RMR Calculator

BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) measure the same thing - the calories your body burns at rest - but under different conditions. True BMR requires 12+ hours fasted, post-sleep, lying still in a thermally neutral room. RMR allows light wakefulness and a shorter fast, which is far easier to measure in the real world. As a result, RMR readings usually run 6-10% higher than strict BMR. The three formulas this calculator uses (Mifflin-St Jeor, Harris-Benedict, Katch-McArdle) were derived from BMR studies but are used interchangeably for RMR in nutrition and clinical practice, which is the convention this calculator follows.

Harris-Benedict (1919, revised by Roza and Shizgal in 1984) was the standard for most of the 20th century, but it tends to overestimate by 5-15% in modern populations. Mifflin-St Jeor (1990) was built on a more representative sample of body weights and ages. A 2005 American Dietetic Association systematic review (Frankenfield, Roth-Yousey, Compher) compared every published equation against indirect calorimetry and found Mifflin-St Jeor was within 10% of measured values for about 82% of nonobese and 70% of obese adults, beating every other formula tested. That is why modern nutrition software and clinical guidelines default to it.

Katch-McArdle is the most accurate formula when you have a reliable body fat percentage from a DEXA scan, hydrostatic weighing, BodPod, or a calibrated multi-site skinfold protocol. Unlike Mifflin and Harris-Benedict, it ignores sex and age entirely and works from lean body mass: RMR = 370 + 21.6 x lean mass in kg. For lean and muscular people whose body composition differs from the average adult, Katch-McArdle usually beats sex- and age-based formulas. The catch: it is only as good as your body fat number. A bathroom scale BIA reading can be off by 5+ percentage points, which can shift the RMR estimate by 100-150 calories. The auto setting picks Katch-McArdle whenever you enter a body fat percentage, otherwise it defaults to Mifflin.

Total Daily Energy Expenditure (TDEE) is your RMR multiplied by an activity factor that accounts for movement, exercise, and the thermic effect of food. The five-step ladder this calculator uses comes from the Katch-McArdle exercise physiology textbook and is the de facto industry standard: 1.2 for sedentary (desk job, no exercise), 1.375 for lightly active (light exercise 1-3 days/week), 1.55 for moderately active (3-5 days/week), 1.725 for very active (6-7 days/week), and 1.9 for extra active (physical job plus daily training). Most office workers who train a few times a week land between 1.375 and 1.55. The multipliers are rough estimates - real-world activity tracking with a fitness watch can shift the number by 10-15% either way.

The older 3,500-calorie rule treats weight loss as linear, but real energy needs change as body weight and metabolism change. A 500-calorie gap may be used as a rough planning estimate from total daily energy expenditure, not from RMR alone. It does not guarantee a specific weekly result. Avoid setting an intake below your basic needs without guidance from a qualified clinician or registered dietitian.