Allometric Heart Rate Calculator
Predict a mammal's or bird's resting heart rate from body mass with published allometric equations (a × M^b), plus beat interval, beats per day and year, breathing rate, heart mass, ECG intervals and the near-constant billion-beat lifetime.
How it works
Heart rate (bpm) = a × M^b, M in kg · Mammals: a = 241, b = −0.25 (Stahl/Brody) or a = 221.7, b = −0.22 (Storlund 2021) · Birds: a = 155.8, b = −0.282 (Calder) · Breathing rate ≈ 53.5 × M^−0.26 · Heart mass ≈ 5.88 g × M^0.983 · ECG: PR ≈ 0.066 × M^0.21 s, QRS ≈ 0.029 × M^0.19 s (marine 0.039), QT ≈ 0.121 × M^0.23 s (marine 0.100) · Lifespan at 10⁹ beats = 10⁹ ÷ (bpm × 525,960)
Resting heart rate scales allometrically with body mass to roughly the −¼ power: bigger animals have slower hearts. A mouse's heart races at several hundred beats per minute while an elephant's beats around 30. This mirrors the mass^¾ scaling of metabolic rate — heart rate tracks the metabolic rate per kilogram. Pick the classic mammal equation (241 × M^−0.25, from Brody and Stahl, which you can edit), the newer fit by Storlund and colleagues from ECGs of 73 mammal species (221.7 × M^−0.22), or Calder's bird equation. For mammals the calculator also gives the predicted breathing rate (about one breath per 4–5 heartbeats at any size), heart mass (close to 0.6% of body mass) and typical ECG intervals, which lengthen with size and differ between land and marine mammals. Because lifespan rises roughly as mass^+¼ while heart rate falls, the total number of heartbeats in a lifetime is about the same across mammals, on the order of a billion; you can enter an actual lifespan to test it, or a measured heart rate to see how far a species sits from the trend.
Worked example
For a 70 kg human with a = 241 and b = −0.25: heart rate = 241 × 70^(−0.25) = 241 × 0.3458 ≈ 83.3 bpm, a beat every 720 ms. That is about 120,000 beats a day and 43.8 million a year, implying a lifespan near 1,000,000,000 ÷ 43.8 million ≈ 22.8 years of pure resting beats — real humans live far longer than the trend. The same body would breathe about 53.5 × 70^(−0.26) ≈ 18 times a minute and carry a heart of about 5.88 × 70^0.983 ≈ 380 g. The Storlund ECG equation gives 221.7 × 70^(−0.22) ≈ 87 bpm.
Sources: Heart rate — Wikipedia · Storlund, Rosen & Trites (2021), Frontiers in Physiology 12:690029 — allometric heart rate and ECG intervals · George Mason University — Size and animal biology (allometric equations table)
Frequently asked questions
Why do small animals have faster heartbeats?
Heart rate scales with the metabolic rate per kilogram, which rises as animals get smaller (mass^−¼). Small bodies lose heat quickly and must burn energy fast, so their hearts pump much more rapidly to supply oxygen.
Is it true every mammal gets about a billion heartbeats?
Roughly. Heart rate falls as mass^−¼ while lifespan rises as mass^+¼, so the product — total lifetime beats — is approximately constant across mammals, on the order of 10⁹. Humans are an outlier: at 70 beats a minute for 80 years a person has about 3 billion.
Which equation should I use?
The classic 241 × M^−0.25 is the textbook mammal law. The 2021 fit (221.7 × M^−0.22) comes from ECG recordings of 73 species and predicts slightly faster rates for large animals. Use the bird equation for birds, which have bigger hearts and slower resting rates than mammals of the same size.
Does this predict a specific animal exactly?
No. It is a population-level scaling law with real scatter (often ±25%), and it applies to resting rates. Enter a measured rate under More options to see how far a species sits from the trend, and expect activity, stress and temperature to raise the actual rate.
Why does the breathing rate scale the same way?
Breathing supplies the oxygen the heart delivers, so it follows the same −¼ power. The ratio stays near 4–5 heartbeats per breath from mice to elephants, which is a useful sanity check for a measured animal.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.