BMR Calculator
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — just to keep organs functioning — and it forms the foundation for every calorie and weight management calculation you make.
Use this BMR calculator to find your Basal Metabolic Rate using both the Mifflin-St Jeor (most accurate) and Harris-Benedict formulas. See how your BMR compares and get your TDEE at every activity level in one clear table.
Use this BMR calculator to calculate your resting calorie burn and see exactly how your TDEE changes as your lifestyle becomes more or less active.
Basal Metabolic Rate Explained
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest — no movement, no digestion, just keeping your vital organs running. It's the foundation of all calorie calculations and accounts for roughly 60–75% of total daily calorie expenditure in sedentary individuals. Understanding your BMR is the first step to building an effective nutrition strategy.
Mifflin-St Jeor vs. Harris-Benedict: Which to Use
The Mifflin-St Jeor equation (1990) is the most accurate standard formula: Men: BMR = 10W + 6.25H − 5A + 5; Women: BMR = 10W + 6.25H − 5A − 161 (W = weight in kg, H = height in cm, A = age). The original Harris-Benedict equations (1919, revised 1984) tend to overestimate BMR by 5–15% compared to measured values in modern clinical studies. For most people without body composition data, Mifflin-St Jeor is the recommended starting point — and what this calculator uses as its primary result.
What Affects Your BMR
Muscle mass is the biggest controllable factor — muscle burns approximately 6 cal/lb/day at rest vs. fat at only 2 cal/lb/day. Age reduces BMR through muscle loss. Sex matters because men typically have more lean mass. Body size directly scales BMR. Genetics can cause ±15% individual variation from formula estimates. Hormones (especially thyroid) can significantly alter BMR — hypothyroidism can reduce it by 30–40%.
BMR Estimates Across Different Profiles
BMR varies widely by age, sex, and body composition. Here's how Mifflin-St Jeor estimates differ across realistic profiles — and what their corresponding sedentary and moderate TDEE look like:
| Profile | Weight / Height | BMR (est.) | Sedentary TDEE | Moderate TDEE |
|---|---|---|---|---|
| Woman, 25, active | 130 lbs / 5'4" | 1,360 cal | 1,632 cal | 2,108 cal |
| Woman, 45, moderate | 155 lbs / 5'5" | 1,410 cal | 1,692 cal | 2,185 cal |
| Man, 25, active | 175 lbs / 5'10" | 1,870 cal | 2,244 cal | 2,898 cal |
| Man, 45, moderate | 190 lbs / 5'11" | 1,870 cal | 2,244 cal | 2,898 cal |
| Man, 65, lightly active | 170 lbs / 5'9" | 1,610 cal | 1,932 cal | 2,495 cal |
BMR and Weight Management: A Critical Distinction
To manage weight, you need to know your TDEE (BMR × activity factor), not just BMR. Eating at TDEE maintains weight. Many people make a dangerous mistake: they eat at their BMR level when trying to lose weight. BMR is the absolute minimum for survival — eating at or below it for extended periods causes muscle loss, fatigue, hormonal disruption, and metabolic adaptation. A healthy deficit is 300–500 cal below TDEE, not below BMR.
3 Medical Conditions That Significantly Alter BMR
- Hypothyroidism: Underactive thyroid reduces BMR by 15–30%. Symptoms include fatigue, cold sensitivity, weight gain despite normal eating. Diagnosed with a TSH blood test and treated with daily levothyroxine.
- PCOS (Polycystic Ovary Syndrome): Affects up to 10% of women of reproductive age. Insulin resistance reduces the body's ability to metabolize calories efficiently, meaning standard formulas may consistently underestimate true calorie needs.
- Cushing's syndrome: Excess cortisol promotes fat storage while causing muscle wasting — reducing BMR even as body weight increases. Often underdiagnosed.
How to Use Your BMR to Build a Weekly Meal Plan
The practical workflow: (1) Calculate your TDEE using your BMR and activity multiplier. (2) Set your calorie target based on your goal — TDEE for maintenance, TDEE minus 300–500 for fat loss, TDEE plus 150–300 for lean muscle gain. (3) Divide your daily target across 3–5 meals. (4) Set a protein floor first (0.7–1g per lb of bodyweight), then fill remaining calories with carbs and fat based on preference. Track for 2–3 weeks before adjusting.
Key Insight: Building 10 lbs of muscle through 1–2 years of consistent resistance training raises your BMR by approximately 60 cal/day. That's 21,900 extra calories burned per year without any change to diet or exercise frequency — the equivalent of about 6 lbs of fat. The compounding effect of muscle on long-term metabolism is one of the strongest arguments for prioritizing strength training at any age.
Crash Dieting and Metabolic Adaptation
Severe calorie restriction causes the body to reduce BMR by 15–30% through muscle loss, reduced thyroid output, and adaptive thermogenesis — a phenomenon called metabolic adaptation. This explains why people who lose weight on crash diets regain it rapidly when they resume normal eating. The lower BMR persists for months to years after dieting ends. Use our calorie calculator to set a moderate, sustainable deficit and preserve your metabolic rate.
Know your BMR? Calculate your full calorie target and macro breakdown with our calorie calculator, or read our guide on using a calorie deficit for fat loss.