Calculate your Basal Metabolic Rate — the calories your body burns at complete rest.
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Basal Metabolic Rate (BMR) is the number of calories your body needs to maintain basic life functions — breathing, circulation, cell repair — while completely at rest. This calculator uses the Mifflin-St Jeor equation, considered the most reliable standard formula for estimating BMR in most adults. Your BMR is the foundation used to calculate total daily calorie needs once activity level is factored in. The most historically-used formula is the Harris-Benedict equation from 1919, though the newer Mifflin-St Jeor equation (1990) is now considered more accurate by most comparison studies and is what this calculator uses by default. BMR typically makes up 60-75% of total daily calorie burn for most people, which is why it's the essential starting point for any calorie or weight-loss calculation — without it, a meaningful daily calorie target can't be estimated. Since no simple at-home formula can perfectly replicate a metabolic lab test, treat the result as a solid working estimate rather than an exact figure — it becomes genuinely useful once it's combined with an activity multiplier to build a full daily calorie target, which is the practical reason most people look this number up in the first place.
Basal Metabolic Rate is the amount of energy your body needs just to stay alive at complete rest — keeping your heart beating, lungs breathing, brain active, and cells repairing themselves — with no digestion, movement, or exercise factored in at all. In a research lab, BMR is measured after 8 hours of sleep and 12 hours of fasting, lying perfectly still in a temperature-controlled room, which is why in everyday use the very similar Resting Metabolic Rate (RMR) is what's usually estimated instead — the two numbers differ by only a few percent for most people.
This calculator uses the Mifflin-St Jeor equation, published in 1990, which is now generally recommended over the older Harris-Benedict formula (from 1919, revised in 1984) because multiple validation studies have found Mifflin-St Jeor more accurate for the modern population, particularly for people with overweight or obesity, where the older formula tends to overestimate. Both formulas use the same basic inputs — weight, height, age, and biological sex — because these four variables explain most of the person-to-person variation in resting energy needs; sex is included because, on average, men carry more metabolically active muscle mass per kilogram of body weight than women at the same size.
BMR by itself isn't a target number to hit — it's the foundation for calculating total daily energy expenditure once activity is added on top (see the TDEE calculator), and it's the "floor" that most nutrition guidelines warn against eating below for extended periods, since chronically eating near or under BMR can trigger metabolic adaptation, muscle loss, and nutrient deficiencies rather than healthy fat loss.
It's also worth remembering that BMR naturally declines with age, largely because muscle mass — the body's most metabolically active tissue at rest — tends to decrease over the decades unless deliberately preserved through resistance training and adequate protein intake, which is one reason the same weight and height can produce a meaningfully lower BMR estimate at age 55 than at age 25.
No — BMR is calories at rest only, while TDEE adds your daily activity on top of BMR.
Muscle mass naturally declines with age unless actively maintained through resistance training, and since muscle burns more energy at rest than fat, this gradual muscle loss lowers BMR even at a stable body weight.
Yes, muscle tissue burns more calories at rest than fat tissue, so higher muscle mass generally means a higher BMR.