BMR calculator

What BMR is

Basal metabolic rate is the energy your body burns over 24 hours doing absolutely nothing: keeping your heart beating, your lungs breathing, your organs running and your body temperature stable, while lying still and fasted. It is measured, strictly, first thing in the morning after an overnight fast and eight hours of sleep, in a warm, quiet room. Almost nobody measures it under real lab conditions, which is why we estimate it with an equation instead.

BMR is not a small number. For most adults it is 60 to 70 percent of total daily energy expenditure, more than everything else combined: digestion, walking around, and training all sit on top of it. That makes it the single biggest input into any calorie target, which is why getting a reasonable estimate matters even though no equation is exact.

Because it is the largest single piece of the calculation, a small error in BMR carries through to every number built on top of it, from maintenance calories to a cutting or bulking target. That is also why comparing a few equations against each other, rather than trusting one blindly, is worth the extra minute.

It is also not the number to eat. BMR describes a body doing nothing for a day; you are not doing nothing. Add an activity multiplier to BMR to get TDEE, and use that for any calorie target. Confusing the two is one of the most common reasons a self prescribed diet fails before it properly starts.

How BMR is calculated

This calculator uses the Mifflin St Jeor equation by default, and switches to Katch McArdle if you enter a body fat percentage.

Mifflin St Jeor, by weight

For men: 10 times weight in kilograms, plus 6.25 times height in centimetres, minus 5 times age in years, plus 5. For women, the same calculation with 161 subtracted instead of 5 added. A 40 year old man at 85 kg and 180 cm works out to 850 plus 1,125 minus 200 plus 5, which is 1,780 kcal.

Katch McArdle, by lean body mass

Lean body mass is weight multiplied by one minus body fat percentage. BMR is then 370 plus 21.6 times lean mass in kilograms. The same man at 20 percent body fat has a lean mass of 68 kg, giving 370 plus 1,468.8, or about 1,839 kcal, higher than the weight based result because none of that lean mass is being discounted as fat.

Both equations need accurate inputs to be worth anything. Height and weight are easy to get right; body fat percentage is not, and a bad guess there does more damage to Katch McArdle than an ordinary weight measurement does to Mifflin St Jeor. Without a trustworthy body fat number, Mifflin St Jeor is the safer default.

Comparing the four common BMR equations

Mifflin St Jeor is not the only option, and it is worth knowing what the alternatives do differently and where each one is a better or worse fit.

Mifflin St Jeor (1990)

Built from 498 adults across a wide weight range. A 2005 systematic review by the American Dietetic Association found it the most accurate of the common equations for both lean and obese adults, which is why it is the default here and in most clinical dietetics practice.

Harris Benedict, revised 1984

The original dates to 1919 and was recalculated by Roza and Shizgal in 1984 using better statistical methods on the same type of data. It still runs higher than Mifflin St Jeor, typically by around 5 percent, because the underlying sample skewed leaner than the general population. Useful as a cross check, not as the primary number.

Katch McArdle

Uses lean body mass instead of total weight, so it does not penalise someone who is heavier because of muscle rather than fat. Well suited to lean, resistance trained people with a body fat reading from a DEXA scan or consistent skinfold measurements. A guessed body fat percentage removes the advantage.

Cunningham (1980)

Also lean mass based, derived in 1980 by reanalysing the original Harris Benedict data on 223 normal adults, and it runs noticeably higher than Katch McArdle. It has since been confirmed as the most accurate of the established equations in recreational athletes (ten Haaf and Weijs, 2014); the study's authors recommend their own new weight based equation only as a fallback for when body composition cannot be measured, not because it beat Cunningham. That makes Cunningham well suited to people carrying a lot of muscle, though it tends to overestimate for the general population.

When the error actually matters

A 5 to 10 percent gap between equations sounds small until it is attached to a real decision. For someone maintaining weight on a rough estimate, that gap mostly washes out, because the weekly weight trend corrects for it within a couple of weeks either way. It matters far more in three specific situations: a tight pre-competition cut where a physique or weight class athlete is working with a narrow calorie window and little room for error; a clinical or refeeding context, where a chronically underfed patient is being brought back onto food and overshooting resting energy needlessly risks refeeding syndrome; and a very small or very large body, where every equation here was built on adults broadly in the middle of the size range and extrapolates further from its own data the more a person sits outside that range.

In those three situations, the equation choice is not a rounding error, it is a meaningful part of the plan. A dietitian working with a hospitalised patient, or a coach working with a bodybuilder in the final weeks before a show, has good reason to prefer a measured number over any predicted one, which is where indirect calorimetry comes in.

Measured RMR versus a predicted BMR

Every equation on this page is a prediction, built from population averages and applied to an individual who was never actually measured. Indirect calorimetry is the alternative: a real measurement, taken while you breathe into a mask or hood for 10 to 30 minutes, usually first thing in the morning, fasted, after resting quietly. The device measures how much oxygen you consume and how much carbon dioxide you produce, and from that ratio calculates the energy your body is actually burning at rest, no formula, no population average, no guesswork about your muscle mass or metabolism.

This is the gold standard for a reason: it accounts for individual variation that no equation can see, including genuinely faster or slower metabolisms that fall outside what body weight, height, age and sex predict. Research comparing measured resting metabolic rate against predictive equations consistently finds that even the best performing equations, Mifflin St Jeor included, are within 10 percent of the measured value for most people but can be off by considerably more for some individuals, with no way to tell in advance who those individuals are.

In practice, indirect calorimetry is not something most people need. It requires specialised equipment usually found in hospitals, university exercise physiology labs, and some sports performance or metabolic health clinics, it costs money, and it takes an appointment rather than thirty seconds on a calculator. For the average person adjusting a diet based on their own weight trend, a predicted BMR combined with two to three weeks of real world tracking gets close enough, because the tracking data ends up doing the same job a measurement would: telling you what your body is actually doing, rather than what a formula expects it to do.

It is worth paying for a measurement in a narrower set of cases: a suspected metabolic or thyroid condition where a doctor wants a real baseline rather than an estimate confounded by symptoms, a competitive athlete or physique competitor where a percent or two of accuracy changes the plan, or someone who has dieted for a long time and suspects their metabolism has adapted downward by more than the standard equations would predict. Outside those cases, retesting a prediction periodically as weight and body composition change is a reasonable substitute for a lab measurement most people will never need, and it costs nothing but a couple of minutes with a calculator.

BMR at a glance: four equations, one person

To show how much the choice of equation matters, here is the same person run through all four: a 30 year old woman, 65 kg, 165 cm, 25 percent body fat.

What affects your BMR

Muscle mass. Muscle is more metabolically active than fat, so two people at the same weight can have meaningfully different BMR depending on body composition. It is the main reason lean mass based equations exist.

Age. BMR declines with age, roughly 1 to 2 percent per decade past early adulthood, largely because muscle mass tends to fall unless it is actively maintained through resistance training.

Sex. Men typically carry more muscle mass at the same weight, which is why every equation here uses a different constant for men and women.

Genetics and hormones. Thyroid function in particular can shift BMR meaningfully in either direction. A number that looks unusually far from these estimates, in either direction, alongside symptoms like fatigue or unexplained weight change is worth checking with a doctor rather than adjusting calories around.

Body temperature and illness. Fever raises BMR; being in a calorie deficit for a long time can lower it, an adaptive response sometimes called metabolic adaptation.

Body size. A taller or heavier person has more tissue to maintain and generally burns more at rest, which is why weight and height are inputs in every equation on this page, not just a starting weight on its own.

Sleep and stress. Poor sleep and chronically elevated stress hormones are linked to small shifts in resting energy in both directions in different studies, and are more relevant to how consistently you can stick to a plan than to the equation itself.

What to do with your BMR

On its own, BMR only tells you what your body needs to keep functioning. It is not a calorie target and it should not be treated as one; eating at BMR ignores every calorie spent on movement and digestion and is a target almost nobody can sustain or should try to.

Turn it into TDEE

Multiply BMR by an activity factor from 1.2 for a sedentary lifestyle to 1.9 for an athlete to get total daily energy expenditure, the number that actually governs your weight. The TDEE calculator does this step for you and adds cut, maintain and bulk targets on top.

Use it to sanity check other numbers

If a calorie target from another source looks implausible, checking it against your BMR is a quick reality check. A daily target below your BMR is almost never appropriate outside of medical supervision.

Retest it periodically, not constantly

BMR does not move week to week in any way you would notice, so there is no benefit to recalculating it daily. Revisit it after a meaningful change: 5 kg of weight lost or gained, a body recomposition phase that changes your body fat percentage, or roughly every couple of months during an extended diet or bulk. Anything more frequent is just noise.

Common mistakes

Eating at or below BMR. This ignores all the energy spent on daily activity and training and is not a sustainable or healthy target for most people.

Guessing body fat percentage. A rough guess plugged into Katch McArdle or Cunningham can move the result by hundreds of calories, undermining the reason to use those equations in the first place.

Comparing numbers across different equations without noticing. A BMR from Harris Benedict and one from Mifflin St Jeor for the same person can differ by 5 percent or more; that is expected, not a sign either tool is broken.

Treating BMR as fixed. It changes with weight, muscle mass, age and a sustained calorie deficit. Recalculate periodically rather than relying on a number from months ago.

Confusing BMR with TDEE. BMR has no activity built in. Any calorie target should come from TDEE, not BMR directly.

Ignoring the equation you started with. Switching between a weight based equation and a lean mass based one from one recalculation to the next, without a reason such as a new, trustworthy body fat reading, makes the trend line meaningless. Pick one and stay with it unless something real has changed.

How coaches use a BMR calculator

For a coach, BMR is rarely the number handed to a client. It is the building block used to sanity check a calorie target, especially when a client arrives having self prescribed a very low target from another app or a generic diet plan.

A quick check at onboarding, comparing a proposed target against BMR and TDEE, catches the two most common problems early: targets set too low to be sustainable, and targets that were never adjusted as the client's weight changed.

Scraler builds that check into onboarding and keeps it live afterwards. Set a calorie and macro target per client, track weight and adherence trends over time, and review them together at check-ins instead of relying on a number calculated once and never revisited. When a client's weight, body fat or training volume shifts, the target can be updated from the same view rather than starting a new calculation from scratch. See nutrition coaching and client check-ins.