What Is BMR (Basal Metabolic Rate)?

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to stay alive. Even when you are sleeping, sitting still, or watching TV, your body is hard at work — pumping blood, breathing, repairing cells, and keeping your organs functioning. All of that background work requires energy, and BMR is the measurement of that energy.

For most people, BMR accounts for 60–70% of total daily calorie burn. That means even if you never exercised, your body would still burn the majority of your calories just keeping you alive. This is why understanding BMR is the foundation of any weight management plan — whether you want to lose, gain, or maintain weight.

You can get your personalized number in seconds with our BMR calculator.

BMR vs RMR: What's the Difference?

People often use BMR and RMR (Resting Metabolic Rate) interchangeably, but they are measured under slightly different conditions:

  • BMR is measured under strict laboratory conditions — after 8+ hours of sleep, 12+ hours of fasting, in a thermally neutral room.
  • RMR is measured under less strict conditions and is typically 5–10% higher than BMR.

In everyday practice, the difference rarely matters. Most online calculators (including ours) use the Mifflin-St Jeor formula, which estimates resting energy expenditure closely enough for practical use. See our RMR calculator for a side-by-side comparison.

How Is BMR Calculated?

The most widely recommended formula is Mifflin-St Jeor, developed in 1990 and validated against direct calorimetry:

  • Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
  • Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

An older alternative is the Harris-Benedict formula (1918), which tends to overestimate BMR by 5–10%, especially in obese individuals. For most people, Mifflin-St Jeor is the better choice.

What Factors Affect BMR?

BMR is not a fixed number. It changes based on several factors:

  1. Body size and composition — Larger bodies and more muscle mass burn more calories at rest.
  2. Age — BMR decreases about 1–2% per decade after age 30, mostly due to muscle loss.
  3. Gender — Men typically have higher BMR because of greater muscle mass.
  4. Genetics — Some people naturally burn more or fewer calories at rest.
  5. Thyroid function — Hormonal imbalances can raise or lower BMR significantly.
  6. Body temperature — Fever or cold exposure increases calorie burn.

How to Use BMR for Your Goals

Your BMR is the floor — you should never eat below it for extended periods. To find your actual daily needs, multiply BMR by an activity factor to get TDEE:

  • Sedentary: BMR × 1.2
  • Lightly active: BMR × 1.375
  • Moderately active: BMR × 1.55
  • Very active: BMR × 1.725
  • Extra active: BMR × 1.9

From there, adjust for your goal: subtract 300–500 kcal for fat loss, add 200–300 kcal for muscle gain, or match TDEE for maintenance. Use our calorie deficit calculator for a personalized target.

Quick Tip Recalculate your BMR every 4–6 weeks or after losing 5–10% of your body weight. As you lose weight, your BMR drops, so your calorie target should drop too.