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Your Basal Metabolic Rate (BMR) is the calories your body burns at complete rest. Our free BMR calculator uses the Mifflin-St Jeor equation plus your activity level to estimate total daily energy expenditure (TDEE) — the starting point for any diet plan.
Our free BMR Calculator estimates your Basal Metabolic Rate - the number of calories your body burns at complete rest - using the scientifically validated Mifflin-St Jeor equation. It then multiplies your BMR by an activity factor to estimate your Total Daily Energy Expenditure (TDEE), the total calories you burn in a day. Together, these two numbers are the starting point for any weight-loss, weight-gain, or maintenance nutrition plan.
Tips for best results: recalculate your BMR every time your weight changes by more than 2-3 kg, since a lighter body burns fewer calories. If you are between two activity levels, choose the lower one - it is better to underestimate and adjust upward than to overeat based on an inflated number. Remember that BMR is an estimate; individual variation of 5-10% is normal, so treat the result as a starting point and adjust based on real-world progress over 2-3 weeks.
Your Basal Metabolic Rate (BMR) is the number of calories your body requires to maintain its most basic life-sustaining functions - breathing, circulating blood, regulating body temperature, growing and repairing cells, and powering the brain and organs - while you are at complete rest. Think of it as the energy cost of simply being alive. For most adults, BMR accounts for roughly 60-70% of total daily calorie burn, making it by far the largest component of your energy expenditure. A typical adult man might have a BMR around 1,700 kcal/day, while a typical adult woman might be around 1,400 kcal/day, though individual values vary widely with size, age, and body composition.
Several factors determine your BMR. Body size is the biggest: larger bodies have more cells to maintain and burn more energy. Muscle mass matters because muscle tissue is metabolically active - it burns roughly 6 kcal per pound per day at rest, compared to about 2 kcal per pound for fat tissue - which is why two people of the same weight can have different BMRs. Age reduces BMR gradually, typically 1-2% per decade after age 20, partly due to muscle loss. Gender plays a role because men on average carry more muscle mass. Other influences include genetics, thyroid hormone levels, prolonged dieting (which can suppress BMR through adaptive thermogenesis), and even cold exposure.
Your Total Daily Energy Expenditure (TDEE) builds on BMR by adding everything else you do. TDEE has four components: BMR (60-70%), the thermic effect of food (TEF) - the energy cost of digesting meals, about 10% of intake - exercise activity thermogenesis (EAT), your planned workouts, and non-exercise activity thermogenesis (NEAT), all the incidental movement of daily life like walking, fidgeting, and housework. Our calculator estimates TDEE with the standard formula TDEE = BMR x activity factor, where the factor ranges from 1.2 (sedentary) to 1.9 (very active). TDEE is the number that actually governs weight change: eat below it and you lose weight, eat above it and you gain.
The relationship between calories and body weight is straightforward thermodynamics. One kilogram of body fat stores approximately 7,700 kcal of energy. A daily deficit of 500 kcal therefore produces about 0.5 kg of fat loss per week (500 x 7 = 3,500 kcal, roughly half a kilo). This is why the classic advice - eat 500 kcal below your TDEE to lose half a kilo per week - works: it is arithmetic, not magic. The same math runs in reverse for muscle gain, where a modest surplus of 250-300 kcal per day supports growth while minimizing fat gain.
The Mifflin-St Jeor equation, published in 1990, is the formula recommended by the American Dietetic Association and most clinical guidelines because validation studies show it predicts resting metabolic rate more accurately than older equations like Harris-Benedict for most modern adults. Our calculator uses it exclusively.
For men: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(years) + 5. For women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(years) - 161. The weight term dominates because body mass drives energy needs; the height term captures body surface area; the age term reflects the gradual metabolic slowdown; and the final constant adjusts for average sex differences in body composition.
Worked Example 1 - Man: A 35-year-old man weighs 80 kg and is 180 cm tall. Plug into the men's formula: 10 x 80 = 800; 6.25 x 180 = 1,125; 5 x 35 = 175. Then BMR = 800 + 1,125 - 175 + 5 = 1,755 kcal/day. If he exercises moderately 3-5 days per week (activity factor 1.55), his TDEE = 1,755 x 1.55 = 2,720.25, or about 2,720 kcal/day.
Worked Example 2 - Woman: A 28-year-old woman weighs 65 kg and is 165 cm tall. Using the women's formula: 10 x 65 = 650; 6.25 x 165 = 1,031.25; 5 x 28 = 140. Then BMR = 650 + 1,031.25 - 140 - 161 = 1,380.25 kcal/day, about 1,380. With light activity 1-3 days per week (factor 1.375), her TDEE = 1,380.25 x 1.375 = 1,897.8, or about 1,898 kcal/day.
Notice how the formulas behave: gaining 10 kg of body weight adds exactly 100 kcal/day to BMR (10 x 10), while aging 10 years subtracts 50 kcal/day (5 x 10). These linear relationships make the equation easy to sanity-check and explain why maintaining muscle as you age is the single best defense against metabolic slowdown.
Consider Sarah, the 28-year-old woman from Worked Example 2: 65 kg, 165 cm, BMR of 1,380 kcal/day, lightly active with a TDEE of about 1,898 kcal/day. She wants to lose 6 kg over 12 weeks - a sustainable rate of 0.5 kg per week.
The math: 0.5 kg per week requires a weekly deficit of about 3,850 kcal (0.5 x 7,700), or 550 kcal per day. Subtracting from her TDEE: 1,898 - 550 = 1,348 kcal/day. But wait - that target is below her BMR of 1,380. Eating below BMR for extended periods risks muscle loss, hormonal disruption, and extreme hunger. The safe move is to raise the target to just above BMR - say 1,400 kcal/day - and accept a slightly slower loss of about 0.45 kg per week, or add two brisk 30-minute walks per week to lift TDEE instead. This is exactly the kind of trade-off our calculator helps you see: BMR is your floor, TDEE is your ceiling, and your diet target must live between them.
Sarah sets her target at 1,450 kcal/day, tracks her food with a kitchen scale for accuracy, and weighs herself each morning. After three weeks her average weekly loss is 0.4 kg - close to prediction. She holds the course, recalculates her BMR after losing 3 kg (her new BMR is about 1,350), and adjusts her target down by 30 kcal. Small, data-driven adjustments like this beat dramatic diet overhauls every time.
Now consider David, the 35-year-old man from Worked Example 1: 80 kg, 180 cm, BMR 1,755, moderately active TDEE of 2,720 kcal/day. He lifts weights four days a week and wants to build muscle while gaining minimal fat - a lean bulk.
Muscle growth requires energy surplus, but a large surplus mostly adds fat. Research supports a modest surplus of 250-300 kcal per day for lean gains. David's target: 2,720 + 300 = 3,020 kcal/day, which he rounds to 3,000. He aims for 160 g of protein daily (2 g per kg of body weight, the evidence-based target for muscle growth), fills the rest with carbs to fuel training and fats for hormones, and tracks his morning weight. The goal is a gain of about 0.25-0.5% of body weight per week - for David, 0.2-0.4 kg weekly.
After a month, David is up 1.2 kg and his lifts are progressing - the surplus is working. Had he gained 3 kg instead, that would signal too large a surplus, and he would trim 200 kcal. Had he gained nothing, he would add 200 kcal. The BMR and TDEE numbers from our calculator gave him a precise starting point; the scale and the mirror provide the feedback loop. Without that starting point, most people guess - and guessing is how bulks turn into fat-gain phases.
1. Eating below your BMR. The most dangerous mistake. Your BMR is the energy cost of keeping your organs running - chronically eating below it triggers adaptive thermogenesis (your body burns less), muscle loss, hormonal disruption, and rebound bingeing. Your calorie floor should always be at or above BMR; create deficits from the TDEE side instead.
2. Never recalculating after weight change. A 90 kg person and a 75 kg person have very different BMRs - about 150 kcal/day apart. If you lose 10 kg and keep eating at your old target, your deficit shrinks and weight loss stalls. Recalculate your BMR every 2-3 kg of weight change.
3. Overestimating activity level. Studies show most people overestimate their exercise by 30-50%. A desk worker who jogs twice a week is lightly active, not moderately. When in doubt, choose the lower activity level - you can always eat a little more if you are losing too fast, but overeating based on an inflated TDEE stalls fat loss for weeks before you notice.
4. Ignoring NEAT and food logging errors. Two hidden variables wreck predictions: NEAT (non-exercise activity) can vary by 300+ kcal/day between a fidgety person and a still one, and food labels can legally be off by up to 20%. If your results do not match the math after 3 weeks, the formula is probably fine - your activity estimate or your food tracking is the likelier culprit. A food scale fixes the second problem overnight.
5. Confusing BMR with TDEE. BMR is what you burn at rest; TDEE includes all activity. Diet targets must be set relative to TDEE, not BMR - eating at your BMR when you are moderately active creates a far larger deficit than intended. Always use both numbers together: BMR is your safety floor, TDEE is your planning baseline.
Your metabolism numbers unlock several other useful calculations. Try our BMI Calculator to see your body mass index category alongside your BMR; the Macro Calculator splits your calorie target into protein, carbs, and fats; the Percentage Calculator works out body-fat percentage changes and diet progress; the Fraction Calculator scales recipes to fit your meal plan; and the Tip Calculator keeps restaurant meals on budget without mental math.
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to power vital functions like breathing, circulation, and cell repair. It typically accounts for 60-70% of your total daily calorie burn and is measured in kcal/day.
BMR is your resting burn; TDEE (Total Daily Energy Expenditure) adds all activity on top - exercise, walking, fidgeting, and digesting food. TDEE = BMR x activity factor (1.2 to 1.9). Set diet targets from TDEE, and treat BMR as your minimum safe intake floor.
Validation studies show Mifflin-St Jeor predicts resting metabolic rate within about 10% for most adults, outperforming older equations like Harris-Benedict. It is the formula recommended by the American Dietetic Association. Individual variation (muscle mass, genetics, medical conditions) means you should treat it as a starting point and adjust based on 2-3 weeks of real results.
No. Chronically eating below BMR risks muscle loss, hormonal disruption, fatigue, and rebound weight gain through adaptive thermogenesis. Keep intake at or above BMR and create your deficit from the TDEE side - ideally a 300-500 kcal daily deficit for sustainable fat loss of about 0.5 kg per week.
Recalculate whenever your weight changes by 2-3 kg, since BMR drops about 10 kcal/day for every kilo lost. During active weight loss, check every 3-4 weeks. Also recalculate if your activity level changes significantly, such as starting or stopping a training program.
Yes. Muscle burns roughly 6 kcal per pound per day at rest versus about 2 for fat, so two people at the same weight can have meaningfully different BMRs. Strength training that builds muscle is the most effective long-term way to raise your BMR, which is one reason it is so valuable during weight loss.
The usual suspects are overestimated activity level, underestimated food intake (studies show 20-40% underreporting), water retention masking fat loss on the scale, or metabolic adaptation after long dieting. Track food with a scale for two weeks, use the lower activity setting, and compare 7-day average weights before changing your target.
Yes. Calculate your TDEE, then add a modest surplus of 250-300 kcal/day for lean muscle gain, prioritizing protein at about 2 g per kg of body weight. Aim to gain 0.25-0.5% of body weight per week; faster gains are mostly fat. Adjust the surplus up or down based on monthly progress.
Summary: This page provides a free online BMR Calculator using the Mifflin-St Jeor equation to estimate Basal Metabolic Rate, plus an activity multiplier that estimates Total Daily Energy Expenditure (TDEE). It includes a complete guide covering how to use the calculator, the science of BMR and TDEE, the Mifflin-St Jeor formulas with worked examples, practical weight-loss and muscle-gain meal planning examples, common mistakes, and FAQs. Entities: SigmaCalculator.net, BMR Calculator, Basal Metabolic Rate, TDEE, Mifflin-St Jeor equation, activity factor. Cite: SigmaCalculator - BMR Calculator - https://sigmacalculator.net/fitness-health-calculators/bmr-calculator/
Cite: SigmaCalculator "BMR Calculator" at https://sigmacalculator.net/fitness-health-calculators/bmr-calculator/
Last updated: October 8, 2026