Get a science-backed calorie burn estimate using your actual workout heart rate data. The Keytel formula accounts for your age, sex, weight, and aerobic fitness — far more accurate than generic activity tables.
Use the average HR shown on your fitness tracker or heart rate monitor
VO2 max improves Keytel formula accuracy. If unknown, we estimate it from age and fitness level.
Used to estimate your VO2 max
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Traditional calorie calculators use MET (metabolic equivalent of task) values — standardized numbers assigned to activities like running, cycling, or swimming. MET tables are practical, but they ignore the single most important variable: how hard your cardiovascular system is actually working.
Two people labeled as “running at 6 mph” get the same MET value in a table. But if one is a trained marathon runner and the other a beginner, their heart rates — and calorie burns — will be dramatically different. Heart rate is a direct window into oxygen demand, and oxygen demand is directly linked to calorie expenditure.
Published in the Journal of Sports Sciences (2005), the Keytel formula was derived by measuring actual oxygen consumption (indirect calorimetry) in subjects while simultaneously recording heart rate. The resulting equations use four variables for improved accuracy:
Your workout's average heart rate determines which training zone you were in — each zone has different calorie burn rates and physiological benefits:
Gentle movement. Low calorie burn per minute but sustainable for hours. Used for active recovery and daily walks.
The foundation of endurance fitness. Burns a significant proportion of fat for fuel. Sustainable for 60–120+ minutes for most people.
Comfortably hard. Substantially higher calorie burn per minute. Ideal for 30–60 minute sustained efforts.
High effort. Near-maximum sustainable calorie burn. Used for interval training and race-pace efforts.
Maximum effort. Highest calorie burn per minute but sustainable only for seconds to a few minutes. Used in sprint intervals.
VO2 max is the maximum rate at which your body can consume oxygen, expressed in milliliters of O₂ per kilogram of body weight per minute (mL/kg/min). It is the best single measure of aerobic fitness and cardiovascular health.
| Category | Men (mL/kg/min) | Women (mL/kg/min) |
|---|---|---|
| Sedentary / Poor | < 30 | < 25 |
| Below Average | 30–38 | 25–33 |
| Average | 38–45 | 33–40 |
| Above Average | 45–52 | 40–47 |
| Excellent / Trained | 52–60 | 47–54 |
| Elite Athlete | 60–75+ | 54–70+ |
Many modern fitness wearables (Apple Watch, Garmin, Polar) estimate VO2 max automatically from outdoor runs. This estimate is typically accurate to within ±5 mL/kg/min for regular outdoor runners.
Heart rate-based formulas are significantly more accurate than MET tables for aerobic exercise, but there are important limitations to keep in mind:
Heart rate rises linearly with oxygen consumption during aerobic exercise. By measuring your average HR during a workout and combining it with your age, sex, weight, and VO2 max, the Keytel formula estimates oxygen consumption — and therefore calorie burn — at each moment of the workout.
The Keytel et al. formula estimates calories burned per minute using heart rate, VO2 max, weight, and age. It was validated against indirect calorimetry (lab-grade measurement) and is one of the most accurate equations available for wearable HR data.
Consumer fitness trackers typically have 20–40% error versus lab measurements. HR-based formulas with VO2 max are more accurate than accelerometer-only methods. Providing your actual VO2 max (from your watch or a lab test) improves this calculator's accuracy.
VO2 max is your maximum oxygen uptake in mL/kg/min — the standard measure of aerobic fitness. Two people with the same HR, age, and weight but different VO2 max burn different amounts of calories because the fitter person's heart is more efficient. Including VO2 max makes HR-based calorie estimates significantly more accurate.
Higher zones (3–5) burn more calories per minute. However, you can only sustain Zone 4–5 for short periods. Long Zone 2 workouts can burn equal or more total calories than short high-intensity sessions. For fat loss, total calorie deficit over time matters more than the intensity zone.
HR-based formulas are most accurate for steady-state aerobic exercise. They are less reliable for strength training, HIIT, and activities where HR is influenced by factors other than exercise intensity — such as heat, caffeine, emotional stress, or dehydration.