BMR Calculator
This free BMR calculator helps you estimate your basal metabolic rate — the number of calories your body burns at rest each day. Using evidence-based formulas including Mifflin–St Jeor, Harris-Benedict, and Katch-McArdle, this tool provides accurate calorie estimates based on your age, sex, height, weight, and activity level. You can also view your TDEE (Total Daily Energy Expenditure) to understand how many calories you burn including daily movement. Whether you're planning a diet, adjusting macros, losing weight, or building muscle, this BMR calculator gives you science-based numbers to guide your nutrition planning. All calculations run instantly in your browser — private, fast, and easy to use.
What Is a BMR Calculator?
A BMR (Basal Metabolic Rate) calculator estimates the number of calories your body burns at complete rest — just to sustain breathing, circulation, temperature regulation, and cellular repair. BMR is the starting point for all calorie and nutrition planning. Multiply BMR by an activity factor to get TDEE (Total Daily Energy Expenditure), which is your actual daily calorie need. For a 30-year-old woman, 5'6", 140 lbs: BMR ≈ 1,389 cal/day; sedentary TDEE ≈ 1,667 cal/day.
BMR Formula Comparison (35M, 5'9", 180 lbs)
| Formula | BMR Result | Best For | Accuracy |
|---|---|---|---|
| Mifflin-St Jeor | 1,767 cal | General population | ±5–10% |
| Harris-Benedict (revised) | 1,824 cal | Alternative estimate | ±10–15% |
| Katch-McArdle | 1,645 cal* | Athletes (known BF%) | ±3–5% |
| Original Harris-Benedict | 1,892 cal | Historical reference | ±15–20% |
*Katch-McArdle based on 15% body fat assumption. Use Mifflin-St Jeor for most people; Katch-McArdle for athletes with known body fat %.
From BMR to TDEE: Activity Multipliers
| Activity Level | Multiplier | Description | TDEE (BMR 1,500) |
|---|---|---|---|
| Sedentary | ×1.2 | Desk job, little exercise | 1,800 cal |
| Lightly Active | ×1.375 | Exercise 1–3 days/week | 2,063 cal |
| Moderately Active | ×1.55 | Exercise 3–5 days/week | 2,325 cal |
| Very Active | ×1.725 | Hard exercise 6–7 days/week | 2,588 cal |
| Extremely Active | ×1.9 | Hard exercise + physical job | 2,850 cal |
Most people overestimate activity level by one category. When in doubt, choose Sedentary or Lightly Active and adjust based on 2–3 weeks of real weight data.
Worked Example: Sarah's BMR Calculation
30F, 5'6" (168 cm), 140 lbs (63.5 kg) — Mifflin-St Jeor
BMR = (10 × 63.5) + (6.25 × 168) − (5 × 30) − 161 = 1,389 cal/day
Sedentary TDEE: 1,667 | Lightly Active: 1,910 | Moderately Active: 2,153
Weight loss target (1 lb/week): 1,167–1,653 cal/day depending on activity
Factors That Affect BMR
Muscle mass is the single largest controllable variable: muscle burns ~6 cal/lb/day at rest vs. fat at ~2 cal/lb. A person who gains 10 lbs of muscle permanently raises their BMR by ~60 cal/day — equivalent to a 20-minute walk. BMR declines ~2–3% per decade after 30, mostly due to muscle loss (sarcopenia). Men have higher BMRs than women of the same weight due to greater muscle mass. Prolonged calorie restriction causes metabolic adaptation (metabolism slows by 5–15%), which is why diet breaks and maintenance phases are important.
Frequently Asked Questions About BMR
What is Mifflin-St Jeor and why is it preferred?
Mifflin-St Jeor (1990) was derived from a larger, more representative sample than the original Harris-Benedict (1919). In clinical studies, it predicts measured resting metabolic rate within ±10% for 82% of adults. Harris-Benedict tends to overestimate by 5–15%, leading to excess calorie recommendations. Mifflin-St Jeor is now the standard recommended by most dietitians.
What is the Katch-McArdle formula?
Katch-McArdle calculates BMR from lean body mass only (fat-free mass), not height and weight. BMR = 370 + (21.6 × lean mass in kg). It's the most accurate formula for people with known body fat percentage, because fat mass doesn't contribute to resting metabolic rate. Required inputs: weight and body fat %. Typically 5–10% lower than Mifflin-St Jeor for lean athletes.
How much does age affect BMR?
BMR declines approximately 2–3% per decade, primarily due to loss of muscle mass (sarcopenia). From age 30 to 60, a person may lose 150–250 cal/day in BMR with no change in weight. Resistance training that preserves muscle mass largely counteracts this decline — one more reason strength training is critical for metabolic health in aging.
Can I boost my BMR naturally?
Yes. 1) Build muscle through resistance training (most effective). 2) Eat adequate protein (prevents muscle breakdown). 3) Avoid prolonged severe calorie restriction (causes metabolic adaptation). 4) Sleep 7–9 hours (sleep deprivation reduces BMR by 5–10%). 5) Don't do excessive fasted cardio, which can accelerate muscle loss over time.
What is the difference between BMR, RMR, and TDEE?
BMR (Basal Metabolic Rate) is measured under strict conditions (12-hour fast, complete rest). RMR (Resting Metabolic Rate) is measured after 4–6 hours of rest and is typically 5–10% higher than BMR — this is what most calculators actually estimate. TDEE (Total Daily Energy Expenditure) is RMR/BMR plus all activity and digestion, representing your full daily calorie burn.
How often should I recalculate my BMR?
Recalculate whenever your weight changes by 5+ lbs, your activity level changes significantly, or after 3–6 months. BMR tracks with weight: losing 20 lbs typically reduces BMR by 80–150 cal/day, requiring adjustment of your calorie target. Failing to recalculate is why weight loss plateaus are common — what was a deficit becomes maintenance.