Common Errors in BMR & TDEE Calculation
Most BMR and TDEE mistakes are invisible: the number comes out clean, looks plausible, and quietly leads a plan astray by a hundred calories a day or more. Because a daily error of 150 kcal compounds into over a kilogram of body weight every couple of months, understanding where these errors come from is more valuable than the formula itself. Here are the failures that trip up even experienced trackers.
The most common and most damaging error is a silent unit mix-up. Mifflin-St Jeor is strictly metric: weight in kilograms, height in centimetres. A person who enters 165 pounds as if they were kilograms inflates BMR by a factor of 2.2 — roughly 1600 instead of 750 kcal for that variable alone — producing a TDEE that can be 1000 kcal too high. Equally, someone who types 5 feet 9 inches as 5.9 expecting inches to convert will short their height badly. Always verify that the selected unit matches the number entered.
Overstating the activity factor is the second biggest source of error, and it is almost universal. People tend to describe their busiest week rather than their average week, and the descriptors are easy to inflate: "moderately active" sounds modest, but it actually means structured exercise three to five days per week. A person who walks to a desk job and lifts twice a week is closer to 1.375 than 1.55. The difference is roughly 350 kcal per day on a 2000 kcal TDEE — a comfortable weekly deficit or surplus hiding inside one dropdown selection.
Double-counting exercise is a related failure. Many people set a generous activity tier and then additionally "eat back" workout calories from a fitness watch. Since the tier already includes training, this counts the same energy twice and can turn a 500 kcal deficit into a surplus by Friday. Trackers should either use a lower tier and add half of measured exercise energy, or use the tier and ignore the watch. Mixing both systems reliably sabotages a cut.
Treating the output as a law rather than an estimate creates a subtler but equally common problem. Mifflin-St Jeor was validated on a general adult population; it is routinely off by several hundred calories for very athletic individuals, for people with high muscle mass, and for those who have dieted aggressively in the past. When the scale does not move after two to three weeks of adherence, the correct response is to adjust the target by 100–150 kcal — not to conclude that dieting "does not work" and abandon the plan.
Failing to recompute after weight change guarantees eventual drift. Every 5 kg of lost weight lowers the BMR estimate by about 50 kcal per day, and every 5 kg gained raises it. A person who started a cut at 90 kg and reaches 80 kg is operating on a stale budget unless they recalculate. This is why a calculator that lives in your browser and takes ten seconds to re-run outperforms a printed diet sheet from six months ago.
Weighing inconsistently is a measurement error that pollutes the input itself. A single Friday weigh-in after a salty restaurant meal can read 2 kg higher than the true morning weight, and entering that value inflates every downstream number. The fix is the protocol: fasted morning weigh-ins, averaged over seven days, and only that average goes into the calculator. Reacting to a single weigh-in is noise-trading against yourself.
Some users apply the wrong formula variant, typically by using the male constant for everyone or by reaching for the older Harris-Benedict equation and then comparing it to Mifflin-St Jeor results. The two equations disagree by hundreds of calories for some body sizes, so mixing methodologies confuses any longitudinal record. Pick one validated equation — this page uses Mifflin-St Jeor — and stick with it so that week-over-week comparisons stay meaningful.
Finally, ignoring the context that shifts real metabolism. Sleep deprivation, illness, thyroid changes, and certain medications can move actual expenditure far from any formula. If weight behavior diverges sharply from the budget while food tracking is honest, that is a signal to consider a non-dietary cause — and, when relevant, to raise it with a clinician. The BMR & TDEE Calculator is a powerful, private, free starting tool, but the scale and the trends remain the ultimate referee.