Why your watch says you burned 600 calories and you did not

You finish a 45 minute run and the watch says 600 calories. That feels like permission. A large muffin is about 420, so the muffin is basically free, and you have earned it.

The number is wrong in two specific ways, and both of them push in the same direction. Not because your watch is broken, but because of what it is counting and what it cannot know.

Error one: it counts the calories you would have burned anyway

Sitting on the sofa for 45 minutes burns calories. Your heart runs, your brain runs, you stay warm. For a 72 kilogram adult that background burn is roughly 55 calories over that time.

Most trackers report the gross total, which includes that background burn. So the extra you actually spent by running rather than sitting is around 55 calories less than the headline. It sounds small. It is not the big error. It is the one that stacks on top of the big error.

Error two: the model does not know your body

Underneath almost every tracker is the same arithmetic: a MET value for the activity, multiplied by your body weight, multiplied by time. MET values come from a published compendium built from population averages.

Averages are the problem. A trained runner is more economical than an untrained one, which means they burn less covering the same distance at the same pace. Fitness is efficiency, and efficiency means spending less. So the fitter you get, the more your watch overstates you.

How big is the gap in practice

Independent testing has repeatedly found consumer devices overstating energy expenditure by roughly 20 to 30 percent, with heart rate measured far more accurately than calories. Heart rate is a measurement. Calories are a model wrapped around that measurement, and the model is where the error lives.

The 600 calorie run is closer to 410 of genuinely extra energy. The muffin was not free. It was most of the run.

What to do instead

The fix is not a better tracker. It is using the number for a different job.

1. Stop eating back exercise calories, or eat back at most half. This single change resolves most stalled deficits.
2. Use the burn figure to compare sessions against each other, not to compare against food. It is reasonably good at telling you today was harder than Tuesday.
3. Trust the scale trend over three weeks more than any daily estimate. Your own weight data beats any model within a month.
4. Judge training by whether the session happened, which is a fact, rather than by what it burned, which is a guess.

The honest limit of all of this

A MET-based estimate is not more accurate than your watch. It is the same class of model with the same population averages behind it. What it does is show its working, and show the resting burn separately, so you can see which part of the number is real extra effort.

Nothing available to a consumer measures your actual energy expenditure. That takes a metabolic cart and a lab. Everything else, including this, is arithmetic on assumptions.

The number is a rough guide. It is not a budget you have earned.

The short version

• Trackers report gross calories, including what you would have burned resting anyway.
• MET models use population averages, and getting fitter makes you more efficient, so the overstatement grows as you improve.
• Independent testing puts the typical overestimate at 20 to 30 percent.
• Eating back the reported figure is the most common reason a working deficit quietly stops working.
• Use burn numbers to compare your own sessions, and use the scale trend to judge whether the diet is working.

None of this is a reason to stop exercising or to stop wearing the watch. Exercise earns its place on cardiovascular health, sleep, and mood without needing to win the calorie argument. It just should not be the thing you count on to create a deficit.