How Your Heart Rate Becomes a Calorie Number
Your watch turns heart beats into calories using a chain of guesses: heart rate to oxygen to calories. It is an indirect estimate, not a real measurement, which is why Burnie does not use heart rate at all.
Your fitness watch shows a calorie number after a workout. It looks like a fact. But your watch cannot actually count calories. It only counts heart beats. Then it turns those beats into calories using a chain of maths and guesses. This article explains that chain in plain words, so you can see why the number is only an estimate.
The chain: heart rate to oxygen to calories
Your body burns calories by using oxygen. More oxygen in means more calories burned. When you exercise, your heart beats faster to push more oxygen to your muscles. So heart rate and oxygen use go up together. Scientists found this is a fairly straight-line link when you exercise at a steady pace. The heart rate method is based on the linear relationship between HR and oxygen uptake with increased work rates under steady state conditions. So the watch measures heart beats, then uses that straight-line link to guess how much oxygen you used, then turns oxygen into calories.
The maths your watch quietly does
Exercise scientists wrote formulas that turn speed and body weight into oxygen use. These are the ACSM metabolic equations. For walking, the formula is VO2 = 3.5 + (0.1 x speed) + (1.8 x speed x grade). For running it is VO2 = 3.5 + (0.2 x speed) + (0.9 x speed x grade). The 3.5 part is your resting oxygen, because resting is conventionally treated as 3.5 mL per kg per minute. Then oxygen becomes calories, because calories = absolute VO2 x about 5 kcal per litre of oxygen. So a watch hides three maths steps inside one number: beats to oxygen, oxygen to calories.
A heart-rate calorie number is a real measurement of the calories I burned.
mostly falseMostly false. Heart rate is measured directly, but calories are only guessed indirectly through proxy calculations. A Stanford study of seven wrist-worn devices found the heart rate part was good, but the calorie part was poor. Six of the devices achieved a median error for HR below 5 percent during cycling, which is quite accurate. But for energy expenditure, no device achieved an error in EE below 20 percent. So your beats are measured, while your calories are an indirect proxy estimate that can easily be off by a quarter or more.
Measured directly vs guessed indirectly
Burnie should read my heart rate to give me better calorie numbers.
mostly falseMostly false, and Burnie does not do this. Heart-rate calorie numbers are an indirect guess that can be off by more than 20 percent even on good devices. Burnie never reads heart rate, sleep, blood pressure, or location, by design. Burnie uses your phone or Android Health Connect (Samsung Health) active calories, or a manual override with PUT /api/summary/daily/active-calories. The honest science says heart rate does not make calorie numbers exact, so Burnie does not pretend it does. Burnie is free, with no paid tier.
Smart ways to use the heart-rate calorie number
The bottom line
Your watch turns heart beats into calories through a chain: heart rate to oxygen, oxygen to calories. Heart rate is measured, but calories are an indirect guess that can be off by 20 percent or more. That is exactly why Burnie does not read heart rate. Burnie uses your phone or Android Health Connect (Samsung Health) active calories, or a manual override at PUT /api/summary/daily/active-calories, and Burnie is free with no paid tier. Put your real effort into logging food before you eat, the side you truly control.