Why your fitness tracker's calorie burn is mostly fiction

If you wear a fitness tracker and trust the calorie burn number, you're being slightly misled.

The active-energy number on your wrist is, on average, 20–40% inaccurate. Sometimes more. Here's why, and what to do.

What trackers measure (and don't)

A wrist-worn tracker has heart rate (sometimes), accelerometers (always), GPS (sometimes), and skin temperature (rarely). From these, it estimates:

• Steps (reasonably accurate)
• Heart rate (reasonably accurate during light activity, less so during weights)
• Active calories (extrapolated from heart rate + movement, often via proprietary algorithms)

The active calorie estimate is the output of a model, not a measurement. The model has assumptions baked in.

Where the error comes from

1. Resting metabolic rate (RMR) extrapolation. Trackers estimate your RMR from age, sex, height, weight. As we covered in the TDEE post, this estimate can be off by ±15%.

2. Activity-type confusion. The accelerometer doesn't know if your wrist is moving because you're running, or because you're typing fast, or because you're stirring a pot. Heart rate helps disambiguate, but imperfectly.

3. Strength training is hard. Your heart rate during a heavy deadlift looks identical to a 30-second sprint to the algorithm. The actual energy expenditure is very different.

4. Cooking, cleaning, walking around the house. All chronically under-counted.

5. NEAT (fidgeting, posture). Essentially invisible to wrist trackers.

How to actually use the data

Two principles:

1. Trust trends within the same activity. If your "Apple Watch run calories" is consistently 600 for a 5k, the absolute number might be wrong but the trend (today's run was harder/easier than usual) is informative.

2. Don't trust the absolute number. Particularly: do not "earn" food based on the active calorie reading. The classic mistake is to do a 400-calorie workout and eat back 400 calories. The workout was probably 250–300 actual calories, and you're now in surplus.

A safer rule: if you're training and trying to lose weight, eat back zero exercise calories. Set your target based on your sedentary TDEE. Treat any movement as a buffer, not a budget.

The watch number that is useful

Resting heart rate is the single most useful health number that consumer wearables produce. Trends in RHR correlate well with cardiovascular fitness and recovery state. A consistently rising RHR is one of the earliest warnings of overtraining or illness.

Sleep estimates are better-than-nothing but not great. The "sleep stages" calls are noisy. The total sleep duration estimate is reasonably trustworthy.

What our app does with wearable data

We sync with Apple Health and Whoop. We import workouts (for context) and do not automatically increase your daily calorie target based on the workout. Users can manually toggle "eat back exercise calories" if they really want to. We hide the toggle by default because most users get better results without it.

The watch is great for noticing patterns. It is bad for granting you snacks.