RMRResting metabolic rate

Resting metabolic rate is the energy used to keep you alive while at rest: breathing, circulation, brain activity, cell maintenance. It is closely related to basal metabolic rate but measured under less strict conditions — rested rather than fully post-absorptive and undisturbed — which puts it typically a few per cent higher.

It matters because it makes up the largest single slice of daily energy expenditure, usually somewhere around 60 to 70 per cent for a moderately active person. It scales mainly with fat-free mass, which is the mechanistic reason muscle is worth keeping during a diet, though the per-kilogram effect is smaller than gym folklore suggests.

In practice the two terms are used interchangeably in most calculators and diet advice, and the equations behind them are population estimates with meaningful individual scatter. Treat the output as a starting point to adjust from, not a measurement.