BMR Calculator
Your Basal Metabolic Rate (BMR)
This is the number of calories your body burns at complete rest to maintain vital functions.
Total Daily Energy Expenditure (TDEE)
This is your estimated daily calorie requirement based on your activity level.
Calorie Guidance for Different Goals
How to Use This Calculator
Select your gender and preferred unit system (metric or imperial). Enter your age, weight, and height accurately. Choose the calculation formula—Mifflin-St Jeor is recommended for most people as research shows it provides the most accurate estimates. Select your activity level based on your typical weekly exercise routine. Click “Calculate BMR” to see your results, including your basal metabolic rate and personalised calorie recommendations for various goals.
What is Basal Metabolic Rate?
Basal Metabolic Rate (BMR) represents the minimum number of calories your body requires to maintain essential physiological functions whilst at complete rest. These functions include breathing, circulation, cell production, nutrient processing, and temperature regulation. Your BMR accounts for approximately 60-75% of your total daily energy expenditure, making it the largest component of your calorie burn.
BMR varies significantly between individuals based on several factors including age, gender, body composition, and genetics. Men typically have higher BMRs than women due to greater muscle mass, whilst BMR naturally decreases with age as muscle mass declines. People with more lean muscle tissue burn more calories at rest than those with higher body fat percentages.
Calculation Methods Explained
Men: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) – (5 × age) – 161
The Mifflin-St Jeor Equation is currently recognised as the most accurate prediction formula for BMR in healthy adults. Research published in medical journals demonstrates that this equation estimates resting metabolic rate within 10% of indirect calorimetry measurements in approximately 82% of cases, making it more reliable than older formulas.
Men: BMR = 13.397 × weight(kg) + 4.799 × height(cm) – 5.677 × age + 88.362
Women: BMR = 9.247 × weight(kg) + 3.098 × height(cm) – 4.330 × age + 447.593
The Harris-Benedict Equation was originally developed in 1919 and revised in 1984. Whilst widely used historically, studies show it tends to overestimate BMR by approximately 5% compared to measured values, particularly in overweight and obese individuals.
Activity Level Multipliers
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Little or no exercise, desk job |
| Lightly Active | 1.375 | Light exercise or sports 1-3 days per week |
| Moderately Active | 1.55 | Moderate exercise or sports 3-5 days per week |
| Very Active | 1.725 | Hard exercise or sports 6-7 days per week |
| Extra Active | 1.9 | Very hard exercise and physical job or training twice daily |
Choose your activity level honestly for accurate results. Many people overestimate their activity level. Sedentary applies even if you walk occasionally—it refers to structured exercise. If uncertain, choose the lower category to avoid overestimating calorie needs.
Factors Affecting Your BMR
Age
BMR decreases approximately 1-2% per decade after age 20. This decline occurs primarily due to loss of muscle mass and hormonal changes. Maintaining muscle through resistance training can help preserve BMR.
Gender
Men typically have BMRs 5-10% higher than women of the same age and weight. This difference stems from higher testosterone levels and greater proportion of muscle mass to body fat.
Body Composition
Muscle tissue burns approximately 13 calories per kilogram daily, whilst fat tissue burns only 4.5 calories. Higher muscle mass directly increases BMR, making strength training valuable for metabolic health.
Genetics
Genetic factors can cause BMR variations up to 20-30% between individuals of similar size and composition. Some people naturally have more efficient metabolisms than others.
Hormones
Thyroid hormones significantly regulate metabolic rate. Hypothyroidism can reduce BMR by 15-40%, whilst hyperthyroidism can increase it by 50-100%. Medical conditions affecting hormones require professional assessment.
Temperature
Environmental temperature affects energy expenditure. Cold exposure increases calorie burn as the body works to maintain core temperature. Fever also elevates BMR significantly—each degree Celsius increases metabolism by approximately 10%.
Comparing Measurement Methods
| Method | Accuracy | Cost | Availability |
|---|---|---|---|
| Indirect Calorimetry | Gold Standard (most accurate) | High (£75-150) | Specialist clinics only |
| Mifflin-St Jeor Formula | Within ±10% for 82% of people | Free | Widely available |
| Harris-Benedict Formula | Within ±10% for 64% of people | Free | Widely available |
| Bioelectrical Impedance | Moderate (varies by device) | Moderate (£20-500) | Home scales or clinics |
Indirect calorimetry measures oxygen consumption and carbon dioxide production to calculate energy expenditure with the highest accuracy. However, this method requires expensive equipment and controlled laboratory conditions. Prediction equations like Mifflin-St Jeor offer practical alternatives with reasonable accuracy for most healthy adults planning dietary strategies.
Practical Applications
Weight Management
To lose weight, create a calorie deficit by consuming fewer calories than your TDEE. A deficit of 500 calories daily typically results in weight loss of approximately 0.5kg per week. Never eat below your BMR for extended periods, as this can slow metabolism and cause muscle loss.
Building Muscle
Muscle gain requires a calorie surplus—typically 10-20% above TDEE—combined with resistance training. Excessive surplus leads to fat gain, so moderate increases work best. Protein intake of 1.6-2.2g per kilogram body weight daily supports muscle synthesis.
Athletic Performance
Athletes often require significantly higher calorie intakes than standard activity multipliers suggest. Endurance athletes may need 1.5-2× their BMR or more depending on training volume. Individual monitoring and adjustment based on performance and recovery indicators proves essential.
Medical Conditions
Certain conditions affect metabolic rate. Thyroid disorders, PCOS, diabetes, and some medications alter BMR. If you have medical conditions or take medications, consult healthcare professionals before making dietary changes based on BMR calculations.
Common Questions
No. Your BMR represents calories burned at complete rest, but you burn additional calories through daily activities. Eating only your BMR could create too large a deficit. Instead, calculate your TDEE (BMR × activity level) and reduce that by 10-20% for sustainable weight loss.
Age-related BMR decline occurs primarily through sarcopenia—the progressive loss of muscle mass. Hormonal changes, reduced physical activity, and decreased organ size also contribute. Regular resistance training and adequate protein intake can significantly slow this decline.
Yes, though changes tend to be modest. Building muscle through strength training provides the most effective approach, as muscle tissue burns more calories than fat. High-intensity interval training may temporarily elevate metabolism. Adequate sleep, stress management, and avoiding extreme calorie restriction also support healthy metabolic function.
The Mifflin-St Jeor equation estimates BMR within ±10% of measured values for approximately 82% of healthy adults. Accuracy decreases in very lean individuals, obese individuals, elderly people, and those with medical conditions. Use calculated BMR as a starting point and adjust based on real-world results over 2-4 weeks.
No. Research consistently shows that meal frequency does not significantly affect total daily energy expenditure. Whether you eat three large meals or six small meals, the thermic effect of food remains similar. Choose an eating pattern that suits your lifestyle and helps manage hunger.
Metabolic adaptation occurs during calorie restriction, with BMR decreasing beyond what body composition changes predict. This adaptation typically ranges from 5-15%. However, it doesn’t prevent weight loss—it may slow the rate. Taking diet breaks and minimising extreme deficits can help mitigate adaptation.
The Mifflin-St Jeor and Harris-Benedict equations don’t account for body composition, which limits accuracy for very lean or obese individuals. The Katch-McArdle formula uses lean body mass if you know your body fat percentage, potentially providing better accuracy for people outside average body composition ranges.
Limitations and Considerations
BMR calculators provide estimates based on population averages and may not perfectly reflect individual metabolic rates. Genetic variations, previous dieting history, medication use, hormonal status, and medical conditions all influence actual metabolic rate. People who have repeatedly dieted may experience metabolic adaptation that reduces BMR below predicted values.
The equations work best for healthy adults of average body composition. Accuracy decreases for children, elderly individuals, pregnant or breastfeeding women, athletes with very high muscle mass, and obese individuals. These groups should seek personalised guidance from healthcare professionals or registered dietitians.
When using BMR calculations for dietary planning, monitor real-world outcomes over several weeks. If weight change doesn’t match expectations, adjust calorie intake by 100-200 calories and reassess. Individual variation means some trial and adjustment typically proves necessary.
References
- Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. The American Journal of Clinical Nutrition. 1990;51(2):241-247. doi:10.1093/ajcn/51.2.241
- Harris JA, Benedict FG. A Biometric Study of Human Basal Metabolism. Proceedings of the National Academy of Sciences. 1918;4(12):370-373. doi:10.1073/pnas.4.12.370
- Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. The American Journal of Clinical Nutrition. 1984;40(1):168-182. doi:10.1093/ajcn/40.1.168
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of Predictive Equations for Resting Metabolic Rate in Healthy Nonobese and Obese Adults: A Systematic Review. Journal of the American Dietetic Association. 2005;105(5):775-789. doi:10.1016/j.jada.2005.02.005
- Sabounchi NS, Rahmandad H, Ammerman A. Best-fitting prediction equations for basal metabolic rate: informing obesity interventions in diverse populations. International Journal of Obesity. 2013;37(10):1364-1370. doi:10.1038/ijo.2012.218
- Johnstone AM, Murison SD, Duncan JS, Rance KA, Speakman JR. Factors influencing variation in basal metabolic rate include fat-free mass, fat mass, age, and circulating thyroxine but not sex, circulating leptin, or triiodothyronine. The American Journal of Clinical Nutrition. 2005;82(5):941-948. doi:10.1093/ajcn/82.5.941
- Müller MJ, Bosy-Westphal A, Klaus S, et al. World Health Organization equations have shortcomings for predicting resting energy expenditure in persons from a modern, affluent population: generation of a new reference standard from a retrospective analysis of a German database of resting energy expenditure. The American Journal of Clinical Nutrition. 2004;80(5):1379-1390. doi:10.1093/ajcn/80.5.1379
- Galgani J, Ravussin E. Energy metabolism, fuel selection and body weight regulation. International Journal of Obesity. 2008;32(Suppl 7):S109-S119. doi:10.1038/ijo.2008.246