Metabolic Rate Scaling Calculator (Kleiber’s Law)

How it works

BMR = a · mass^b · Kleiber: a ≈ 70, b = 0.75 (kcal/day, mass in kg) · Mass-specific rate = BMR ÷ mass ∝ mass^(b−1)

Kleiber's law is one of biology's most robust patterns: across animals spanning many orders of magnitude in size, basal metabolic rate scales not with mass itself but with mass raised to about the ¾ power. Doubling body mass therefore raises energy demand by only about 2^0.75 ≈ 1.68 times, not 2. The consequence is that the mass-specific rate — energy burned per kilogram — falls as mass^(−¼), so a mouse's tissues run far hotter metabolically than an elephant's. You can adjust the coefficient and exponent to fit other taxa or to explore the ⅔ surface-area hypothesis.

Worked example

For a 70 kg mammal with Kleiber constants (a = 70, b = 0.75): BMR = 70 × 70^0.75 = 70 × 24.2 = 1,695 kcal/day ≈ 82 W. Per kilogram that is 1,695 ÷ 70 ≈ 24 kcal/kg/day — far lower than a 0.02 kg mouse, whose per-kg rate is many times higher.

Sources: Kleiber's law — Wikipedia

Frequently asked questions