My Cocker Spaniel

Cat & Dog BSA & Dosage Calculator

By Manuel Villanueva | Published 2003 | Updated January 4, 2024

For example "mg"

Body Surface Area Formula For Cats and Dogs

K m × BW 100 = BSA m 2

The K constant, also known as Meeh's constant, represents the factor used to estimate the relative surface area of animals. It varies greatly among species and due to size differences. Many animals have a K value around 10, with specific values for cats and dogs being 10.4 and 10.1 respectively. Body weight (BW) is measured in kilograms, and body surface area (BSA) is expressed in square meters. The body weight exponent may be written as 0.67 facilitating calculation but ⅔ is more accurate. Calculating your cat or dog's BSA is important because it helps determine their energy and fluid requirements as well as appropriate drug dosages.

Convert Human Dose into Cat or Dog Dose Formula

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

1.73 m2 is the average BSA of an adult human.


1. 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? Round your answers to 3 decimal places. First let's convert the dog's weight in kilograms (kg). See footnote about this conversion.

  • Convert pounds to kilograms and get the value of K.
    • 1 kilogram ≈ 2.205 pounds
    • 23 ÷ 2.205 ≈ 10.431
    • K = 10.1 because this is a dog.
  • Now plug your numbers into the body surface area formula:
BSA = K m × BW 100
= 10.1 m × 10.431 100
  • To calculate 10.431 use the xy exponent calculator key. Enter 10.431, xy key, ( key, 2 ÷ 3, ) key.
= 10.1 m × 4.774 100
= 48.217 100
  • Next calculate the dose of medication using the dosage formula:
D = BSA × HD 1.73   m 2
= 0.482 × 25 1.73   m 2
= 12.050 1.73   m 2

This cocker spaniel's body surface area is approximately 0.482 m2 and the dosage is approximately 6.965 milligrams.

2. Calculate the BSA of a 7 kilogram cat. If a person takes 650 milligrams of buffered aspirin how much would this cat take? Round your answer to 3 decimal places. Please note this is just an example. Buffered aspirin is not safe on cats. The cat's weight is already in kilograms and the K value for cats is 10.4.

  • Now plug your numbers into the body surface area formula:
BSA = K m × BW 100
= 10.4 m × 7 100
  • To calculate 7 use the xy exponent calculator key. Enter 7, xy key, ( key, 2 ÷ 3, ) key.
= 10.4 m × 3.659 100
= 38.054 100
  • Next calculate the dose of medication using the dosage formula:
D = BSA × HD 1.73   m 2
= 0.381 × 650 1.73   m 2
= 247.650 1.73   m 2

This cat's body surface area is approximately 0.381 m2 and the dosage is approximately 143.150 mg.


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.

Drug Overdose & Highly Toxic Medications

Potent medications require careful dosing to avoid the risk of overdose. A drug's ratio of the toxic dose to the effective dose is called its therapeutic index. For example, if a medication has toxic effects at 50 milligrams and the desired effect at 5 milligrams, it would have a therapeutic index of 10 (a ratio of 10:1). Medications with lower therapeutic index values are more toxic. For example, the heart medication digoxin is extremely potent with a therapeutic index of 1.5-2 increasing the likelihood of accidental overdose. On the other hand, penicillin is relatively safe with a therapeutic index greater than 100

The following is a top 10 list of human medications involved in pet fatalities. These medications have a low therapeutic index:

  1. Fluorouracil
  2. Phenylbutazone
  3. Metronidazole
  4. Amlodipine
  5. Methimazole
  6. Hydroxyurea
  7. Minoxidil
  8. Mirtazapine
  9. Calcitriol
  10. Meloxicam


  • Hand, M. S., Thatcher, C. D., Rimillard, R. L., & Roudebush, P. (Eds.). (2000) Small Animal Clinical Nutrition. (4th ed.). Marceline, MO: Walsworth.
  • Gouma, E., Simos, Y., Verginadis, I., Lykoudis, E., Evangelou, A., & Karkabounas, S. (2012). A simple procedure for estimation of total body surface area and determination of a new value of Meeh's constant in rats. Laboratory animals, 46(1), 40-45.
  • Hill, R. C., & Scott, K. C. (2004). Energy requirements and body surface area of cats and dogs. Journal of the American Veterinary Medical Association, 225(5), 689-694.
  • Saganuwan, S. A. (2017). Derivation of a unique body surface area (bsa) formula for calculation of relatively safe doses of dog and human anticancer drugs. J Cancer Sci Ther, 9(10), 690-704.
  • Gerald, M. C., & Lemmi, F. O. (1988). Nursing Pharmacology and therapeutics. Appleton & Lange.
  • Swirski, A. L., Pearl, D. L., Berke, O., & O'Sullivan, T. L. (2020). Companion animal exposures to potentially poisonous substances reported to a national poison control center in the United States in 2005 through 2014. Journal of the American Veterinary Medical Association, 257(5), 517-530.
  • ROWING, J. E. (2000). APPENDIX Mathematics, Symbols, and Physical Constants.


Most clinicians use a simple formula to convert pounds to kilograms: pounds divided by 2.2 equals kilograms. While 2.2 is easy to remember and work with, it's not entirely precise. In reality, one kilogram is approximately 2.204622622 pounds, which is the value used in this calculator. Keep this in mind when comparing the results if you used 2.2 in your calculations.