BMR Calculator

About the BMR Calculator

Your Basal Metabolic Rate (BMR) is the number of calories your body needs to perform essential life-sustaining functions — breathing, circulating blood, regulating body temperature, producing cells, and maintaining organ function — while completely at rest. BMR typically accounts for 60–75% of your total daily energy expenditure (TDEE), which is why it is often used as a starting estimate for broader calorie planning.

This calculator compares four established equations. Mifflin-St Jeor is commonly used as the default estimate for the general population. Harris-Benedict is older and still widely referenced. Katch-McArdle and Cunningham use lean body mass, which can be useful when body fat percentage is known and body composition differs meaningfully from the average assumed by weight-based formulas.

Once your BMR is estimated, the page applies an activity factor (1.2–1.9) to estimate TDEE — the total calories burned each day including exercise and general movement. From there, you can sketch out rough calorie ranges for maintenance, fat loss, or weight gain, then refine them from real-world results.

When This Page Helps

Knowing your BMR helps turn a vague calorie goal into a more structured estimate. Whether your goal is maintenance, fat loss, or weight gain, comparing several equations side by side gives you a starting range that can then be adjusted from your weight trend, appetite, and training response.

How to Use the Inputs

1. Enter your age in years — BMR decreases approximately 1–2% per decade after age 20.
2. Select your biological sex, as the equations have sex-specific coefficients.
3. Enter your weight and select kg or lbs.
4. Enter your height and select cm or inches.
5. Optionally enter your body fat percentage to enable the Katch-McArdle and Cunningham equations.
6. Select your activity level to calculate TDEE from your BMR.
7. Compare results across formulas and use the calorie goals table to plan your diet.

Example Calculation

Tips & Best Practices

• Use Mifflin-St Jeor as a practical default unless you know your body fat percentage.
• Track your weight weekly for 2–4 weeks using the calculated TDEE, then adjust calories up or down by 100–200 kcal based on actual trends.
• If you are obese, Katch-McArdle with measured body fat may be more accurate than Harris-Benedict.
• Resistance training increases lean mass and can raise BMR over time — consider it part of your metabolic strategy.
• Very low-calorie intakes are better reviewed with a clinician or dietitian rather than set from a calculator alone.
• NEAT (fidgeting, walking, standing) can account for 200–900 kcal/day — it matters more than you think.

How BMR Equations Were Developed

The original Harris-Benedict equations were published in 1919 and later revised in 1984. Mifflin-St Jeor was developed in 1990 and is widely used in modern nutrition calculators. Katch-McArdle and Cunningham were built around lean body mass, which is why they are often more useful when body-composition data is available.

Why Estimates Still Need Adjustment

BMR equations describe average resting energy use from a limited set of inputs. They do not capture every factor that can influence energy expenditure, such as medication use, illness, hormones, spontaneous movement, or major differences in lean mass. That is why calculator results work best as starting estimates rather than fixed calorie prescriptions.

Beyond BMR: Components of TDEE

TDEE includes BMR plus the thermic effect of food, exercise activity, and non-exercise activity. Non-exercise activity is often the most variable part, which is why two people with the same BMR can still maintain weight on noticeably different calorie intakes.

Frequently Asked Questions

• What is the difference between BMR and RMR?

  BMR is measured under strict resting conditions (12-hour fast, complete rest, thermoneutral environment). RMR (Resting Metabolic Rate) is measured under less strict conditions and is typically 10–20% higher than BMR. Many formula-based estimators are closer to RMR, even when they are labeled as BMR.

• Which BMR formula is the most accurate?

  Mifflin-St Jeor is commonly used as a general-purpose starting estimate for adults. If you know your body fat percentage, Katch-McArdle may fit some leaner or more muscular individuals better because it uses lean body mass instead of total weight.

• Does BMR change with age?

  Yes. BMR decreases approximately 1–2% per decade after age 20, primarily due to loss of muscle mass (sarcopenia). Resistance training can slow this decline by preserving lean tissue.

• How does muscle mass affect BMR?

  Muscle tissue is metabolically active and burns about 6 kcal/lb/day at rest, compared to fat tissue which burns about 2 kcal/lb. More muscle mass = higher BMR, which is why the Katch-McArdle formula using lean body mass can be more accurate for muscular individuals.

• Can I eat below my BMR to lose weight faster?

  Eating far below estimated energy needs for extended periods can increase the risk of fatigue, muscle loss, nutrient shortfalls, and other problems. Very low-calorie plans are better reviewed with a qualified clinician or dietitian than set from a calculator alone.

• How accurate are activity level multipliers?

  Activity multipliers are population averages and can vary by ±10–20% between individuals. Factors like NEAT (Non-Exercise Activity Thermogenesis), genetics, and metabolic adaptation affect actual TDEE. Use the calculated TDEE as a starting point, then adjust based on real-world weight trends over 2–4 weeks.