Your RMR Calculator
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RMR vs BMR — What's the Difference?
Basal Metabolic Rate (BMR) is measured under strict laboratory conditions: lying completely still in a temperature-controlled room after 12 hours of fasting and 8 hours of sleep. Resting Metabolic Rate (RMR) is measured at rest but without the strict fasting and temperature requirements. RMR is typically 10–20% higher than BMR due to the thermic effect of food from recent meals and minor activity. In practice, most "BMR calculators" actually estimate RMR — the distinction is primarily academic.
Which Formula Is Most Accurate?
A 2005 systematic review in the Journal of the American Dietetic Association by Frankenfield et al. compared five major equations against measured metabolic rate. The Mifflin-St Jeor equation was most accurate within 10% for the majority of adults. However, it was developed on a population without stratification for very high or very low body fat — for muscular athletes and very lean individuals, Katch-McArdle is significantly more accurate because it uses lean body mass rather than total weight.
Concrete example: a 90 kg man with 10% body fat (lean, muscular) has 81 kg of lean mass. Mifflin-St Jeor treats his 90 kg the same way it would treat a sedentary 90 kg man with 30% body fat. Katch-McArdle only "sees" the 81 kg of metabolically active lean tissue and produces a significantly different estimate — and is more accurate for the athlete.
Why Does RMR Matter for Weight Management?
Your RMR accounts for 60–75% of your Total Daily Energy Expenditure (TDEE) in most sedentary adults. This means that even without any exercise, your body burns the majority of its calories just keeping you alive — heart beating, lungs breathing, brain functioning, cells repairing. Crash diets that cause rapid weight loss lower RMR through two mechanisms: reduced lean mass (muscle is metabolically expensive) and adaptive thermogenesis (the body deliberately slows metabolism to conserve energy). This "metabolic adaptation" is why most crash dieters regain weight — their RMR has been suppressed.
Yes — and resistance training is by far the most effective method. Each kilogram of lean muscle tissue burns approximately 13 kcal/day at rest (fat burns ~4.5 kcal/day). Gaining 5 kg of muscle therefore increases RMR by roughly 65 kcal/day — modest on its own, but meaningful when compounded with higher activity-based calorie burn from having more muscle. High-protein diets also temporarily elevate RMR through the thermic effect of protein (25–30% of protein calories are burned in digestion). Cold exposure, adequate sleep, and avoiding extreme caloric restriction also help preserve RMR.
The gold-standard test is indirect calorimetry — a metabolic cart test available at many sports medicine facilities and research institutions. For self-estimation: if you have an accurate body fat percentage, use Katch-McArdle. If you don't know your body fat %, use Mifflin-St Jeor. If your calculated TDEE (RMR × activity multiplier) matches your observed maintenance calories after 3–4 weeks of careful tracking, the formula is working well for you. If you consistently gain or lose weight at maintenance, adjust your estimate accordingly.