BMR Calculator

Basal metabolic rate (BMR) is the number of calories your body burns at complete rest — just to keep your heart beating, lungs breathing, organs functioning, and body temperature regulated. It typically accounts for roughly 60–75% of total daily calorie burn for most people, making it the single largest component of daily energy expenditure, even before any deliberate activity or exercise is factored in. Three formulas — Mifflin-St Jeor, Harris-Benedict, and Katch-McArdle — dominate BMR estimation today, each built from a different research population and using slightly different inputs.

What BMR actually measures

BMR is measured under very specific laboratory conditions: complete physical and mental rest, a thermoneutral environment, and a 12-hour fast, typically first thing in the morning before any food, caffeine, or physical activity. Because these conditions are impractical to replicate outside a lab, virtually every “BMR calculator” — including this one — actually estimates BMR using population-based regression formulas built from height, weight, age, and sex, rather than measuring it directly.

A closely related but distinct term, resting metabolic rate (RMR), is measured under slightly less strict conditions and tends to run a few percent higher than true BMR in practice, though the two terms are frequently used interchangeably in casual and even some clinical contexts. The formulas below are technically BMR estimation formulas, but their outputs are commonly treated as reasonable RMR estimates too, given how close the two figures typically sit for most people.

Several factors drive real individual variation in BMR beyond what any formula captures. Muscle mass is one of the largest — muscle tissue burns meaningfully more calories at rest than fat tissue does, which is exactly why two people of identical height, weight, age, and sex can have genuinely different actual BMRs if one carries more lean mass than the other. Sex differences are built into all three formulas below (reflecting that men typically carry more muscle mass and less body fat than women at an equivalent height and weight), and age is included because both muscle mass and metabolic rate tend to decline gradually over adulthood. Genetics, thyroid function, and even climate (BMR can run modestly higher in colder climates, since more energy goes toward maintaining body temperature) all contribute additional variation that a height-weight-age-sex formula simply cannot see.

A commonly cited illustration of the muscle-mass effect: two people with identical height, weight, age, and sex can still have real BMRs differing by 100–200 calories per day if one carries substantially more muscle than the other — a gap none of the height-weight-age-sex inputs above can detect, since all four are identical between them. This is the exact scenario Katch-McArdle is designed to handle better than the other two, provided an accurate body fat percentage is available to calculate lean mass from.

The three formulas compared

(W = weight in kg, H = height in cm, A = age in years, LBM = lean body mass in kg.)

Mifflin-St Jeor, published in 1990, is generally considered the most accurate of the three for the general population today, with published validation studies showing average accuracy within about 10% of measured BMR for most adults. It’s the formula this calculator uses as its primary/default result. Harris-Benedict was originally published in 1918 and revised in 1984 using updated population data; even the revised version tends to run slightly higher than Mifflin-St Jeor for the same inputs, a pattern attributed to changes in average body composition between the reference populations each formula was built from — people today typically carry a different muscle-to-fat ratio than the populations studied nearly a century ago. Katch-McArdle takes a fundamentally different approach: rather than using total weight, it uses lean body mass directly, which makes it more accurate specifically for people with an atypical (particularly higher) muscle mass relative to their total weight, since it isn’t diluted by however much or little fat that same total weight includes.

Worked example

Using this calculator’s own default example — a 35-year-old man, 170 lbs, 5’9” tall, no body fat percentage provided:

The roughly 120-calorie spread across all three is typical — each formula was built from a different reference population using a slightly different statistical approach, and none of them can see your actual individual metabolism. This is exactly why the calculator shows all three side by side rather than presenting a single number as definitively correct.

Katch-McArdle and the lean body mass estimate

Katch-McArdle’s accuracy depends entirely on how good its lean body mass input is. When you provide an actual measured body fat percentage — from a DEXA scan, the US Navy circumference method, or another source — the calculator computes LBM directly: weight × (1 − body fat % ÷ 100), which is the most reliable version of this formula. When body fat percentage isn’t available, this calculator estimates LBM using the Boer (1984) formula (the same one used by this site’s dedicated Lean Body Mass Calculator) rather than assuming a single flat body fat percentage for every visitor regardless of sex, height, or build. This produces a meaningfully more individualized fallback estimate than a one-size-fits-all assumption, though it’s still an estimate — providing a real measured body fat percentage will always give the most accurate Katch-McArdle result.

From BMR to TDEE: adding activity

BMR alone only describes calories burned at complete rest. Total Daily Energy Expenditure (TDEE) — the number that actually matters for weight management — adds everything else: digesting food, daily movement, and deliberate exercise. TDEE is calculated by multiplying BMR by an activity multiplier:

These multipliers, sometimes called the Harris-Benedict activity factors even when paired with Mifflin-St Jeor’s BMR figure, are widely used estimates rather than a precisely individualized number — actual daily activity varies enough between people at the “same” nominal activity level that TDEE estimated this way carries meaningful uncertainty even when BMR itself is accurate. It’s a solid starting point for setting a calorie target, best refined over a few weeks by comparing predicted TDEE against actual observed weight change and adjusting from there.

Why the formulas disagree

Each formula was derived from a different study population at a different point in time, using different measurement technology and statistical methods. Mifflin-St Jeor’s 1990 reference population reflects more recent average body composition than Harris-Benedict’s, which partly explains why Harris-Benedict tends to run higher — average body fat percentage in the general population has shifted over the intervening decades, and formulas built from population averages inherit whatever the reference population looked like at the time.

None of the three regression formulas can see an individual’s actual metabolic rate, which is influenced by genetics, thyroid function, muscle mass, and other factors no simple formula captures. This is precisely why Katch-McArdle’s lean-mass-based approach can outperform the other two for specific individuals — particularly athletes and bodybuilders — even though it isn’t universally “more accurate” for the general population.

Limitations of BMR estimates

All BMR formulas are population averages, and averages don’t perfectly predict any specific individual. Medical conditions affecting metabolism — thyroid disorders being the most common — can shift actual BMR meaningfully above or below what any formula predicts, and none of these formulas account for medication use, which can also affect metabolic rate. Significant recent weight change also affects accuracy: someone who has recently lost a large amount of weight typically has a measured BMR somewhat lower than these formulas would predict for their current weight, a well-documented phenomenon called adaptive thermogenesis or “metabolic adaptation,” where the body’s metabolism partially compensates for a sustained calorie deficit beyond what reduced body mass alone would explain.

For most people without a diagnosed metabolic condition, these formulas — especially Mifflin-St Jeor — provide a reasonably close starting estimate. As with any calculated health number, the most reliable approach is to use the formula result as a starting point, then adjust based on real-world observed results (actual weight change over several weeks at a given calorie intake) rather than treating the formula output as an unchangeable fact.

A brief history of BMR estimation

James Arthur Harris and Francis Gano Benedict published the original Harris-Benedict equation in 1918, based on data from just over 200 subjects — a remarkably small sample by modern standards, but groundbreaking at the time as one of the first attempts to predict metabolic rate from simple, easily measured variables rather than requiring direct calorimetry for every individual. The formula remained the standard reference for the better part of a century until Roza and Shizgal published a revised version in 1984 using an expanded dataset, correcting some of the original’s known biases — the “Harris-Benedict, revised 1984” figures used throughout this page and calculator refer specifically to that updated version, not the original 1918 coefficients.

Mifflin, St Jeor, and colleagues published their competing formula in 1990, explicitly designed to improve on Harris-Benedict using a larger, more contemporary reference population reflecting how average body composition had shifted over the preceding decades. Subsequent validation research comparing both formulas against directly measured metabolic rate — using techniques like indirect calorimetry — has generally found Mifflin-St Jeor to track more closely with actual measured BMR for most adults today, which is why it has gradually displaced Harris-Benedict as the more commonly recommended default in clinical nutrition practice, even though Harris-Benedict remains widely cited and used.

Using BMR as a reference point, not a floor to force

BMR is often discussed alongside advice not to eat below it, and this general guidance is worth understanding correctly. The concern isn’t that eating below BMR for a single day causes harm — it’s that sustained, significant calorie intake below BMR for extended periods deprives the body of the minimum energy needed for basic organ function, which can contribute to muscle loss, nutrient deficiencies, hormonal disruption, and a slowed metabolic rate over time. This is general population-level guidance rather than a precise personal threshold, since actual safe minimum intake varies by individual health status, activity level, and other factors a simple formula can’t assess.

Because BMR and calorie planning touch directly on nutrition and body image, it’s worth a direct note: these formulas are informational tools meant to support a broader conversation about health and activity, not a basis for restrictive eating patterns. Anyone with a complicated relationship with food, exercise, or body image — or anyone considering a significant, sustained calorie restriction — is better served working with a doctor or registered dietitian who can account for individual health history in a way no formula can.