Free BMR Calculator – Basal Metabolic Rate

What Is Basal Metabolic Rate?

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain essential life functions — breathing, circulation, cell production, temperature regulation, brain activity, and organ function. It represents the minimum energy your body needs to survive if you did nothing but lie still in a temperature-controlled room all day.

BMR typically accounts for 60–70% of total daily energy expenditure (TDEE) in moderately active individuals. This means the majority of calories you burn each day are used not for exercise but for keeping your body alive. Understanding your BMR provides the foundation for all calorie-based planning because it sets the absolute floor below which sustained calorie restriction becomes physiologically risky.

The Mifflin-St Jeor Equation

This calculator uses the Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate predictive BMR formula for the general population. For men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161.

Worked example for a 35-year-old woman, 65 kg, 168 cm: BMR = (10 × 65) + (6.25 × 168) − (5 × 35) − 161 = 650 + 1050 − 175 − 161 = 1,364 kcal per day. This means her body burns approximately 1,364 kcal just to maintain basic functions at rest.

Alternative equations include the Harris-Benedict formula (1919, revised 1984), which tends to overestimate by about 5% compared to Mifflin-St Jeor, and the Katch-McArdle formula, which uses lean body mass instead of total weight and may be more accurate for muscular or lean individuals. PrimeMacros defaults to Mifflin-St Jeor as the most broadly validated option.

Factors That Influence BMR

Lean body mass is the strongest predictor of BMR — more muscle means higher resting energy expenditure. This is why resistance training is valuable even for fat loss: building or maintaining muscle directly increases the number of calories burned at rest. Every kilogram of muscle burns approximately 13 kcal per day at rest, compared to about 4.5 kcal per kilogram of fat.

Age reduces BMR by roughly 1–2% per decade after age 20, primarily because of gradual muscle loss (sarcopenia). Sex also influences BMR: men typically have higher BMR than women of similar height and weight due to greater lean mass. Genetics, thyroid function, and hormonal status also play roles — conditions like hypothyroidism can lower BMR by 10–15%.

Sustained calorie restriction can lower BMR beyond what weight loss alone would predict, a phenomenon called metabolic adaptation or adaptive thermogenesis. Studies on contestants from extreme weight-loss programs found BMR reductions of 500+ kcal below expected values even years after the diet ended. More moderate deficits (300–500 kcal) and periodic diet breaks help minimize this effect.

From BMR to TDEE: The Complete Picture

BMR alone does not tell you how many calories to eat — you need to multiply it by an activity factor to get your TDEE. Common PAL (Physical Activity Level) multipliers are: Sedentary 1.2, Lightly Active 1.375, Moderately Active 1.55, Very Active 1.725, and Extra Active 1.9. Using the 35-year-old woman from above: TDEE = 1,364 × 1.55 = 2,114 kcal for moderate activity.

Never eat below your BMR for extended periods without medical supervision. Doing so risks metabolic slowdown, muscle loss, nutrient deficiency, hormonal disruption, and impaired immune function. If your fat loss target requires eating below BMR, it is a sign that the deficit is too aggressive and should be moderated.

BMR estimates by age and sex (75 kg, 175 cm)

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