What this calculator does
Heart rate is a reasonable proxy for exercise intensity, and the Keytel equations convert it into an energy estimate using heart rate, weight, age and sex. Forty-five minutes at 140 bpm for a 30-year-old man of 75 kg estimates 583 kcal.
The equations were fitted to measured oxygen consumption during steady aerobic exercise, and that is where they work. They are considerably less reliable for interval work, for resistance training, and whenever heart rate is elevated by heat, caffeine, dehydration or stress rather than by effort.
The formula
Separate regression equations for men and women combine heart rate, weight and age into a kilojoule rate, which is multiplied by the duration and divided by 4.184 to give kilocalories. The equations come from Keytel and colleagues, published in 2005, and were fitted against measured oxygen uptake in steady-state aerobic exercise.
| Term | Meaning |
|---|---|
| Keytel equation | The 2005 regression relating heart rate to energy expenditure. |
| kcal | Kilocalorie, what is meant by a food Calorie. |
| Steady state | Sustained aerobic exercise at a stable intensity, which is what the equations were fitted to. |
| Cardiac drift | The gradual rise in heart rate during prolonged exercise at constant effort, which inflates the estimate. |
The inputs explained
| Field | What to enter |
|---|---|
| Sex | Sex, which selects the equation. |
| Age (years) | Age in years. |
| Weight (kg) | Body weight in kilograms. |
| Average heart rate during exercise (bpm) | Average heart rate over the session in beats per minute. |
| Duration (minutes) | Session duration in minutes. |
When to use it
Estimating energy use for a session
A heart rate monitor gives the average, and this converts it to an energy figure without needing lab equipment.
Comparing two sessions
The relative comparison is more reliable than the absolute number, since the same systematic error applies to both.
Sanity-checking a device estimate
Fitness trackers use similar regressions, and an independent calculation shows whether a device figure is in a plausible range.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does heart rate change the estimate?
A fixed session length across a range of average heart rates.
| Average heart rate | Calories burned | Per minute | Formula used |
|---|---|---|---|
| 120 bpm | 447 kcal | 9.9 kcal | Keytel et al. (2005) heart-rate regression equation |
| 140 bpm | 583 kcal | 13.0 kcal | Keytel et al. (2005) heart-rate regression equation |
| 160 bpm | 719 kcal | 16.0 kcal | Keytel et al. (2005) heart-rate regression equation |
| 180 bpm | 854 kcal | 19.0 kcal | Keytel et al. (2005) heart-rate regression equation |
Questions
How accurate is this estimate?
Reasonable for steady aerobic exercise, within perhaps 10 to 20% for most people. It is considerably worse for intervals, resistance training and anything where heart rate is driven by something other than aerobic effort. Treat it as a comparison tool rather than a measurement.
Why does it need age and sex?
Because both affect the relationship between heart rate and oxygen consumption. The equations were fitted separately by sex and include an age term, which reflects how maximum heart rate and cardiac output change over a lifetime.
Does it include resting metabolism?
It estimates total energy expenditure during the session, which includes the resting baseline you would have burned anyway. For the additional cost of exercising, subtract roughly 1 kcal per kilogram per hour of resting metabolism.
Why is my watch figure different?
Different devices use different equations, and many incorporate additional data such as VO₂max estimates, movement patterns or GPS speed. None of them measures energy directly. Disagreement between estimates is expected and does not mean one is right.
For a VO₂max estimate, see the Bruce protocol calculator. For weekly activity against guidelines, see the MET-minutes calculator.