Estimates an animal’s basal metabolic rate from its body mass using Kleiber’s law, a scaling relationship that holds across species from mice to elephants.
How it works
Body mass in kilograms is raised to the 3/4 power and multiplied by 70, the standard Kleiber’s law coefficient.
What this does not include
This does not include human-specific calorie calculators that factor in height, age, and sex — for those, see this site’s health-category calorie calculators, which use different, human-tailored formulas.
How to use this calculator
- Enter the animal’s body mass in kilograms.
A worked example
A 70 kg animal: estimated basal metabolic rate ≈ 1,694.03 kcal/day, using Kleiber’s law (mass raised to the ¾ power).
A 10 kg animal: BMR ≈ 393.64 kcal/day — far less than proportional to the 7× size difference, since metabolic rate scales sub-linearly with mass.
What the variables mean
| Variable | Meaning |
|---|---|
| Mass | Body mass, in kilograms |
Edge cases worth knowing
Metabolic rate doesn’t scale directly with mass. A 7× larger animal in the examples above has only about a 4.3× higher metabolic rate — Kleiber’s law’s ¾-power scaling means larger animals are relatively more energy-efficient per unit of body mass.
Zero mass makes the calculation meaningless, so the calculator declines to show a result for that input.
Frequently asked questions
Why does metabolic rate scale with mass to the 3/4 power instead of directly?
Because larger animals have proportionally less surface area relative to their volume, so their metabolism scales more slowly than their body mass does — a relationship Max Kleiber first quantified in 1932.
Does this work for any animal?
Kleiber’s law has been shown to hold remarkably well across a huge range of animal sizes, from mice to elephants, though it’s a general approximation rather than a species-specific measurement.
Is this the same as a human BMR calculator?
No — human-specific formulas like Mifflin-St Jeor also factor in height, age, and sex, which this general cross-species scaling law does not.
