Kleiber Metabolic Rate Calculator
The basal metabolic rate of an animal from its body mass by Kleiber's law — the three-quarter power scaling that holds from a mouse to an elephant — with the daily energy need and the rate per kilogram.
Metabolic rate scales with mass to the power 0.
How the kleiber metabolic rate calculator works
Metabolic rate scales with mass to the power 0.75, not 1: an animal ten times heavier burns only about 5.6 times the energy. Kleiber found this in 1932 and it holds across six orders of magnitude of body mass.
The consequence is that per kilogram, small animals burn far more — a mouse eats its body weight in days, an elephant in months. That is why small animals live fast and die young.
Formula: BMR (W) = 3.4 × M^0.75 (M in kg)
Worked examples
| Inputs | Basal metabolic rate (W) | Note |
|---|---|---|
| A 70 kg human | 82.282 | about 82 W, 1,700 kcal |
| A 20 g mouse | 0.181 | far more per kilogram |
| A 5 tonne elephant | 2,021.652 | far less per kilogram |
FAQFrequently asked questions
What is Kleiber's law?
Basal metabolic rate scales with body mass to the power 0.75. It was found empirically in 1932 and holds from bacteria to whales.
Why three quarters and not one?
Still debated. The leading explanation is the geometry of the branching networks — blood vessels, airways — that supply every cell.
Why do small animals eat so much?
Because per kilogram they burn far more. A shrew must eat almost continuously; an elephant can fast for days.
What is field metabolic rate?
Energy use in a free-living animal, including activity, digestion and thermoregulation — typically two to three times basal.
Does this apply to humans?
Roughly — 70 kg gives about 1,700 kcal a day, close to measured BMR. Species-specific equations like Mifflin–St Jeor are more accurate for people.
Where these figures come from
- NCBI Bookshelf — Molecular Biology of the Cell — growth kinetics and molecular conventions
- Wellcome Sanger Institute — a leading UK genomics institute
Last checked: September 2026. Formulas are the standard textbook forms; assumptions are stated on each page because they are where these models break.