What the number is built from
This uses the Keytel (2005) equation, validated against oxygen-uptake testing. It reads your average heart rate during the session, adjusts for weight, age and sex, and gives calories per minute. Multiply by minutes for the session total.
- Men. kcal/min = (−55.10 + 0.6309 × HR + 0.1988 × kg + 0.2017 × age) ÷ 4.184
- Women. kcal/min = (−20.40 + 0.4472 × HR − 0.1263 × kg + 0.074 × age) ÷ 4.184
HR is average heart rate in beats per minute. The equation is most reliable for steady exercise between roughly 40 and 80% of your capacity.
Heart rate drives the total more than anything else
Weight and age nudge the result, but heart rate is the main lever — push harder and the per-minute burn climbs steeply. As a reference, here is roughly what a 30-minute session burns for an adult around 70 kg.
| Avg heart rate | Effort | ~kcal in 30 min |
|---|---|---|
| 110 bpm | Light | ~180–210 |
| 130 bpm | Moderate | ~250–290 |
| 150 bpm | Hard | ~320–370 |
| 170 bpm | Very hard | ~400–450 |
Bands are approximate and vary with weight, age and sex — use the calculator for your own inputs.
The "220 minus age" max is only a rough anchor
- It can be off by a lot. Max heart rate = 220 − age carries a spread of about ±10–12 bpm and can miss by up to 40 bpm in older adults. The formula 211 − 0.64 × age fits real data better.
- Zones are shares of that max. Zone 2–3 (60–80%) is the endurance range; Zone 4–5 (80–100%) is short, hard interval work.
- The afterburn is not counted. EPOC adds roughly 5–15% more over the following day, especially after hard sessions, and sits on top of the figure here.
Common questions
How does heart rate estimate calories burned?
Heart rate tracks how hard your heart is working, which correlates closely with oxygen use and therefore energy spent. The Keytel equation turns your average heart rate, weight, age and sex into calories per minute, then multiplies by how long you exercised.
Why do men and women get different results?
The formula uses separate coefficients for each sex because, at the same heart rate, differences in muscle mass and body composition change how much energy is actually burned. That is why the calculator asks for sex.
Why does my fitness tracker show a different number?
Trackers use their own mix of heart rate, motion sensors and generic activity tables, so a 10 to 15% gap is normal. None of these are exact — treat any estimate as a ballpark, not a measurement.
