Two people can step on the scale at the identical weight and height and still burn meaningfully different amounts of energy at rest. Understanding why explains both the power and the limits of any BMR formula.
Muscle is more metabolically active than fat. Of two people at the same weight, the one carrying more lean mass and less fat will have a higher resting burn. A standard height/weight formula can't see this difference — which is exactly the gap the lean-mass-based Katch-McArdle equation is designed to fill when you have a body-fat measurement.
Men generally have higher BMRs than women at the same weight, largely due to more lean mass — which is why the formulas have different constants by sex. Age nudges it down over time. And a taller person at the same weight has a different build than a shorter one, which the height term captures.
Beyond the inputs, genuine individual variation remains: genetics, thyroid and hormone levels, and how much your body fidgets and moves unconsciously (non-exercise activity, or NEAT) all shift the number. Two people identical on paper can differ by a couple hundred calories for reasons a formula simply cannot capture.
It's why every BMR and TDEE figure is an estimate, not a measurement, and why the same calorie target won't behave identically for two similar-looking people. The fix isn't a better formula — it's calibration. Start with the calculator, then let your own results refine it: the tracker reads your real maintenance from how your weight actually responds.
Mostly body composition — more muscle means a higher resting burn at the same weight. Sex, age, genetics, hormones, and unconscious movement (NEAT) also differ between people.
Partly. Height/weight formulas use sex and age but can't see body composition. Katch-McArdle uses lean mass if you have a body-fat measurement. Individual genetic variation no formula can capture.