BMRBasal Metabolic Rate
The calories your body burns at complete rest just to stay alive.
BMR (Basal Metabolic Rate) is the energy your body spends at complete rest simply staying alive: breathing, circulating blood, maintaining temperature, repairing cells and running your organs. For most people it is the largest single slice of the day's energy use, commonly around two thirds of it.
Where that energy goes is not where most people assume. The brain, liver, kidneys and heart are small in mass and enormous in metabolic cost per kilogram, which is why resting expenditure does not scale neatly with how muscular someone looks.
It still scales with size, and fat free mass is the best single predictor there is. The paper behind the Mifflin St Jeor equation found fat free mass explained more of the variation in resting expenditure than anything else, and also noted that the older Harris Benedict equations overestimated measured resting expenditure by about 5%.
That gap matters, because the older equations are still embedded in plenty of apps. Any estimate is a population average applied to you, and individual resting rates vary by a few hundred calories a day around the prediction, so treat the output as a starting point rather than a fact about your body.
Dieting lowers it, partly because you are smaller and partly beyond that, which is the phenomenon called metabolic adaptation. This is a real effect and a poor excuse: it slows progress rather than stopping it, and it is one argument for moderate deficits and for keeping muscle while you lose fat.
You cannot meaningfully raise it by eating chilli, drinking cold water or taking a thermogenic supplement. The lever that exists is more lean mass, and its effect is real but modest, on the order of a small number of calories per kilogram of muscle per day rather than the transformation usually advertised.
In practice, our BMR calculator gives the baseline, the TDEE calculator adds activity on top, and your own weight trend over a few weeks corrects both better than any equation can.