RMRResting metabolic rate
The energy your body uses at rest: slightly higher, and easier to measure, than BMR.
Resting metabolic rate is the energy your body uses at rest to keep itself running: breathing, circulation, temperature, organ function and cellular maintenance. It is the largest component of most people's daily expenditure, commonly around two thirds of the total.
It is closely related to basal metabolic rate and slightly higher. BMR is measured under strict conditions, after an overnight fast, at complete rest, in a thermoneutral room on waking. RMR is measured under more relaxed conditions and typically comes out a few percent above it, which is why the two terms are often used interchangeably and should not be.
Fat free mass is the best single predictor of it, which is why larger and more muscular people burn more at rest, and why the paper behind the Mifflin St Jeor equation found fat free mass explaining more of the variance than anything else. That same paper found the older Harris Benedict equations overestimated resting expenditure by about 5%.
Individual variation around any equation runs to a few hundred calories a day, so a predicted RMR is a starting hypothesis rather than a measurement. Real measurement requires indirect calorimetry, a mask and a laboratory.
It falls during weight loss, both because a smaller body costs less and beyond that, which is metabolic adaptation. In the most extreme documented case, six years after a weight loss competition, participants had resting rates hundreds of calories below baseline.
The lever people hope for, raising it substantially, does not really exist. More muscle helps modestly; nothing else you can buy does.
In practice, our BMR calculator gives the baseline estimate, and three weeks of weekly average weight change corrects it better than any equation.