About This Tool
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain essential life functions: breathing, blood circulation, cell repair, hormone production, and temperature regulation. If you did nothing but lie in bed for 24 hours, your BMR represents the energy cost of simply keeping your body alive. For most adults, BMR accounts for 60 to 75 percent of total daily calorie expenditure, making it the single largest component of your energy budget. This calculator computes your BMR using two established formulas side by side: the Mifflin-St Jeor equation (1990) and the Harris-Benedict equation (originally published in 1919, revised in 1984). Showing both results allows you to see the range of estimates and understand that no single formula is perfectly precise. The Mifflin-St Jeor equation is currently recommended by the American Dietetic Association as the most accurate for estimating resting metabolic rate in both normal-weight and overweight individuals. Knowing your BMR is the first step in building an evidence-based nutrition plan. By multiplying BMR by an activity factor, you can estimate your Total Daily Energy Expenditure (TDEE), which tells you how many calories you burn in a typical day including exercise and daily movement. From there, you can set specific calorie targets for weight loss, maintenance, or gain. This calculator includes an activity multiplier reference table so you can estimate TDEE directly from your BMR result.
Mifflin-St Jeor vs. Harris-Benedict
Both formulas estimate BMR from the same four variables (weight, height, age, and gender), but they use different coefficients and produce slightly different results:
Mifflin-St Jeor (1990):
- Men: 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5
- Women: 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161
Harris-Benedict (Revised 1984):
- Men: 88.362 + 13.397 x weight(kg) + 4.799 x height(cm) - 5.677 x age
- Women: 447.593 + 9.247 x weight(kg) + 3.098 x height(cm) - 4.330 x age
The Mifflin-St Jeor equation was developed from a more recent and diverse study population, which is why it tends to be more accurate for modern adults. The Harris-Benedict equation, despite being over a century old, remains widely used and typically produces results within 5 to 10 percent of Mifflin-St Jeor. The difference is usually 50 to 150 calories, which is well within the margin of error for any BMR estimation method.
What Affects Your BMR
Several factors influence your basal metabolic rate beyond the four variables used in these equations:
- Body composition: Muscle tissue burns roughly 6 calories per pound per day at rest, while fat tissue burns about 2 calories per pound. Two people with identical height, weight, age, and gender can have different BMRs if one has more lean muscle mass.
- Genetics: Inherited metabolic variation can account for differences of 200 to 300 calories between individuals with otherwise similar profiles. Some people naturally run "hotter" metabolically.
- Thyroid function: The thyroid gland regulates metabolic rate through hormones T3 and T4. Hypothyroidism (underactive thyroid) lowers BMR, while hyperthyroidism raises it.
- Environmental temperature: Exposure to cold increases BMR as the body works harder to maintain core temperature. This is the principle behind cold exposure research, though the calorie impact for most people is modest.
- Caffeine and stimulants: Caffeine temporarily increases metabolic rate by 3 to 11 percent. The effect is dose-dependent and diminishes with habitual use.
- Hormonal status: Testosterone, growth hormone, and cortisol all influence metabolic rate. Changes during menstruation, pregnancy, and menopause can shift BMR noticeably.
From BMR to Daily Calorie Needs
BMR alone does not tell you how many calories to eat because it excludes all physical activity and the energy cost of digestion. To estimate total daily energy expenditure (TDEE), multiply your BMR by an activity factor:
- Sedentary (x1.2): Desk job, minimal daily movement
- Lightly Active (x1.375): Light exercise 1-3 days per week
- Moderately Active (x1.55): Moderate exercise 3-5 days per week
- Active (x1.725): Hard exercise 6-7 days per week
- Very Active (x1.9): Intense daily training plus physical job
For example, a person with a Mifflin-St Jeor BMR of 1,700 calories who exercises moderately would have an estimated TDEE of 1,700 x 1.55 = 2,635 calories. This is the number of calories needed to maintain current weight. Eating below this number creates a deficit for fat loss; eating above it creates a surplus for muscle gain.
How to Increase Your BMR
While you cannot change your age, height, or genetic predisposition, several strategies can raise your basal metabolic rate:
- Build lean muscle: Resistance training (weightlifting, bodyweight exercises, resistance bands) increases muscle mass, which permanently raises BMR. Adding 5 pounds of muscle increases resting calorie burn by approximately 30 to 35 calories per day.
- Eat adequate protein: Protein has the highest thermic effect of food (20 to 30 percent of calories consumed are used just for digestion), compared to carbohydrates (5 to 10%) and fat (0 to 3%). A high-protein diet also supports muscle retention during calorie deficits.
- Avoid prolonged extreme diets: Severe calorie restriction causes metabolic adaptation, where the body lowers BMR to conserve energy. Moderate deficits (250 to 500 calories below TDEE) with periodic diet breaks minimize this effect.
- Get quality sleep: Sleep deprivation reduces BMR and increases hunger hormones. Aim for 7 to 9 hours of uninterrupted sleep per night.
- Stay hydrated: Studies show that drinking 500 mL of water can increase metabolic rate by 24 to 30 percent for about an hour. The effect is modest but adds up over time.
Frequently Asked Questions
Which BMR formula should I use?
The Mifflin-St Jeor equation is generally recommended by dietitians and the American Dietetic Association because it was developed from a more recent study population and tends to be more accurate across different body types. The Harris-Benedict equation is still useful as a second data point. If the two formulas give you substantially different results (more than 200 calories apart), your true BMR is likely somewhere in between. Use the Mifflin-St Jeor value as your primary reference.
Is BMR the same as resting metabolic rate (RMR)?
Not exactly, but they are closely related. BMR is measured under strict laboratory conditions after 12 hours of fasting and 8 hours of sleep, in a temperature-controlled room, while lying completely still. RMR is measured under less stringent conditions and tends to be about 10 to 20 percent higher than BMR because it includes some minimal activity. In practice, most calculators (including this one) use the terms somewhat interchangeably, and the formulas are close enough for nutritional planning purposes.
Why do men have a higher BMR than women?
On average, men have more lean muscle mass and less body fat than women of similar height and weight. Since muscle tissue is more metabolically active than fat tissue, this composition difference results in a higher resting metabolic rate. The Mifflin-St Jeor equation accounts for this by adding 5 calories for men and subtracting 161 for women, creating a 166-calorie difference when all other variables are equal. Hormonal differences, particularly higher testosterone levels in men, also contribute to greater lean mass and higher BMR.
Does BMR decrease with age?
Yes. Both the Mifflin-St Jeor and Harris-Benedict formulas subtract calories for each year of age. The primary reason is that adults lose approximately 3 to 8 percent of muscle mass per decade after age 30, a process called sarcopenia. Less muscle mass means lower resting calorie expenditure. However, regular resistance training can significantly slow this decline. Studies show that older adults who strength train 2 to 3 times per week can maintain or even increase their muscle mass, partially offsetting the age-related BMR reduction.
Can I measure BMR precisely at home?
True BMR measurement requires indirect calorimetry, a clinical test that analyzes the oxygen you inhale and the carbon dioxide you exhale to calculate your exact energy expenditure at rest. This is typically done in a hospital or research lab setting. At-home estimates using equations like Mifflin-St Jeor are accurate within about 10 percent for most people, which is sufficient for practical nutrition planning. Some fitness centers offer RMR testing with portable metabolic carts, which provides a step up in accuracy from equations alone.
How does body fat percentage affect BMR accuracy?
The standard BMR equations use total body weight without distinguishing between fat and muscle. This means they may overestimate BMR for people with very high body fat percentages and underestimate it for muscular individuals with low body fat. The Katch-McArdle formula addresses this by using lean body mass instead of total weight: BMR = 370 + (21.6 x lean mass in kg). If you know your body fat percentage, you can calculate lean mass as total weight x (1 - body fat %) and use the Katch-McArdle formula for a potentially more accurate result.