Kleiber's Law Animal Metabolic Rate Calculator
Estimate an animal's basal metabolic rate from its body mass using Kleiber's law (rate ∝ mass^0.75) — in kilocalories per day, kilojoules per day and watts.
How it works
Kleiber’s law: BMR (kcal/day) = a · M^0.75, with M in kg and a ≈ 70 for mammals · Power (W) = kcal/day × 4184 ÷ 86400
Kleiber's law is the classic allometric scaling of metabolism: across mammals from mice to elephants, basal metabolic rate rises with body mass to the three-quarter power, not in direct proportion. So doubling mass raises energy needs by about 68%, and larger animals burn far less energy per kilogram than small ones. The coefficient near 70 kcal/day per kg^0.75 fits placental mammals; birds and other groups use different constants.
Worked example
For a 400 kg animal, 400^0.75 ≈ 89.4, so BMR ≈ 70 × 89.4 ≈ 6260 kcal/day. That is about 6260 × 4.184 ≈ 26,190 kJ/day, or roughly 303 watts of continuous power — only about 15.6 kcal/day per kg.
Sources: Kleiber's law — Wikipedia
Frequently asked questions
Why the 3/4 power instead of a simple ratio?
If metabolism scaled with surface area it would follow mass^0.67, and with volume mass^1.0. Empirically metabolic rate scales near mass^0.75 across many species, attributed to the geometry of resource-distribution networks like blood vessels.
Does Kleiber’s law apply to humans?
Roughly. A 70 kg human gives about 70 × 70^0.75 ≈ 1690 kcal/day, close to typical resting metabolic rates. Human-specific BMR equations (Mifflin-St Jeor, Katch-McArdle) are more accurate because they add height, age and body composition.
What coefficient should I use for birds?
Birds generally have higher mass-specific metabolism; passerine and non-passerine birds use coefficients above the mammalian ~70. Adjust the coefficient input to match the group you are modelling.
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Educational estimate, not medical or laboratory advice — verify formulas, units and assumptions for your specific organism or protocol.