Calculate your Basal Metabolic Rate (BMR) — the number of calories your body burns at rest just to maintain vital functions. Also calculates your Total Daily Energy Expenditure (TDEE) based on your activity level. Uses the accurate Mifflin-St Jeor equation.
Basal Metabolic Rate (BMR) represents the minimum number of calories your body requires to perform essential physiological functions while at complete rest. This includes breathing, circulating blood, controlling body temperature, cell growth and repair, brain function, and processing nutrients. BMR accounts for 60-75% of your total daily calorie expenditure for most people, making it the largest component of energy use even if you're sedentary. Understanding your BMR helps establish a baseline for weight management: if you consume fewer calories than your BMR long-term, your body may enter starvation mode, slowing metabolism and catabolizing muscle tissue for energy. For weight loss, aim to eat between BMR and TDEE (total daily energy expenditure, which includes activity) to create a sustainable calorie deficit without metabolic adaptation. BMR varies significantly between individuals based on age, sex, weight, height, and body composition. Men typically have higher BMRs than women due to greater muscle mass. Muscle tissue burns approximately 6 calories per pound daily at rest, whilst fat burns only 2 calories per pound, explaining why muscular individuals have higher BMRs. As you age, BMR naturally declines approximately 1-2% per decade after age 20 due to muscle loss and hormonal changes, meaning a 60-year-old burns roughly 200-300 fewer calories daily at rest than they did at 20 with the same body weight.
The Mifflin-St Jeor equation is considered the most accurate BMR formula for the general population, developed in 1990 to replace the outdated Harris-Benedict equation from 1919. Studies show it predicts BMR within approximately 10% accuracy for most individuals, which translates to about 150-200 calories margin of error for an average person. However, no equation is perfect as they can't account for individual metabolic variations. Factors not captured include thyroid function (hypothyroidism can reduce BMR by 20-40%), genetics (some people are naturally 200-300 calories more or less efficient), medications (certain antidepressants, steroids, and diabetes medications affect metabolism), previous dieting history (yo-yo dieting can suppress BMR by 5-10%), caffeine and stimulant use (temporarily increases BMR by 3-11%), environmental temperature (cold exposure increases BMR as the body works to maintain core temperature), and current health status (illness, inflammation, or stress increase metabolic rate). Body composition matters significantly: the equations use total weight, but someone with 30% body fat versus 15% body fat at the same weight will have different BMRs due to muscle mass differences. For most accurate results, consider DEXA scan or metabolic testing at a sports science lab which measures actual oxygen consumption and CO2 production to determine true metabolic rate, though this costs £80-150. For general diet planning, the Mifflin-St Jeor calculation provides sufficient accuracy.
BMR (Basal Metabolic Rate) is calories burned at complete rest performing only vital functions. TDEE (Total Daily Energy Expenditure) is total calories burned including all activity throughout the day. TDEE = BMR × Activity Multiplier. The activity multipliers are: Sedentary (1.2) for desk jobs with no deliberate exercise, Light (1.375) for light exercise or sports 1-3 days weekly plus normal daily activity, Moderate (1.55) for moderate exercise 3-5 days weekly, Active (1.725) for intense exercise 6-7 days weekly or physical job, Very Active (1.9) for very intense exercise twice daily or extremely physical job like construction or professional athletics. TDEE includes Non-Exercise Activity Thermogenesis (NEAT) — the calories burned through daily activities like fidgeting, walking to the car, climbing stairs, typing, standing, and general movement that isn't formal exercise. NEAT can vary by 2,000+ calories daily between individuals and explains why some people seem to eat anything without gaining weight. For weight management, create calorie deficits or surpluses relative to TDEE, not BMR. A 500 calorie daily deficit from TDEE results in approximately 1 pound weekly fat loss (3,500 calorie deficit per pound). However, never eat below your BMR for extended periods as this triggers metabolic adaptations, muscle loss, hormonal disruptions, decreased bone density, impaired immune function, loss of menstrual cycle in women, and significantly reduced energy levels.
Yes, but it requires strategic effort focused on muscle building. Each pound of muscle added increases BMR by approximately 6 calories daily, so gaining 10 pounds of muscle adds 60 daily calories (21,900 yearly). While this seems modest, combined with NEAT increases from being more muscular and active, the total impact is significant. Resistance training 3-4 times weekly targeting all major muscle groups stimulates muscle protein synthesis and gradually increases lean mass. Consume adequate protein (1.6-2.2g per kg bodyweight) to support muscle growth. Progressive overload — consistently increasing weight, reps, or volume — is essential for continued muscle development. High-Intensity Interval Training (HIIT) temporarily elevates BMR for 24-72 hours post-exercise through Excess Post-Exercise Oxygen Consumption (EPOC), though this doesn't permanently raise baseline BMR. Avoid extreme calorie restriction which decreases BMR by 5-15% through metabolic adaptation as the body conserves energy. Adequate sleep (7-9 hours) maintains healthy metabolism, while chronic sleep deprivation can reduce BMR by 2-8%. Stay well-hydrated as even 2% dehydration can slow metabolism temporarily. Cold exposure through cold showers, outdoor winter activities, or keeping rooms cooler (around 18°C) can increase daily calorie burn by 100-200 calories as the body works to maintain core temperature, though this doesn't directly raise BMR itself. Certain supplements claim to boost metabolism — green tea extract and caffeine can temporarily increase metabolic rate by 3-8%, but tolerance develops quickly making long-term impact minimal. Focus on sustainable muscle building through resistance training rather than quick fixes or supplements.
Using Mifflin-St Jeor: BMR = (10 × 75) + (6.25 × 175) - (5 × 30) + 5 = 750 + 1,093.75 - 150 + 5 = 1,699 calories/day. TDEE sedentary: 1,699 × 1.2 = 2,039 cal. TDEE moderate exercise: 1,699 × 1.55 = 2,633 cal. For weight loss at moderate activity, eat 2,133-2,383 calories (500-250 cal deficit). For muscle gain, eat 2,883-3,133 calories (250-500 cal surplus).
BMR = (10 × 60) + (6.25 × 165) - (5 × 25) - 161 = 600 + 1,031.25 - 125 - 161 = 1,345 calories/day. TDEE light activity: 1,345 × 1.375 = 1,849 cal. TDEE active: 1,345 × 1.725 = 2,320 cal. This demonstrates the significant difference activity level makes — nearly 500 calories between light and active. For sustainable fat loss at active level, eat 1,820-2,070 calories.
BMR = (10 × 90) + (6.25 × 180) - (5 × 50) + 5 = 900 + 1,125 - 250 + 5 = 1,780 calories/day. Despite weighing 15kg more than Example 1's 30-year-old, his BMR is only 81 calories higher due to age-related metabolic decline. This illustrates why maintaining weight becomes harder with age — a 50-year-old needs approximately 200-300 fewer calories than a 30-year-old of similar size. Resistance training becomes increasingly important to preserve muscle mass and BMR.
Track food intake accurately using apps like MyFitnessPal or Chronometer for 1-2 weeks to establish current consumption patterns before making changes. Weigh foods rather than guessing portions as visual estimates are typically 20-50% inaccurate. Calculate your TDEE based on honest activity assessment — most people overestimate activity levels and underestimate food intake. For fat loss, create a 300-500 daily calorie deficit from TDEE (not BMR), aiming for 0.5-1% bodyweight loss weekly. Faster loss increases muscle loss and triggers metabolic adaptation. For muscle gain, create a 250-500 calorie surplus from TDEE, gaining 0.25-0.5% bodyweight weekly. Monitor progress through weekly weigh-ins at the same time (morning, after toilet, before eating), taking weekly average rather than daily fluctuations which can vary 2-5 pounds due to water retention, digestion timing, glycogen storage, and sodium intake. Adjust calories every 2-3 weeks based on actual weight changes rather than predicted calculations, as BMR estimates have margins of error and your body may respond differently than average. Prioritize protein (25-30% of calories) to preserve muscle during fat loss and support muscle growth during gaining phases. Resistance training 3-4 times weekly prevents muscle loss during dieting and maximizes muscle gain during surplus. Don't compare yourself to others — someone your exact size may have 200-300 calorie different BMR due to genetics, muscle mass, or metabolic health. Focus on finding your personal maintenance calories through trial and error, then adjust from there. Remember weight management is 80% nutrition, 20% exercise — you cannot out-exercise a bad diet, but exercise optimizes body composition. Finally, sustainable progress takes months and years, not weeks. Be patient, consistent, and focus on building lifelong healthy habits rather than quick fixes.