BMR Calculator

Basal Metabolic Rate, commonly abbreviated as BMR, represents the minimum number of calories your body requires to sustain basic life-sustaining functions while at complete rest. These functions include breathing, blood circulation, cell production, nutrient processing, and maintaining body temperature. BMR accounts for roughly 60 to 75 percent of your total daily energy expenditure, making it the single largest component of the calories you burn each day.

How BMR Is Calculated

Several scientifically validated equations exist for estimating BMR. The Mifflin-St Jeor equation, published in 1990, is widely considered the most accurate for the general population. It uses weight, height, age, and sex to produce an estimate. The revised Harris-Benedict equation, an update of the original 1919 formula, provides similar results with slightly different coefficients. For individuals who know their body fat percentage, the Katch-McArdle formula offers an alternative approach that uses lean body mass instead of total body weight, which can be more accurate for very lean or very muscular individuals.

Our calculator computes all three formulas simultaneously so you can compare results and choose the estimate that best fits your situation. The Mifflin-St Jeor result is highlighted as the primary recommendation, as research published in the Journal of the American Dietetic Association found it to be the most reliable predictor of actual measured resting metabolic rate.

BMR vs. TDEE: What's the Difference?

BMR measures calories burned at complete rest — essentially what your body would need if you stayed in bed all day. Total Daily Energy Expenditure (TDEE) adds the calories burned through physical activity, the thermic effect of food (energy used to digest meals), and non-exercise activity thermogenesis (NEAT) such as fidgeting and walking. TDEE is the number you actually need for diet planning because it reflects your real-world calorie needs. To estimate your TDEE, multiply your BMR by an activity factor typically ranging from 1.2 for sedentary individuals to 1.9 for extremely active people.

Factors That Affect Your BMR

Several factors influence your Basal Metabolic Rate. Body composition is the most significant: muscle tissue is metabolically active and burns more calories at rest than fat tissue. This is why individuals with more muscle mass tend to have higher BMRs. Age plays a role as well — BMR typically decreases by about 1 to 2 percent per decade after age 20, largely due to gradual muscle loss (sarcopenia).

Sex affects BMR because men generally carry more muscle mass and less body fat than women. Genetics also contribute to metabolic rate variation between individuals. Hormonal factors, particularly thyroid function, can significantly impact BMR: an underactive thyroid (hypothyroidism) can lower BMR, while an overactive thyroid (hyperthyroidism) can raise it. Environmental temperature, stress, illness, and certain medications can also cause temporary fluctuations.

Using BMR for Diet Planning

Understanding your BMR is a critical first step in any evidence-based nutrition plan. For weight loss, you need to create a caloric deficit — consuming fewer calories than your TDEE. A common recommendation is a deficit of 500 calories per day, which translates to roughly half a kilogram (about one pound) of weight loss per week. However, it is important never to eat below your BMR for extended periods, as this can slow your metabolism, cause muscle loss, and lead to nutrient deficiencies.

For weight gain or muscle building, you need a caloric surplus above your TDEE. A moderate surplus of 250 to 500 calories per day, combined with resistance training, supports muscle growth while minimizing fat gain. For weight maintenance, aim to match your calorie intake to your TDEE. Regularly recalculating your BMR as your weight, age, or activity level changes ensures your nutrition plan stays aligned with your body's actual needs.

Limitations of BMR Estimates

All BMR equations produce estimates, not exact measurements. The gold standard for measuring BMR is indirect calorimetry, a clinical test that measures oxygen consumption and carbon dioxide production. Equation-based estimates can differ from measured values by 5 to 15 percent in either direction. Factors like genetics, hormonal status, medications, and metabolic adaptation from previous dieting can all cause your actual BMR to diverge from calculated values. Use your BMR estimate as a starting point and adjust based on real-world results over several weeks.