BMR Calculator — Free Online Basal Metabolic Rate Tool

How to Use the BMR Calculator

1. Select your preferred unit system: Click the toggle at the top to switch between Metric (kilograms and centimeters) and Imperial (pounds, feet, and inches). The calculator adjusts all input fields based on your selection. Metric is the international standard, while Imperial is commonly used in the United States.
2. Enter your age: Type your current age in years in the age field. BMR naturally decreases with age at a rate of roughly 1% to 2% per decade after age 20, primarily due to muscle mass loss. Accurate age input is essential for a reliable BMR estimate. The calculator accepts ages from 15 to 120 years.
3. Select your biological sex: Choose Male or Female. Males typically have a higher BMR than females of the same age, height, and weight because of greater average muscle mass. The Mifflin-St Jeor equation uses different constants for each sex to account for these physiological differences.
4. Enter your weight and height: Input your current weight and height using the fields appropriate for your selected unit system. For the most consistent results, weigh yourself at the same time each day, ideally in the morning before eating. The calculator accepts weights from 20 to 300 kg (44 to 660 lbs) and heights from 50 to 300 cm (1 ft 8 in to 9 ft 10 in).
5. Review your results: The results panel immediately displays your calculated BMR in calories per day, along with hourly calorie burn estimates for sleeping, resting, and light activity. Below those values, you will see your estimated total daily calorie needs at five different activity levels, from sedentary to very active. All results update automatically when you change any input.

The calculator provides default values (30-year-old male, 75 kg, 175 cm) so you can see results immediately. Simply replace these with your own measurements for a personalized BMR estimate.

BMR Formula and Calculation Method

This calculator uses the Mifflin-St Jeor equation, which a 2005 systematic review in the Journal of the American Dietetic Association identified as the most accurate predictive equation for estimating resting metabolic rate in both normal-weight and obese adults.

Variables Explained

• Weight (kg): Your body weight in kilograms. Each kilogram of body weight contributes 10 calories to your daily BMR. This is the most heavily weighted variable in the equation because larger bodies require more energy for basic metabolic processes.
• Height (cm): Your height in centimeters. Each centimeter of height adds 6.25 calories to your BMR. Taller individuals have more body surface area and generally more lean tissue, which increases energy requirements.
• Age (years): Your age in years. Each additional year reduces BMR by 5 calories, reflecting the natural decline in metabolic rate that occurs with aging due to progressive loss of lean muscle mass.
• Sex constant: Males receive a +5 adjustment while females receive a -161 adjustment. This 166-calorie difference accounts for the average differences in body composition between biological sexes, as males typically carry more metabolically active muscle tissue.

Step-by-Step Calculation Example

Consider a 30-year-old male who weighs 80 kg and is 178 cm tall:

1. Weight component: 10 x 80 = 800
2. Height component: 6.25 x 178 = 1,112.5
3. Age component: 5 x 30 = 150
4. Sex constant (male): +5
5. BMR = 800 + 1,112.5 - 150 + 5 = 1,768 calories/day

This means the body of a 30-year-old male at 80 kg and 178 cm burns approximately 1,768 calories per day at complete rest. This is the minimum energy needed for vital organ function, breathing, temperature regulation, and cellular maintenance. Any physical activity adds calories on top of this baseline.

Practical Examples

Example 1: Sarah Plans a Weight Loss Diet

Sarah is a 35-year-old woman who weighs 72 kg (159 lbs) and is 165 cm (5 feet 5 inches) tall. She works a desk job and walks three times a week. She wants to understand her calorie needs to lose weight safely:

• BMR = 10(72) + 6.25(165) - 5(35) - 161 = 720 + 1,031.25 - 175 - 161 = 1,415 calories/day
• With light activity (x1.375): TDEE = 1,415 x 1.375 = 1,946 calories/day
• Weight loss target (-500 cal): 1,446 calories/day

Sarah learns that her body burns about 1,415 calories just to stay alive. With her light activity level, she needs approximately 1,946 calories daily to maintain her current weight. To lose about one pound per week, she could aim for roughly 1,446 calories daily, which is above the recommended minimum of 1,200 calories for women. She can further refine her plan using the macro calculator to determine her ideal protein, carb, and fat split.

Example 2: Marcus Optimizes Athletic Performance

Marcus is a 24-year-old male who weighs 85 kg and is 183 cm tall. He trains six days per week, combining weight training and cardio. He wants to know his baseline calorie needs for muscle building:

• BMR = 10(85) + 6.25(183) - 5(24) + 5 = 850 + 1,143.75 - 120 + 5 = 1,879 calories/day
• With high activity (x1.725): TDEE = 1,879 x 1.725 = 3,241 calories/day
• Muscle gain target (+500 cal): 3,741 calories/day

Marcus discovers his high BMR of 1,879 calories reflects his larger frame and young age. With intense training, he needs over 3,200 calories just to maintain weight. To support muscle growth, he should aim for approximately 3,741 calories daily. He uses the protein calculator to determine that at 1.5-2.0 g/kg body weight, he needs 128 to 170 grams of protein daily as an athlete.

Example 3: Helen Understands Age-Related Changes

Helen is a 62-year-old woman who weighs 65 kg and is 162 cm tall. She noticed she gains weight more easily now than in her 30s despite eating similar amounts. She uses the BMR calculator to understand why:

• BMR at age 62: 10(65) + 6.25(162) - 5(62) - 161 = 650 + 1,012.5 - 310 - 161 = 1,192 calories/day
• BMR at age 32 (same weight/height): 10(65) + 6.25(162) - 5(32) - 161 = 650 + 1,012.5 - 160 - 161 = 1,342 calories/day
• Difference: 1,342 - 1,192 = 150 fewer calories burned at rest per day

Helen discovers that aging alone has reduced her BMR by 150 calories per day compared to her 30s. Over a year, that represents approximately 54,750 calories, equivalent to about 15.6 pounds of body weight. This explains her gradual weight gain. Her doctor recommends strength training two to three times per week to preserve muscle mass and help maintain her metabolic rate as she ages.

Example 4: David Compares Activity Level Impact

David is a 28-year-old male weighing 78 kg at 180 cm. He recently started a new office job after years of physical labor and wants to understand how his reduced activity changes his calorie needs:

• BMR = 10(78) + 6.25(180) - 5(28) + 5 = 780 + 1,125 - 140 + 5 = 1,770 calories/day
• Previous job (active, x1.725): 1,770 x 1.725 = 3,053 calories/day
• New job (sedentary, x1.2): 1,770 x 1.2 = 2,124 calories/day
• Daily difference: 3,053 - 2,124 = 929 fewer calories needed

David realizes that switching from an active to a sedentary lifestyle reduces his daily calorie needs by nearly 930 calories. If he continues eating the same amount, he could gain roughly 1.9 pounds per week. He decides to use the TDEE calculator regularly and adjust his nutrition to match his new activity level while adding structured exercise to his routine.

BMR Reference Table by Age, Sex, and Weight

Values calculated using the Mifflin-St Jeor equation. Individual results may vary based on body composition, genetics, and health conditions.

Tips and Complete Guide to Understanding BMR

The Science Behind Basal Metabolism

Your body never truly rests. Even during sleep, billions of biochemical reactions occur simultaneously to maintain life. Your brain alone consumes about 20% of your BMR despite being only 2% of body weight. Your liver processes nutrients and detoxifies substances, using roughly 25% of BMR energy. Skeletal muscles, even at rest, account for about 20% as they maintain tone and readiness. The heart beats approximately 100,000 times per day, and your kidneys filter about 180 liters of blood daily. All of these processes require continuous energy, which is what BMR measures.

Why the Mifflin-St Jeor Equation Is Preferred

Among the many equations developed to estimate BMR, the Mifflin-St Jeor formula has emerged as the most reliable for modern populations. The older Harris-Benedict equation, originally published in 1919 and revised in 1984, tends to overestimate calorie needs by 5% to 15%. The WHO/FAO equations are less accurate for individuals outside the weight ranges studied. The Katch-McArdle formula requires body fat percentage, which most people do not know accurately. The Mifflin-St Jeor equation, published in 1990, was developed using more recent data and diverse populations, making it the recommended choice by the Academy of Nutrition and Dietetics for most adults.

Using BMR for Weight Management

Understanding your BMR is the foundation of effective weight management. Since BMR represents calories burned at rest, you can estimate your Total Daily Energy Expenditure (TDEE) by multiplying your BMR by an activity factor. The difference between your TDEE and calorie intake determines whether you gain, lose, or maintain weight. A deficit of 500 calories per day typically results in about one pound of weight loss per week. However, never eat fewer than your BMR unless supervised by a healthcare professional, as consuming fewer calories than your body needs for basic functions can be harmful. The minimum safe calorie intake is generally 1,200 calories per day for women and 1,500 for men.

BMR and Exercise Planning

Athletes and fitness enthusiasts can use BMR to optimize training nutrition. During intense training phases, calorie intake should exceed TDEE to support recovery and performance. During rest periods, calorie needs decrease. Tracking BMR over time helps identify metabolic adaptation — a natural decrease in metabolic rate that occurs during prolonged calorie restriction. If you notice your weight loss plateauing despite maintaining a calorie deficit, metabolic adaptation may be the cause. Strategic diet breaks (eating at TDEE for 1-2 weeks) can help restore metabolic rate. For personalized exercise calorie planning, pair your BMR results with the TDEE calculator.

Common Mistakes to Avoid

• Eating below your BMR without medical supervision: Your BMR represents the minimum energy your body needs for survival. Consistently eating below this level can lead to muscle loss, nutrient deficiencies, hormonal disruption, and metabolic damage. Always maintain calorie intake above BMR unless directed by a healthcare provider.
• Confusing BMR with total calorie needs: BMR is only the resting component. Your actual daily calorie needs include energy for physical activity, digestion (thermic effect of food), and non-exercise activity thermogenesis (NEAT). Total needs are always higher than BMR, typically 20% to 90% higher depending on activity level.
• Not recalculating after significant weight changes: BMR changes as your body weight changes. If you lose or gain 5 kg or more, recalculate your BMR to ensure your nutrition plan remains appropriate for your current body.
• Ignoring body composition: Two people of the same weight and height can have very different BMRs if one has more muscle mass. If you are muscular, your actual BMR may be higher than the equation predicts. Consider body composition testing for a more complete picture.
• Treating the result as exact: BMR calculators provide estimates, not precise measurements. Individual variation of 10% or more is normal. Use your BMR as a starting point and adjust based on how your body actually responds over 2-4 weeks of consistent eating and activity patterns.

Frequently Asked Questions