Kleiber Metabolic Rate Calculator

Kleiber's law is a biological scaling relationship which states that the basal metabolic rate of an animal scales with the three-quarter power of its body mass. In equation form, metabolic rate is approximately proportional to mass raised to the power 0.75. Named after Max Kleiber, who described it in the 1930s, the law holds approximately across an enormous range of body sizes, from microbes to large mammals.

It means that as body mass increases, metabolic rate increases more slowly than mass itself. A doubling of mass increases the basal metabolic rate by a factor of 2 to the power 0.75, about 1.68, not by a factor of 2. As a result, larger animals are more energy-efficient per unit of body mass: an elephant uses far less energy per kilogram than a mouse.

The coefficient that multiplies mass to the three-quarter power depends on the units and the taxonomic group, so this calculator provides it as a user-editable input. A commonly cited value for mammalian basal metabolic rate is about 70 when mass is in kilograms and the result is in kilocalories per day. Because the exact value varies by source and species, enter the coefficient that matches your reference and units.

The classic Kleiber exponent is three-quarters (0.75), and it fits a vast range of data well. However, the precise best-fit exponent is debated and can differ slightly between taxonomic groups and studies, with values sometimes closer to two-thirds (0.67). This calculator lets you edit the exponent so you can apply the value appropriate to your data, while defaulting to the classic 0.75.