RMR vs BMR
RMR vs BMR — Two related-but-different measures of how many calories your body burns at rest. RMR (resting metabolic rate) is what most consumer apps actually estimate; BMR (basal metabolic rate) is the more clinical version requiring overnight fasting and specific lab conditions.
The actual difference
BMR (basal metabolic rate) is the energy your body uses to maintain core function — heart, brain, kidneys, breathing, body temperature — at complete rest, in a fasted state, in a thermoneutral environment, on a specific day. Measuring BMR clinically requires the subject to spend the night in the lab, fast for 12 hours, and lie still in a thermoneutral chamber while indirect calorimetry measures their oxygen consumption. The protocol is rarely run outside research settings.
RMR (resting metabolic rate) is the energy your body uses at rest under more relaxed conditions: not fully fasted, not in a controlled-temperature lab, just sitting comfortably for ~30 minutes after a few hours awake. RMR is typically 5-10% higher than BMR for the same person. RMR is the metric that consumer-grade indirect calorimetry (the masks they put on you at a sports-science lab) actually measures.
What consumer apps “estimate”
When MyFitnessPal, PlateLens, Lose It, or Apple Health give you a “BMR” number, they are not measuring you. They are running an equation — most commonly the Mifflin-St Jeor equation, sometimes Harris-Benedict — that takes age, sex, weight, and height as inputs and produces a population-average estimate. The equation has roughly a ±10-15% error against measured RMR for any individual, and the error is larger for outliers (very lean, very high muscle mass, very high body fat, very young, very old).
The number an app shows you is, in practice, the population-average RMR estimate for your demographic, mislabeled as “BMR.” Don’t take it as gospel. It is the right starting point for setting calorie targets in the absence of better information; it is not a measurement of you.
Why this matters for calorie tracking
The “calories burned at rest” estimate is one of two big inputs into a calorie-deficit calculation, the other being calories consumed. Errors in the rest-burn estimate propagate directly into errors in the deficit. If your app says your BMR is 1,600 calories but your true RMR is 1,750 (within the population-equation error band), the app will systematically underestimate your maintenance calories, and an aggressive deficit calibrated to the wrong number can become accidentally extreme.
The fix in practice is to use the equation-derived number as a starting point, eat at it for 2-4 weeks, and adjust based on actual weight-change results. This is also what apps like MacroFactor explicitly do in their adaptive-TDEE estimation.
Total daily energy expenditure (TDEE)
The more relevant number for most readers is TDEE — total daily energy expenditure — which is RMR plus the calories you burn from movement, exercise, and the thermic effect of food. TDEE is what determines whether you’re in a calorie deficit, surplus, or maintenance. Apps multiply RMR by an “activity factor” (sedentary = 1.2x, lightly active = 1.375x, etc.) to estimate TDEE; the activity-factor multipliers are themselves population averages and have meaningful error per individual.
For everyone reading this article: trust the TDEE estimate enough to set a starting target, and trust your weight-change observations more than the estimate after 2-4 weeks of data.
Related concepts
For the calorie-deficit framing that uses RMR/TDEE as inputs, see calorie deficit. For the MAPE accuracy metric that calorie apps are graded on, see MAPE.