BMR Calculator

BMR (Basal Metabolic Rate) measures the number of calories your body burns in 24 hours purely to keep you alive at complete rest — no digestion, no movement, no thinking effort beyond baseline brain activity. It covers the energy cost of breathing, pumping blood, maintaining body temperature, cell repair and turnover, and basic organ function (brain, liver, kidneys, and heart together account for the majority of resting energy use despite being a small fraction of body weight). It is measured under strict laboratory conditions: fasted for 12 hours, lying still, in a thermoneutral room, immediately after waking. In everyday use, "BMR" from an online calculator is really an estimate of this value using population-derived formulas, since true laboratory BMR testing (indirect calorimetry) is rarely accessible outside research or clinical settings.

BMR is the foundation of every calorie calculation you'll ever do for weight loss, maintenance, or muscle gain. It typically represents 60–75% of your Total Daily Energy Expenditure (TDEE) — the total calories you burn including movement and exercise. Since TDEE is calculated by multiplying BMR by an activity factor, an inaccurate BMR estimate throws off every downstream number: your maintenance calories, your deficit for fat loss, and your surplus for muscle gain. Understanding your BMR also explains why very low-calorie diets backfire — eating below your BMR for extended periods signals your body to conserve energy, often by reducing non-essential functions and increasing hunger hormones, which undermines sustainable fat loss.

Mifflin-St Jeor, published in 1990, is now considered the more accurate formula for the general population by most dietitians and sports scientists, and is the formula the Academy of Nutrition and Dietetics recommends. It was validated against indirect calorimetry (the gold-standard lab measurement) in a more modern, more diverse sample than the original 1919 Harris-Benedict equation. Studies comparing formula predictions to measured BMR generally find Mifflin-St Jeor has a smaller average error (often within 10% of measured values for most people) compared to the original Harris-Benedict, which tends to overestimate BMR by 5–15%, particularly in individuals with excess body fat since the original equation doesn't account for body composition differences as well. The revised 1984 Harris-Benedict formula improved on the original but Mifflin-St Jeor still edges it out in most validation studies. That said, both are useful starting estimates — the real-world test is tracking your actual weight change against your calculated intake for 2–3 weeks and adjusting from there.

Katch-McArdle is a third BMR formula that uses lean body mass (total weight minus fat mass) rather than total weight, height, and age. The formula is: BMR = 370 + (21.6 × lean mass in kg). Because it directly accounts for the metabolically active tissue driving your resting energy needs, Katch-McArdle can be more accurate than Mifflin-St Jeor or Harris-Benedict specifically for people who are noticeably leaner or more muscular than average — bodybuilders, physique athletes, and very lean individuals. Its accuracy, however, depends entirely on having a reliable body fat percentage measurement, since lean mass is derived from it. If you only have a rough body fat estimate (from a bioelectrical impedance scale, for example), the error in that estimate can outweigh any theoretical accuracy gain over Mifflin-St Jeor. Use our Body Fat Calculator to get a reasonable lean mass estimate first if you want to try this approach.

The two formulas were built from different research populations decades apart and use slightly different weightings for age, height, and weight, so they will rarely produce an identical number — a difference of 50–150 kcal/day between the two is completely normal and expected. This is precisely why we show both: it gives you a realistic range rather than false precision from a single number. Most nutrition professionals treat Mifflin-St Jeor as the more reliable default for the general population, but if your Harris-Benedict result is meaningfully higher, treating your true BMR as somewhere between the two (or leaning toward the Mifflin-St Jeor figure) is a reasonable, evidence-informed approach.

Yes, meaningfully. Skeletal muscle is metabolically active tissue — each kilogram of muscle burns approximately 13 kcal per day at rest, compared to roughly 4.5 kcal per kilogram for fat tissue. This means two people of identical height, weight, age, and sex can have noticeably different true BMRs if one carries significantly more muscle mass than the other. This is also why population-formula BMR estimates (which use total weight, not body composition) tend to underestimate BMR in very muscular individuals and slightly overestimate it in those with higher body fat percentages at the same total weight. Over months and years of consistent resistance training, gradually increasing your muscle mass will measurably raise your resting metabolic rate — one of the most underrated long-term benefits of strength training for weight management.

Yes. BMR tends to decline by approximately 1–2% per decade after age 20, accelerating somewhat after age 40, and more noticeably after 60. This decline happens for two main interacting reasons: age-related loss of skeletal muscle mass (sarcopenia), which begins subtly in the 30s and accelerates without resistance training, and hormonal changes (including declining growth hormone, testosterone in men, and oestrogen in women after menopause) that influence body composition and metabolic rate. The practical implication is that maintenance calorie needs genuinely fall as you age even if activity level stays constant, which is why many people notice gradual weight gain in their 40s and 50s despite eating "the same as always." Resistance training is the single most effective countermeasure, since it directly opposes the muscle loss driving much of the age-related metabolic decline.

Men typically have a higher BMR than women of identical height and weight primarily because men carry proportionally more skeletal muscle and less essential body fat. Both the Mifflin-St Jeor and Harris-Benedict formulas include a sex-specific constant precisely to account for this average difference in body composition. It's worth noting these formulas reflect population averages — an individual woman with high muscle mass could have a higher true BMR than an individual man with low muscle mass at the same total bodyweight, since body composition, not sex itself, is the underlying mechanism driving the difference.

Sustained, severe caloric restriction can trigger "adaptive thermogenesis" or "metabolic adaptation" — a reduction in BMR beyond what would be predicted purely from the loss of body weight and muscle mass. Research (including data from the famous "Biggest Loser" contestant follow-up study) has documented cases where measured BMR dropped substantially below formula-predicted values even years after weight loss, in some cases by several hundred calories per day. The mechanisms include reduced thyroid hormone conversion, lower circulating leptin, increased hunger hormone signalling, and reduced non-exercise activity (spontaneous fidgeting and movement, often called NEAT). The practical takeaway is that moderate, gradual caloric deficits (300–500 kcal/day below TDEE) combined with adequate protein intake and resistance training are far less likely to produce this adaptive slowdown than aggressive crash dieting, and are more likely to preserve muscle mass and long-term metabolic health.

BMR (Basal Metabolic Rate) and RMR (Resting Metabolic Rate) are closely related but technically distinct. True BMR is measured under very strict laboratory conditions — fully fasted for at least 12 hours, having slept a full night in the lab facility, lying completely still, in a thermoneutral environment, immediately upon waking. RMR is measured under more relaxed, practical conditions — typically after a shorter fast, without requiring an overnight lab stay — and is consequently usually 5–10% higher than true BMR because it includes a small amount of the thermic effect of recent food and slightly less strict resting conditions. In everyday nutrition contexts, the terms are frequently used interchangeably, and the formulas used by calculators like this one (Mifflin-St Jeor, Harris-Benedict) technically estimate something closer to RMR, even though they are conventionally labelled "BMR" in almost all consumer and clinical contexts.

Population-based formulas like Mifflin-St Jeor are generally accurate to within about 10% of measured BMR (via indirect calorimetry) for most people who fall close to "average" body composition for their height and weight. Accuracy decreases at the extremes: very muscular individuals, very lean individuals, people with significant amounts of excess fat, and those with metabolic or hormonal conditions (thyroid disorders, PCOS) may see larger gaps between formula estimates and their true measured BMR. If you have access to indirect calorimetry testing (sometimes offered at sports science labs, some gyms, or specialised metabolic clinics), it will give you a personalised, directly measured number rather than a population estimate. For the vast majority of people, however, the Mifflin-St Jeor estimate combined with 2–3 weeks of real-world tracking (comparing predicted vs. actual weight change) provides a sufficiently accurate and far more accessible starting point.

Yes, significantly. BMR rises progressively throughout pregnancy to support foetal growth, placental development, increased maternal blood volume, and breast tissue changes. Estimates suggest BMR increases by roughly 5–10% in the first trimester and can rise 20–25% above pre-pregnancy levels by the third trimester. This is one of the physiological bases for the additional calorie recommendations during pregnancy (commonly cited as no extra calories needed in the first trimester, approximately +340 kcal/day in the second trimester, and approximately +450 kcal/day in the third trimester, per widely used clinical guidelines). Because standard BMR formulas were not developed using pregnant populations, pregnant women should discuss individualised calorie and nutrition targets with their obstetrician or a registered dietitian rather than relying solely on general calculators.

Yes, substantially. The thyroid gland is one of the primary regulators of metabolic rate through its hormones (T3 and T4). Hyperthyroidism (an overactive thyroid, as in Graves' disease) increases BMR, often causing unintentional weight loss, a fast heart rate, heat intolerance, and anxiety-like symptoms despite normal or increased appetite. Hypothyroidism (an underactive thyroid) reduces BMR, commonly causing fatigue, cold intolerance, weight gain despite normal eating habits, and sluggishness. If your real-world weight trend consistently and significantly diverges from what your calculated BMR and TDEE would predict — despite accurate food tracking — it is worth discussing thyroid function tests (TSH, free T3, free T4) with your doctor, since standard population-based BMR formulas cannot detect or account for thyroid dysfunction.

You should almost never eat below your BMR for sustained periods, even when trying to lose weight — your BMR represents the minimum energy your body needs just to keep vital organs functioning, and consistently eating below it increases risk of nutrient deficiency, muscle loss, hormonal disruption, and the metabolic adaptation discussed above. Instead, calculate your TDEE (BMR × activity factor) as your maintenance calorie baseline, then create a deficit of 300–500 kcal/day below TDEE for sustainable fat loss, or a surplus of 200–400 kcal/day above TDEE for lean muscle gain. Use our TDEE Calculator to get this activity-adjusted figure, since BMR alone does not account for the calories you burn through daily movement and exercise.

Not permanently in most cases, but the recovery can take longer than people expect. Severe or prolonged caloric restriction reduces BMR through both muscle loss (a smaller "engine" burns fewer calories) and adaptive thermogenesis (hormonal changes that suppress metabolic rate beyond what muscle loss alone would predict). For most people, BMR gradually recovers toward its formula-predicted level over weeks to months once adequate caloric intake, sufficient protein, and resistance training resume — a process sometimes called "reverse dieting." However, some research suggests that in cases of extreme, repeated, or very prolonged restriction (such as multiple aggressive diet cycles or clinical eating disorders), some degree of metabolic suppression can persist longer term. The safest approach is to avoid extreme restriction in the first place by using a moderate, sustainable deficit calculated from your TDEE rather than arbitrary very-low-calorie targets.

No — this is one of the most common misunderstandings. BMR only represents the calories burned at complete rest, which is typically 60–75% of your total daily calorie burn. Your full daily expenditure (TDEE) adds the thermic effect of food (calories used to digest what you eat, roughly 10% of intake), non-exercise activity thermogenesis or NEAT (walking, fidgeting, standing, everyday movement — highly variable between individuals), and exercise activity thermogenesis (deliberate workouts). For most sedentary-to-moderately-active people, TDEE is roughly 1.2–1.55 times BMR; for very active individuals or athletes, it can be 1.7–2.4 times BMR or higher. Always use your TDEE, not your raw BMR, when setting calorie targets for weight management — use our TDEE Calculator to get this complete figure.

A few common explanations: if you're very muscular, standard formulas (which use total weight rather than body composition) will underestimate your true BMR because they can't detect that a higher proportion of your weight is metabolically active muscle rather than fat. If you carry a higher body fat percentage, standard formulas may slightly overestimate true BMR for the same reason in reverse. If you have dieted aggressively in the past, your true measured BMR could be genuinely lower than formula predictions due to metabolic adaptation. If you have an undiagnosed thyroid condition, PCOS, or are taking certain medications (some antidepressants, beta-blockers, and steroids can influence metabolic rate), your real BMR could diverge from the population-based estimate in either direction. If none of these apply and the number still feels off, remember that ±10% is the normal expected error range for any population formula — track your actual weight trend against your calculated intake for 2–3 weeks as the most reliable individual calibration method.

Athletes and highly trained individuals with above-average muscle mass and below-average body fat percentage are precisely the group for whom standard weight-based formulas (Mifflin-St Jeor, Harris-Benedict) are least accurate, because these formulas cannot see body composition — only total weight. For this population, the Katch-McArdle formula, which uses lean body mass directly, is often considered more accurate, provided a reasonably reliable body fat percentage is available (via DEXA, skinfold testing, or our Body Fat Calculator). Alternatively, many sports dietitians simply use Mifflin-St Jeor as a starting estimate and then rely heavily on tracking real-world weight and performance trends over 2–4 weeks to fine-tune actual calorie targets, since individual variation among athletes can be substantial even with the "correct" formula.

There is a kernel of truth here, but it's often overstated in fitness marketing. Chronic, severe under-eating can suppress BMR through the mechanisms discussed above (muscle loss, hormonal adaptation), so correcting a genuinely too-low intake back toward an appropriate maintenance level can restore BMR toward its formula-predicted value — this is the basis of "reverse dieting" protocols. However, if you are already eating at or above your maintenance calories, further increasing intake will not meaningfully "boost" BMR beyond what your body composition and other physiological factors determine — the extra calories will simply be stored as fat (if in a surplus) or used to support other functions like muscle building if combined with resistance training. The single most evidence-backed way to genuinely and sustainably raise your BMR over time is building muscle mass through progressive resistance training, not simply eating more in the absence of a training stimulus.

Your BMR alone isn't an actionable calorie target — it's an input for the next calculation. The recommended next step is to plug your BMR (or simply your details) into our TDEE Calculator, which multiplies your BMR by an activity multiplier reflecting your typical weekly exercise and movement level to give you your true maintenance calories. From there, subtract 300–500 kcal/day for a sustainable fat-loss deficit, or add 200–400 kcal/day for a lean muscle-building surplus. Pair this with the Protein Calculator to set an adequate protein target (generally 1.6–2.2 g per kg of bodyweight for people who train), and the Macro Calculator if you want a full breakdown of protein, carbohydrate, and fat targets tailored to your goal.