This is not the best way to calculate a drug dosage for your pet. Drug manufactures have specific dosages just for animals. Drugs that are safe for humans may not be safe for your cat or dog. For example, Tylenol should not be used on pets as it may damage their liver.


  • Hand, M. S., Thatcher, C. D., Rimillard, R. L., & Roudebush, P. (Eds.). (2000) Small Animal Clinical Nutrition. (4th ed.). Marceline, MO: Walsworth.
  • Gerald, M. C., & O'Bannon, F. V. (1988). Nursing pharmacology and therapeutics. (2nd ed.). Englewood Cliffs, NJ: Prentice Hall.