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BMR Calculator

Check your Basal Metabolic Rate with our free BMR calculator.

  • Sources: NIH
  • Last updated 3rd April 2026
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Table of Contents

What BMR measures

BMR is the calorie cost of being alive — the energy your body burns just to keep organs running, blood pumping, and cells maintained, without any movement or digestion. It's the floor of your daily calorie needs. Knowing it is the starting point for almost every diet calculation: weight maintenance, calorie deficits, surpluses, and macronutrient planning all begin from BMR.

How BMR is calculated

This calculator uses the Mifflin-St Jeor equation — the most accurate of the common BMR formulas according to the American Dietetic Association. It needs four inputs: weight, height, age, and biological sex.

Men: BMR = 10W + 6.25H − 5A + 5
Women: BMR = 10W + 6.25H − 5A − 161

Where W = weight in kg, H = height in cm, A = age in years.

Worked example using the calculator's default values (male, 74 kg, 170 cm, 30 years):

  • 10 × 74 = 740
  • 6.25 × 170 = 1,062.5
  • 5 × 30 = 150
  • BMR = 740 + 1,062.5 − 150 + 5 = 1,658 calories/day

That's the calorie cost of keeping this person alive at complete rest — not a recommended daily intake, just the metabolic floor. For a recommended intake, see the calorie calculator which adds an activity multiplier to give TDEE.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

Three formulas are commonly used to estimate BMR. They differ in how they account for body composition:

  • Mifflin-St Jeor (1990). Used by this calculator. Validated in 2005 as the most accurate predictive equation for non-obese adults. Uses weight, height, age, and sex.
  • Harris-Benedict (1919, revised 1984). The original BMR formula and still widely cited. Tends to overestimate BMR by 5–15% compared to indirect calorimetry. Same inputs as Mifflin-St Jeor but different coefficients.
  • Katch-McArdle. Uses lean body mass instead of total weight, which makes it more accurate for very lean or very muscular people — but only if you know your body fat percentage. Formula: BMR = 370 + (21.6 × lean body mass in kg).

If you know your body fat percentage with reasonable accuracy (DEXA scan, hydrostatic weighing, or BodPod), Katch-McArdle will likely give you a better estimate. Otherwise, Mifflin-St Jeor is the safer default for most people.

What actually affects your BMR

BMR varies between people of identical age, height, and weight — sometimes by 200–400 calories per day. The biggest sources of variation:

  • Body composition (the dominant factor). Muscle tissue burns roughly 13 cal/kg/day at rest; fat tissue burns about 4 cal/kg/day. Two people at the same weight but different body fat percentages will have noticeably different BMRs.
  • Age. BMR declines roughly 1–2% per decade after age 30, primarily because muscle mass declines unless you actively maintain it.
  • Sex. Men typically have higher BMR than women of the same age, height, and weight — mostly explained by higher average lean body mass (15–20% more muscle), not anything mysterious.
  • Thyroid function. Hypothyroidism reduces BMR by 10–30% in untreated cases; hyperthyroidism raises it. If your weight isn't responding to a reasonable calorie deficit, thyroid testing is worth discussing with a doctor.
  • Sustained dieting. Long calorie deficits cause adaptive thermogenesis — BMR can drop 5–15% beyond what weight loss alone would predict. This is reversible with periods of maintenance eating.
  • Genetics. Identical-twin studies show ~40–50% of BMR variation between individuals is heritable.
BMR calculator showing calorie needs

Using your BMR for weight management

BMR alone isn't your daily calorie target — you need to add the calories burned through activity and digestion. The standard approach:

  1. Get BMR from the calculator above.
  2. Calculate TDEE by multiplying BMR by an activity factor (1.2 for sedentary, 1.375 for light activity, up to 1.725 for very active). See the calorie calculator which automates this.
  3. Adjust from TDEE, not BMR:
    • Maintenance: eat at TDEE
    • Loss: eat 300–500 calories below TDEE
    • Gain: eat 200–500 calories above TDEE
  4. Track for 2–3 weeks and adjust based on actual weight trend. The formulas are estimates — your real metabolism is the only thing that matters in the end.

One common mistake: setting calorie intake at BMR (or below) thinking it'll cause "extra" weight loss. Below-BMR intakes for sustained periods cause adaptive metabolic slowdown, muscle loss, and rebound — they don't accelerate fat loss in the long run.

Sources & references

  • NIH NIDDK — Factors Affecting Weight and Health
  • Mifflin MD et al. (1990). "A new predictive equation for resting energy expenditure in healthy individuals." American Journal of Clinical Nutrition 51(2): 241–247.
  • Frankenfield DC et al. (2005). "Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults." Journal of the American Dietetic Association 105(5): 775–789 — meta-analysis showing Mifflin-St Jeor as the most accurate predictive equation.
  • Bouchard C et al. (1989). "Genetic effects in resting and exercise metabolic rates." Metabolism 38(4): 364–370 — twin study quantifying genetic contribution to BMR.

FAQs

BMR (Basal Metabolic Rate) is measured under strict conditions: after 8+ hours of sleep, 12 hours of fasting, complete physical and emotional rest, and in a thermally neutral room. RMR (Resting Metabolic Rate) is measured under less strict conditions — sitting quietly with normal recent meals. RMR is typically 5–10% higher than BMR because of digestion and minor activity. In casual use, the terms are often interchangeable. This calculator estimates BMR, but in practice it's close enough to RMR for diet planning.

Modestly, yes — but less than fitness marketing suggests. Adding 5 kg of muscle increases BMR by roughly 50–75 calories per day — useful over years but not enough to out-eat a poor diet. The bigger lever is preserving the BMR you have during weight loss: strength training while in a calorie deficit prevents the BMR drop that normally accompanies losing weight, because you keep your muscle mass instead of losing it alongside fat.

Two common reasons. First, formula accuracy varies by ±10–15% even for healthy adults — you may simply be on the low side of normal. Second, sustained dieting and weight loss cause adaptive thermogenesis — your BMR drops more than weight loss alone would predict, sometimes by 100–300 calories. Other factors: hypothyroidism, certain medications, chronic under-eating, and (paradoxically) being very lean can all suppress BMR below predicted levels.

The Mifflin-St Jeor equation is accurate to within ±10% for around 80% of healthy non-obese adults. For obese individuals, accuracy drops — the equation tends to overestimate BMR in obesity. The most accurate single way to measure your actual BMR is indirect calorimetry (measuring oxygen consumption in a clinic), but that requires equipment most people don't have access to. For everyday diet planning, the formula's estimate is good enough — adjust by 100–200 calories after watching your weight trend for 2–3 weeks.

Generally no — eating at BMR means eating less than even a sedentary person burns in a day, because you also burn calories through activity and digestion. Most people lose weight steadily by eating 300–500 calories below their TDEE (Total Daily Energy Expenditure: BMR plus activity), which is typically still well above BMR. Eating at or below BMR for extended periods can cause adaptive metabolic slowdown, muscle loss, and rebound eating.