BMR Calculator — Basal Metabolic Rate

What Is Basal Metabolic Rate?

Your Basal Metabolic Rate (BMR) is the number of calories your body burns every day just to stay alive. No walking, no exercise, no digesting food — just the raw energy cost of keeping your heart beating, your lungs breathing, and your brain running.

BMR accounts for 60-75% of your total daily calorie burn. Even if you spent the entire day in bed, your body would still burn this many calories. That makes BMR the single largest component of your daily energy expenditure — and the foundation for any nutrition plan.

How Is BMR Calculated?

Three formulas dominate the research. Each approaches the problem differently:

Mifflin-St Jeor (1990) — Recommended

Developed by Mifflin et al. (1990) and validated as the most accurate predictive equation by the American Dietetic Association (2005):

• Males: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) + 5
• Females: (10 × weight in kg) + (6.25 × height in cm) - (5 × age) - 161

This is the default formula in our calculator and the one RepStack uses internally. It's accurate within 10% for most adults.

Harris-Benedict Revised (Roza & Shizgal, 1984)

The original Harris-Benedict equation dates to 1919. Roza and Shizgal (1984) revised it with updated coefficients. It tends to estimate slightly higher than Mifflin-St Jeor, particularly for overweight individuals.

Katch-McArdle (1996)

Unlike the other two, Katch-McArdle uses lean body mass instead of total weight:

• BMR = 370 + (21.6 × lean body mass in kg)

This makes it the most accurate formula for people who know their body fat percentage — especially muscular individuals whose BMR would be underestimated by weight-based formulas, and very overweight individuals whose BMR would be overestimated.

Where Do BMR Calories Go?

Your resting metabolism isn't distributed evenly. Research published in the Journal of Applied Physiology breaks it down:

• Liver (21%): The metabolic powerhouse. Detoxification, protein synthesis, and bile production run 24/7.
• Brain (20%): Your brain weighs ~1.4kg but consumes a fifth of your resting calories. Almost entirely glucose.
• Skeletal Muscle (22%): Even at rest, muscle tissue is metabolically expensive. This is why gaining muscle raises your BMR.
• Heart (9%): Never stops. Beats ~100,000 times per day, pumping ~7,500 liters of blood.
• Kidneys (8%): Filter ~180 liters of blood per day. Small organs, massive energy demand per gram.
• Other (20%): Lungs, digestive tract, immune system, thermoregulation, and cellular repair.

BMR vs RMR — What's the Difference?

Resting Metabolic Rate (RMR) is measured under less strict conditions than BMR. True BMR requires 12+ hours of fasting, 8 hours of sleep, and measurement in a thermoneutral environment. RMR just requires sitting quietly for 15-20 minutes.

In practice, RMR is about 3-10% higher than BMR because you're not in a truly basal state. Most online calculators (including ours) actually estimate something closer to RMR, which is fine for practical nutrition planning.

What Affects Your BMR?

• Muscle mass: The #1 modifiable factor. Muscle burns ~6 kcal/lb/day at rest vs ~2 kcal/lb/day for fat. A 10lb muscle gain adds roughly 40 kcal/day to your BMR.
• Age: BMR decreases ~1-2% per decade after 20, mainly due to muscle loss (sarcopenia). Resistance training slows this significantly.
• Gender: Males average 5-10% higher BMR than females of the same weight, primarily due to higher lean mass.
• Height: Taller people have more tissue to maintain, increasing BMR.
• Thyroid function: T3 and T4 hormones directly regulate metabolic rate. Hypo/hyperthyroidism can shift BMR by 10-20%.
• Dieting history: Prolonged caloric restriction triggers metabolic adaptation — your body becomes more efficient, lowering BMR below predicted values. This is why aggressive crash diets backfire long-term.

How to Use Your BMR

BMR alone doesn't tell you how much to eat. You need to multiply it by an activity factor to get your Total Daily Energy Expenditure (TDEE):

• Sedentary (desk job): BMR × 1.2
• Lightly active (1-3 days exercise): BMR × 1.375
• Moderately active (3-5 days): BMR × 1.55
• Very active (6-7 days): BMR × 1.725
• Extremely active (athlete/physical job): BMR × 1.9

From TDEE, you can then set your calorie deficit or surplus based on your goal.

Tips for Accuracy

• Weigh yourself in the morning, fasted, after using the bathroom — this is your most consistent weight.
• If you know your body fat percentage (from calipers, DEXA, or a good estimate), add it to get the Katch-McArdle result.
• Don't eat below your BMR for extended periods. It signals your body to conserve energy and triggers metabolic adaptation.
• Re-calculate every 4-6 weeks as your weight changes — a 5kg loss reduces BMR by roughly 50-60 kcal/day.
• Remember: these are estimates. Use them as a starting point, then adjust based on real-world results over 2-3 weeks.