Fitness trackers and cardio machines routinely overestimate how many calories you burn, sometimes by a wide margin. They are not measuring your metabolism. They are guessing from heart rate and motion, and the guess tends to flatter you, which is a comfortable kind of wrong that quietly wrecks a lot of diets.
Your watch says the run torched 620 calories. The treadmill swears it was 700. You feel great, you feel earned, you feel entitled to the muffin. Somewhere in the machine, a spreadsheet is making all of it up. This matters because people eat back calories they never actually spent, then wonder why the scale refuses to move.
How is calorie burn actually calculated?
Most estimates lean on a value called a MET, short for metabolic equivalent, which rates how hard an activity is compared to sitting still. Sitting is 1 MET. A steady jog is around 7. The rough math is simple: METs multiplied by your body weight in kilograms multiplied by the time in hours.
So a 70 kilogram person jogging for half an hour burns roughly 245 calories by that formula, not the 620 the watch reported. The calories burned calculator does this honestly and will usually hand you a smaller, less exciting number than your wrist did. Body weight scales the result directly, which is why a heavier person burns more doing the identical activity.
How accurate are fitness trackers, really?
There is hard data here. A widely cited 2017 Stanford study tested seven popular wrist devices, including the Apple Watch and the Fitbit Surge, and found that not one of them measured energy expenditure accurately. The most accurate device was off by about 27 percent on average, and the least accurate missed by 93 percent. Yet those same devices tracked heart rate well, with six of the seven under a 5 percent error rate.
In plain terms, the sensor works and the calorie math bolted on top of it is a guess. That does not make trackers useless. As a consistent, relative measure they are genuinely handy: if your watch says today's workout was harder than yesterday's, it probably was. The mistake is treating the absolute calorie number as fact and then spending it like real money.
Median error across tasks and devices. Heart rate stayed under 5% for most.
- Heart rate: 5%
- Calories (best): 27%
- Calories (worst): 93%
Why does eating back your calories backfire?
Because two errors stack in the same direction. Devices overestimate how much you burn, and humans underestimate how much they eat. The two meet in the middle and quietly erase your deficit.
Picture the math. Your watch inflates a workout by 200 calories, and you reward yourself with a 300 calorie snack you did not need. A genuinely solid session becomes a wash before you have even toweled off. The fix is not to obsess over every calorie. It is to stop treating the burn number as a coupon. The exercise is for your heart, your head, and your strength. The eating is a separate ledger, and it does not forgive itself just because you sweated.
What calorie number should you actually use?
Anchor on your maintenance calories, known as your total daily energy expenditure or TDEE, instead of chasing the burn from any single workout. Estimate your resting rate with the BMR calculator, multiply it by an honest activity level, and you have a daily budget that does not depend on any device lying to you mid run.
From there the plan is straightforward:
- Eat around your TDEE to hold your weight steady.
- Eat roughly 500 calories a day below it to lose about a pound a week.
- Eat a similar amount above it to gain gradually.
- Adjust based on what the scale actually does over two to three weeks, not what your watch promised on a Tuesday.
The bottom line
Your tracker is not measuring calories burned, it is estimating them, and it usually estimates high. Use it for trends and effort, not as a ledger you can spend against. Anchor your eating to a maintenance number you calculate once from your BMR and activity level, then let real results, measured over weeks, tell you whether to adjust. Real feedback beats flattering fiction every time.