One dropdown, and the largest error in the whole calculation
Every calorie calculator asks how active you are, and multiplies your BMR by a factor based on the answer. That dropdown carries more error than every other input combined. Height, weight, age and sex are facts you can look up. Activity level is a self-assessment, and self-assessment of activity is reliably generous.
TDEE = BMR × activity multiplier
The range typically runs from about 1.2 to about 1.9. At a BMR of 1,600, that is the difference between roughly 1,920 and 3,040 calories a day.
| Level | Typical multiplier | What it really means |
|---|---|---|
| Sedentary | ~1.2 | Desk job, little deliberate exercise, most of the day seated |
| Lightly active | ~1.375 | Light exercise 1–3 days a week, or a job with some movement |
| Moderately active | ~1.55 | Moderate exercise 3–5 days a week |
| Very active | ~1.725 | Hard exercise 6–7 days a week, or a physically demanding job |
| Extra active | ~1.9 | Physical job plus daily training, or an athlete in heavy season |
Most people select one level higher than their week justifies. Three gym sessions feels like "moderately active", but if the other four days are largely seated, the weekly average is closer to lightly active. The multiplier is meant to describe the whole week, including the days you did nothing.
This matters because the error compounds. Choosing 1.55 instead of 1.375 on a BMR of 1,600 adds roughly 280 calories a day to the estimate. Apply a 500-calorie deficit to an inflated TDEE and the real deficit is around 220 — less than half of what you intended. The plan then appears to fail for reasons that have nothing to do with the plan.
Pick the level below the one you were about to. If weight moves faster than intended you can eat slightly more, which is a pleasant correction. The reverse — discovering after two months that the target was never a deficit — is not.
Fitness trackers and cardio machines report calorie burn with a precision they do not have. They infer expenditure from heart rate, movement and your profile, and the error can be substantial, generally in the direction of over-estimation. Treat those figures as a relative signal — useful for comparing today to last Tuesday — rather than a number to eat back.
Eating back exercise calories is a reliable way to erase a deficit, because the earned figure is usually inflated while the food logged against it is usually under-counted. If you use a target from a calculator, the simpler approach is to leave the target alone on training days.
No multiplier survives contact with your own data. Eat at the estimate for two to three weeks, weigh under consistent conditions, and look at the trend. If weight is stable, the estimate was close to your real maintenance. If it moves, adjust by 100 to 200 calories and hold again. Two or three of those cycles will land you closer than any dropdown.
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