Animal Basal Metabolic Rate Calculator (Kleiber's Law)
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
Kleiber's law estimates an animal's basal metabolic rate with BMR ≈ 70 × mass^0.75 (mass in kg, result in kcal/day). A 30 kg animal thus has an estimated basal metabolic rate of about 897 kcal/day.
Explanation
In 1932, biologist Max Kleiber observed that when comparing animals of very different sizes (from a mouse to an elephant), their basal metabolic rate (the energy spent at rest for vital functions) follows a remarkably stable power law: it's proportional to body mass raised to the power of 3/4, not to the power of 1 as one might naively expect. In practice, this means an animal twice as heavy as another doesn't spend twice as much energy at rest, but about 1.68 times more (2^0.75): the larger an animal is, the more efficient its metabolism relative to its weight. This relationship, known as Kleiber's law or the metabolic allometry law, has proven remarkably robust across mammals and birds over considerable ranges of body mass, although its exact mechanistic basis (notably linked to distribution networks like the vascular system) remains debated among researchers. This formula gives a general estimate across animal species; for an individualized human calorie needs calculation (accounting for age, sex, and activity level), see our calorie needs calculator, based on a formula specifically established and validated for humans.
Example: a 30 kg animal
Inputs
Body mass: 30 kg.
Calculation
Basal metabolic rate = 70 × 30^0.75 ≈ 70 × 12.819 ≈ 897.3 kcal/day.
Result
This animal's estimated basal metabolic rate is about 897 kcal per day.
Frequently asked questions
Why is the exponent 0.75 and not 1?
If metabolism were simply proportional to mass (exponent 1), an elephant would consume, relative to its weight, as much energy as a mouse — which isn't observed in reality. The 3/4 exponent reflects the fact that larger animals are metabolically more "economical" per kilogram than smaller ones, a phenomenon partly linked to how heat dissipates through body surface area and how nutrients are distributed through vascular networks.
Is this formula accurate for a specific animal?
It's a general law established from averages observed across many species, not an individualized measurement for a specific species. An animal's actual metabolism also depends on factors specific to the species, age, ambient temperature, and activity level, which can noticeably shift it from this general estimate.
Does this law also apply to humans?
It broadly applies to mammals, humans included, but formulas specifically established and validated for humans (like Mifflin-St Jeor, used by our calorie needs calculator) give a more accurate estimate for a person, since they incorporate human-specific factors (height, age, sex) rather than a single general law across species.