Basal Metabolic Rate Calculator
Estimate the calories your body burns at complete rest using the Mifflin-St Jeor equation, then multiply by an activity factor to get Total Daily Energy Expenditure. Use the resulting kcal/day as a baseline for diet planning.
Last updated: May 2026
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About this calculator
Basal Metabolic Rate (BMR) is the energy your body needs at complete rest to keep core functions — breathing, circulation, cell turnover, brain activity — running. This calculator uses the Mifflin-St Jeor equation, currently the most accurate validated BMR formula for most non-obese adults: BMR (kcal/day) = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5 for males, and the same equation ending in − 161 for females. The +5 / −161 constants reflect average sex-based differences in lean body mass. The calculator then multiplies BMR by an activity factor between 1.2 (sedentary) and 1.9 (extremely active, e.g. two hard training sessions per day plus a physical job) to estimate Total Daily Energy Expenditure (TDEE), the total calories you actually burn including movement, exercise and the thermic effect of food. The activity multiplier is the largest source of error in everyday use, since people routinely overestimate how active they are. Edge cases: the equation is validated for adults 19–78, not children; it is less accurate in very lean or very obese individuals; pregnant and lactating women need extra energy not captured by the formula; and people on weight-loss diets see BMR decline beyond what the formula predicts because of metabolic adaptation.
How to use
Example 1 — a 30-year-old man, 80 kg, 175 cm, moderately active. Step 1: 10 × 80 = 800. Step 2: 6.25 × 175 = 1093.75. Step 3: 5 × 30 = 150. Step 4: BMR = 800 + 1093.75 − 150 + 5 = 1748.75 kcal/day. Step 5: TDEE at 1.55 activity factor = 1748.75 × 1.55 ≈ 2710 kcal/day. Verify: BMR for an average adult man typically lands between 1600 and 1900 kcal/day, so 1749 is plausible; multiplying by 1.55 gives roughly 2700 kcal/day, consistent with USDA maintenance estimates for a moderately active man of this size. ✓ Example 2 — a 45-year-old woman, 65 kg, 165 cm, lightly active. Step 1: 10 × 65 = 650. Step 2: 6.25 × 165 = 1031.25. Step 3: 5 × 45 = 225. Step 4: BMR = 650 + 1031.25 − 225 − 161 = 1295.25 kcal/day. Step 5: TDEE at 1.375 activity factor = 1295.25 × 1.375 ≈ 1781 kcal/day. Verify: BMR for a middle-aged woman of this build typically falls between 1200 and 1400 kcal/day, matching the calculator. A 500 kcal/day deficit (≈ 1281 kcal/day intake) would predict roughly 0.45 kg of fat loss per week, although metabolic adaptation usually slows this over time. ✓
Frequently asked questions
What is the difference between BMR and TDEE?
BMR is the calories you would burn lying motionless and awake in a thermoneutral room having fasted for 12 hours — pure baseline metabolism. TDEE adds everything else: the thermic effect of food (≈ 10% of intake), non-exercise activity thermogenesis (walking, fidgeting, posture), and structured exercise. This calculator multiplies BMR by an activity factor of 1.2 to 1.9 to estimate TDEE; the factor is the single largest source of error because most people overestimate how active they really are, especially if they sit at a desk all day and train three times a week. For weight management, TDEE — not BMR — is the number you compare your calorie intake to. Eating below TDEE produces a deficit and weight loss; eating above produces a surplus and weight gain.
How accurate is the Mifflin-St Jeor formula for calculating BMR?
Multiple validation studies have shown Mifflin-St Jeor predicts measured resting metabolic rate within about 10% for roughly 82% of normal-weight non-obese adults, making it the most accurate of the commonly used predictive equations including the older Harris-Benedict and the WHO/Schofield equations. The 1990 paper that introduced it (Mifflin et al., American Journal of Clinical Nutrition) validated the formula against indirect calorimetry in nearly 500 healthy adults across a wide age and BMI range. It tends to slightly overestimate BMR in people with obesity and underestimate it in highly muscular individuals because it uses total body weight rather than fat-free mass, which would be more physiologically accurate but is rarely available outside research settings. For clinical precision — for example in cystic fibrosis, post-bariatric surgery or critically ill patients — indirect calorimetry, which measures oxygen consumption and CO₂ production, remains the gold standard. For everyday diet planning, Mifflin-St Jeor is a more than acceptable starting point that you can refine over 4–6 weeks by tracking intake and weight trend, then adjusting your target calorie figure by ±100–200 kcal/day based on what actually happens on the scale.
Why does gender affect BMR calculations?
On average men carry a higher proportion of lean muscle mass than women of the same weight and height, and muscle is far more metabolically active at rest than fat tissue. Hormonal differences reinforce this: testosterone drives muscle accretion, while estrogen is associated with higher body-fat percentages. The +5 vs −161 constants in the Mifflin-St Jeor equation capture this average difference empirically rather than mechanistically. Because the equation uses binary sex as a coarse proxy for body composition, individuals with atypical composition — a very muscular woman, a slender man, someone on hormone therapy, or someone with PCOS — may find their actual BMR diverges from the formula by 5–15%. If you suspect this, weigh and track intake for a few weeks and adjust your target intake based on the trend rather than the prediction.
What are the most common mistakes when using BMR/TDEE calculators?
Overestimating activity level is by far the biggest mistake — a desk worker who goes to the gym three times a week is 'lightly active' (1.375), not 'moderately active' (1.55) or 'very active' (1.725). That single click can inflate the estimate by 400–600 kcal/day and quietly turn a planned deficit into maintenance. Mixing units is the second pitfall: entering weight in pounds or height in inches into a metric formula yields nonsense. People also forget to update the calculation as they lose weight; BMR drops as body mass falls, so a target set at 90 kg keeps producing slow gain by the time you weigh 80 kg. Another error is treating the daily number as a strict ceiling — energy needs vary 200–400 kcal day to day, so judge progress over 2–4 weeks of trend rather than a single day. Finally, the formula assumes a healthy, fully-fed adult; severe dieters, athletes in heavy training blocks, and those with thyroid disease often need clinical assessment.
When should I not use this calculator?
Do not use this calculator for children, adolescents, or pregnant or lactating women — their energy needs follow different equations (Schofield for kids; pregnancy adds roughly 340 kcal in the second trimester and 450 kcal in the third). It is not validated for adults with significant medical conditions affecting metabolism, including untreated hypothyroidism or hyperthyroidism, recent bariatric surgery, severe burns, cancer cachexia, or critical illness; these patients need indirect calorimetry or clinical equations. If you have a history of disordered eating, do not use BMR/TDEE calculators as targets without professional support — rigid calorie targets can entrench restrictive behaviour. Elite athletes in heavy training blocks and people at the extremes of body composition (very lean physique competitors, very obese individuals) will find the formula systematically off by 10–25% and should track intake versus weight trend to calibrate. Finally, do not use the output to dose medications scaled by metabolic rate; that is a clinical decision.