BMR Calculator: Your Basal Metabolic Rate
Your Basal Metabolic Rate is the number of calories your body burns just to stay alive. It powers your heartbeat, breathing, brain activity, and cellular repair — and it accounts for 60-70% of the calories you burn each day.
What Is Basal Metabolic Rate?
Basal Metabolic Rate (BMR) is the minimum number of calories your body requires to perform essential physiological functions at complete rest in a thermally neutral environment. This includes maintaining body temperature, powering your heart and lungs, supporting brain function, running your immune system, and repairing cells. BMR is measured under strict laboratory conditions — lying still, awake, in a fasted state, in a temperature-controlled room. For practical purposes, your Resting Metabolic Rate (RMR) is nearly identical and is what most equations estimate. BMR is the largest component of your Total Daily Energy Expenditure, typically representing 60-70% of all calories burned. This means that the majority of the calories you consume each day go toward simply keeping your body functioning — not exercise.
The Mifflin-St Jeor Equation
Published in 1990, the Mifflin-St Jeor equation is the gold standard for estimating BMR in the general population. It requires only four inputs: weight, height, age, and sex. Multiple validation studies have found it to be the most accurate predictive equation, outperforming older formulas like Harris-Benedict (1919) and the WHO equation.
- Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) + 5
- Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age) - 161
- Example: A 30-year-old man, 180 lbs (81.6 kg), 5'10" (177.8 cm) = 1,782 calories/day
The Katch-McArdle Formula
If you know your body fat percentage, the Katch-McArdle formula provides a more individualized estimate because it accounts for lean body mass directly. This is particularly useful for people at the extremes — very lean or very overweight individuals — where the Mifflin-St Jeor equation tends to under- or overestimate. The formula is straightforward: BMR = 370 + (21.6 x lean body mass in kg). To calculate lean mass, multiply your total weight in kg by (1 - body fat percentage as a decimal). For example, a 200-pound (90.7 kg) person at 25% body fat has 68 kg of lean mass, giving a BMR of 370 + (21.6 x 68) = 1,839 calories per day. The Katch-McArdle formula is sex-neutral because it relies on lean mass, which already differs between men and women.
Factors That Influence Your BMR
Several factors beyond the formula inputs significantly affect your metabolic rate. Muscle mass is the most modifiable — each pound of muscle burns approximately 6-7 calories per day at rest, compared to 2 calories for a pound of fat. This is why resistance training is crucial for long-term metabolic health. Age causes BMR to decline by roughly 1-2% per decade after age 20, primarily due to loss of muscle mass (sarcopenia). Genetics account for a 5-10% variance between individuals of the same size and composition. Thyroid function plays a direct role — hypothyroidism can reduce BMR by 10-15%. Chronic dieting and extreme calorie restriction can suppress BMR through adaptive thermogenesis, where the body becomes more metabolically efficient to conserve energy.
- Muscle mass: most significant modifiable factor
- Age: ~1-2% decline per decade after 20
- Genetics: 5-10% variance between similar individuals
- Thyroid function: can shift BMR by 10-15%
- Chronic dieting: may suppress BMR through adaptive thermogenesis
- Body temperature: fever increases BMR by ~7% per degree Fahrenheit
Why You Should Never Eat Below Your BMR
Your BMR represents the bare minimum energy your body needs for survival. Consistently eating below your BMR forces your body into conservation mode — metabolic rate drops, muscle is broken down for energy, hormone production decreases, immune function weakens, and cognitive performance suffers. This is not a sustainable fat loss strategy. Instead, calculate your TDEE and create a moderate deficit of 250-750 calories below that number, ensuring you always eat at or above your BMR. If your TDEE minus your desired deficit puts you below your BMR, the deficit is too aggressive and you should either reduce it or increase your activity level to raise your TDEE.
Frequently Asked Questions
BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are often used interchangeably, but there is a technical difference. BMR is measured under strict conditions: complete rest, fasted for 12+ hours, in a dark and temperature-controlled room. RMR is measured under more relaxed conditions and tends to be 5-10% higher than BMR. For practical purposes, the difference is negligible and both terms refer to the calories your body burns at rest.
The most effective way to increase your BMR is to build lean muscle mass through resistance training. Each pound of muscle burns roughly 6-7 calories per day at rest — modest per pound, but significant over 10-20 pounds of added muscle. Avoiding crash diets also helps, as severe calorie restriction suppresses metabolic rate. Adequate sleep (7-9 hours) and managing stress support healthy thyroid and hormonal function, which directly influence BMR. Staying well-hydrated and eating sufficient protein also contribute to a healthy metabolic rate.
Yes, but less dramatically than most people believe. A landmark 2021 study in Science found that metabolism remains relatively stable from age 20 to 60, declining only about 0.7% per year — and much of that decline is attributable to loss of muscle mass rather than an inherent metabolic slowdown. After 60, the decline accelerates to about 1.7% per year. The takeaway: maintaining muscle mass through resistance training is the best defense against age-related metabolic decline.
If you know your body fat percentage with reasonable accuracy (from a DEXA scan, body fat calipers, or Navy method), use the Katch-McArdle formula — it accounts for your individual lean mass and is more accurate at the extremes of body composition. If you do not know your body fat percentage, the Mifflin-St Jeor equation is the best general-purpose option and has been validated as the most accurate formula for the majority of people.
No. The idea that eating six small meals per day "stokes your metabolic fire" is a persistent myth. The Thermic Effect of Food (TEF) is proportional to total calorie intake, not meal frequency. Six 300-calorie meals and two 900-calorie meals produce the same total thermic effect. Meal frequency should be based on what helps you control hunger, maintain energy, and adhere to your calorie targets — not metabolic optimization.
Explore More
Related tools, medications, and guides