RMR Calculator
Resting metabolic rate via Mifflin-St Jeor or Katch-McArdle.
Resting metabolic rate
1,718 cal/day
Mifflin-St Jeor
Sedentary (×1.2)
2,061 cal
Light active (×1.375)
2,362 cal
Very active (×1.725)
2,963 cal
AI Breakdown & Smart Takeaway
Plain-English insight on your numbers
Get a personalized explanation of what these results mean — and how to improve them.
How the RMR Calculator works
The RMR Calculator estimates how many calories your body burns at complete rest using either the Mifflin-St Jeor or Katch-McArdle equation, giving you a precise metabolic baseline that underpins every nutrition and fitness plan. It's ideal for anyone — from beginners tracking calories to athletes fine-tuning body composition — who wants a science-backed starting point rather than a generic guess.
Resting metabolic rate (RMR) represents the number of calories your body requires to sustain essential life functions — breathing, circulation, cellular repair, and thermoregulation — while you are awake but completely at rest. It is slightly higher than the closely related basal metabolic rate (BMR), which is measured under stricter laboratory conditions (post-sleep, fasted, in a temperature-controlled room). In practice, the two terms are often used interchangeably, but RMR is the more practical figure because it reflects real-world resting energy expenditure rather than an idealized clinical state. This calculator gives you RMR because it's the more applicable number for everyday calorie planning.
The calculator offers two industry-leading equations. The Mifflin-St Jeor formula — widely considered the gold standard for the general population — requires your age, sex, height, and weight to produce an estimate with roughly ±10% accuracy in most individuals. The Katch-McArdle formula takes a different approach: it requires your lean body mass (total weight minus fat mass) and ignores sex and age entirely, making it exceptionally accurate for lean or athletic individuals who know their body fat percentage. Choosing the right equation matters; using Mifflin-St Jeor on a highly muscular person often underestimates their RMR, while Katch-McArdle on someone without a reliable body-fat measurement can introduce error.
Several biological factors drive your RMR beyond what any formula can perfectly capture. Muscle tissue is metabolically expensive — each kilogram of lean mass burns approximately 13 kcal per day at rest, compared to roughly 4.5 kcal per kilogram of fat — which is why two people of identical weight can have very different calorie needs at rest. Age progressively lowers RMR (roughly 1–2% per decade after age 20) largely due to muscle loss (sarcopenia), while sex differences arise because men typically carry more lean mass. Thyroid function, hormonal status, chronic illness, and even ambient temperature can shift your actual metabolic rate meaningfully away from any formula's output.
A critical and common mistake is treating your RMR as your daily calorie target. RMR only accounts for calories burned at rest; to find your total daily energy expenditure (TDEE), you must multiply RMR by an activity factor — ranging from about 1.2 for sedentary individuals to 1.9 for very hard training athletes. Eating at your RMR figure without adjustment would place almost anyone in a significant deficit. Instead, use RMR as the foundation: understand that no matter how little you move, your body needs at least this many calories to function, and then layer your activity, thermic effect of food, and exercise on top to build a complete energy budget.
Formula
Mifflin-St Jeor, or Katch-McArdle (370 + 21.6 × LBM) when body fat is given
Pro tips
- Choose Katch-McArdle if you know your body fat percentage from a DEXA scan or hydrostatic weighing — it consistently outperforms Mifflin-St Jeor for athletes and lean individuals because it's built on lean mass, not total weight.
- Measure your weight first thing in the morning after using the bathroom and before eating to get the most consistent, repeatable input values — daily fluctuations from food and water can swing weight by 1–3 kg, skewing your result.
- Rerun your RMR calculation every 8–12 weeks or whenever your weight changes by more than 5 kg, since RMR shifts meaningfully as body composition changes — especially during a prolonged cut or bulk.
- Do not eat at your RMR figure directly; instead, use it as your metabolic floor and multiply by your honest activity factor to find TDEE before setting a calorie surplus or deficit of 250–500 kcal/day for sustainable results.
- If your real-world results consistently deviate from your formula-based estimate (e.g., you gain weight eating at a calculated deficit), consider getting a clinical indirect calorimetry test — it measures actual oxygen consumption to determine your true RMR within ±2%.
Key terms
- Resting Metabolic Rate (RMR)
- — The number of calories your body burns per day while awake and at rest, sustaining core physiological functions without any physical activity factored in.
- Basal Metabolic Rate (BMR)
- — A stricter laboratory measure of minimum caloric need recorded after sleep and in a fasted, temperature-controlled state — in practice, BMR and RMR differ by only 10–15% and are often used interchangeably.
- Lean Body Mass (LBM)
- — The portion of your total body weight that is not fat, including muscle, bone, organs, and water — the primary driver of metabolic rate.
- Mifflin-St Jeor Equation
- — A regression formula published in 1990 that predicts RMR from weight, height, age, and sex, regarded as the most accurate prediction tool for the general population.
- Katch-McArdle Equation
- — An RMR formula that uses only lean body mass as its input, making it the preferred choice for lean or athletic individuals with a known body fat percentage.
- Total Daily Energy Expenditure (TDEE)
- — The total calories you burn in a day, calculated by multiplying RMR by an activity multiplier that accounts for exercise and non-exercise movement.
