Animal Metabolic Rate (Kleiber) Calculator
Kleiber's law is one of the most striking regularities in biology: across animals from shrews to whales, the energy an organism burns at rest does not rise in step with its body mass but instead with that mass raised to the power three quarters. This calculator applies the classic form of the law, basal metabolic rate equals 70 times body mass in kilograms to the power 0.75, to estimate how many kilocalories a day an animal of a given size needs simply to stay alive. Enter a body mass and the tool returns an estimated resting metabolic rate, letting you compare how a 10 kilogram animal and a 100 kilogram animal differ not only in total energy use but in energy use per kilogram. The result is a population-level scaling estimate drawn from comparative physiology, useful for teaching, field ecology and quick sanity checks, rather than a measured value for any single creature. Because real metabolic rates vary with temperature, diet, activity and species, treat the number as an order of magnitude guide. Every figure here is computed deterministically from the scaling formula shown in full below, with a worked example that reconciles exactly to the calculator so you can follow each step.
Kleiber's law sets basal metabolic rate from body mass: BMR = 70 times M to the power 0.75, with mass in kilograms and the result in kilocalories per day. For a 50 kg animal the estimate is 1,316.21 kcal/day. Larger animals use more energy in total but less per kilogram.
Kleiber's law formula
BMR = a times M^0.75
BMR = basal metabolic rate (kcal per day)
a = coefficient (standard value 70)
M = body mass in kilograms
0.75 = the three-quarter scaling exponent
The exponent of 0.75 is what makes the law sub-linear: doubling an animal's mass less than doubles its resting energy use, so big animals are more energy efficient per kilogram than small ones.
Worked example
Estimate the resting metabolic rate of a 50 kg animal using the standard coefficient of 70.
- Raise the mass to the power 0.75: 50^0.75 = 18.8030
- Multiply by the coefficient: 70 x 18.8030 = 1,316.21
- Basal metabolic rate = 1,316.21 kcal per day
These are the calculator's default inputs, so the result above matches the widget exactly.
Estimated rate at common masses
Resting metabolic rate (kcal per day) at coefficient 70 for a range of body masses.
| Mass (kg) | BMR (kcal/day) |
|---|---|
| 1 | 70.00 |
| 10 | 393.64 |
| 50 | 1,316.21 |
| 100 | 2,213.59 |
| 500 | 7,401.60 |
Scaling reference: US Geological Survey (USGS).
Kleiber metabolic rate calculator: frequently asked questions
What is Kleiber's law?
Kleiber's law is the observation that the basal metabolic rate of an animal scales with its body mass raised to the power three quarters (0.75), rather than in direct proportion to mass. It means a larger animal burns more energy in total than a smaller one, but less energy per unit of body mass. The relationship holds remarkably well across a huge range of species, from mice to elephants.
What units does this calculator use?
Body mass is entered in kilograms and the estimated basal metabolic rate is returned in kilocalories per day, using the common coefficient of 70. This is a population-level scaling estimate, not a measured value for any one animal, so treat it as an order-of-magnitude guide rather than a precise figure.
Why is the exponent 0.75 and not 1?
If metabolism scaled in direct proportion to mass the exponent would be 1. Empirically it is close to three quarters. The leading explanation is that the rate at which an organism can deliver oxygen and nutrients through its branching circulatory network scales sub-linearly with body size, which constrains how fast tissue can metabolise.
Is this the same as the metabolic rate for humans?
The 0.75 scaling describes how metabolic rate changes across species of different sizes. Human basal metabolic rate is usually estimated with a dedicated equation that accounts for sex, age and height as well as weight. Use a human BMR calculator for people; this tool is a cross-species scaling estimate.
What is the Kleiber formula?
Basal metabolic rate (kcal per day) equals 70 multiplied by body mass in kilograms raised to the power 0.75. Written out: BMR = 70 times M to the power 0.75. For a 50 kg animal that gives about 1,316.21 kcal per day.
Official sources
- Comparative animal physiology and ecology reference: US Geological Survey (USGS). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.