How the Calculation Works
The calculator uses an equation from a 2005 study by Keytel and colleagues (Keytel et al., 2005), who measured how much oxygen 115 regularly exercising adults used on a treadmill or exercise bike and related it to their heart rate, weight, age, and sex. Calories per minute are:
- Men: (−55.0969 + 0.6309 × heart rate + 0.1988 × weight in kg + 0.2017 × age) ÷ 4.184
- Women: (−20.4022 + 0.4472 × heart rate − 0.1263 × weight in kg + 0.074 × age) ÷ 4.184
Multiply by the length of your workout in minutes to get the total. The result is gross calories: it includes the energy you'd burn at rest during the same time.
Calories per Hour by Heart Rate
For a 40-year-old woman weighing 65 kg (143 lb) and a 40-year-old man weighing 80 kg (176 lb), exercising for one hour:
| Average heart rate | Woman, 65 kg | Man, 80 kg |
|---|---|---|
| 110 bpm | 338 kcal | 549 kcal |
| 120 bpm | 402 kcal | 639 kcal |
| 130 bpm | 466 kcal | 730 kcal |
| 140 bpm | 530 kcal | 820 kcal |
| 150 bpm | 594 kcal | 911 kcal |
| 160 bpm | 658 kcal | 1,001 kcal |
At 40, an estimated max heart rate is 180 bpm, so the table runs from about 61% of max (110 bpm) to 89% (160 bpm), inside the range the study tested.
When the Estimate Works Best
The study behind the formula tested steady exercise at about 57% to 90% of max heart rate, in adults aged 18 to 45. The calculator shows where your heart rate falls and flags efforts outside that range. A few things make any heart rate estimate less accurate:
- Very easy efforts, like slow walking, where heart rate is close to resting
- Strength training and HIIT, where heart rate rises and falls quickly and lags behind the work
- Heat, dehydration, stress, or illness, which raise heart rate without raising energy use
- Wrist readings during intense intervals. A chest strap gives more accurate heart rate
For a different approach that doesn't need heart rate, see our guide to METs and MET minutes.
Using Calories for Weight Goals
Exercise is only part of the calories you burn in a day. Most come from your body's baseline needs and everyday movement. A common starting point for losing about 1 lb (0.5 kg) a week is a 500-calorie daily deficit from diet and activity combined, though real-world loss usually slows over time. Our guide to calories, BMR, and TDEE explains how the pieces fit together, and FITIV's weight management tools help you track the trend.
Frequently Asked Questions
How do you calculate calories burned from heart rate?
A widely used method is the Keytel equation, from a 2005 study that measured oxygen use during exercise. It estimates calories per minute from your average heart rate, weight, age, and sex, with separate equations for men and women. Multiply by your workout's length to get the total.
How accurate is a heart rate calorie estimate?
It's a reasonable estimate, not an exact measurement. It works best for steady cardio between roughly 57% and 90% of your max heart rate, the range the study tested. It's less reliable for very easy efforts, strength training, and short sprints, where heart rate and energy use don't track as closely.
Why do men burn more calories at the same heart rate?
Men tend to be larger and carry more muscle, so they use more energy at a given heart rate. That's why the Keytel formula uses separate equations for men and women.
Is this total or active calories?
Total. The estimate includes the calories your body would burn at rest during the same time. Active calories, like Apple's Active Energy, leave out that resting portion, so they're lower.
Why does my watch show a different number?
Watches and apps use different methods. Apple Watch, for example, combines heart rate with motion and other sensors in its own algorithm, and its Move ring counts active calories, not total. Different methods can easily differ by 10–20% or more for the same workout.
