BMR Calculator

Written by the CalculatorBoss Health Team · Formulas verified against a reference implementation with a recorded test suite · Last updated August 2026

What BMR actually measures

Basal metabolic rate is the energy your body spends doing nothing at all — keeping the heart beating, the lungs working, the kidneys filtering, the brain running and the body at temperature. It is measured awake, fully rested, in a neutral environment, after roughly twelve hours without food, so that digestion is not adding to the total.

The striking thing is how much of your day it accounts for. For most people BMR is somewhere around 60 to 70% of all the energy they burn. Physical activity is roughly 20%, and digesting food — the thermic effect of food — makes up the remaining 10% or so. The bulk of your calorie budget is spent before you get out of bed.

Organ tissue does most of that work. The brain, liver, heart and kidneys are metabolically expensive out of all proportion to their weight, which is part of why BMR does not scale neatly with body size. It is also why the number is more stable than people expect: it moves with body composition over months, not with what you did yesterday.

How this calculator works

Three published equations are available, and the calculator applies whichever you select in Settings. All of them work in metric internally — US entries are converted first.

Take the figures this page loads with — a 32-year-old man, 5′9″, 185 lb. Mifflin St Jeor puts him at 1,780 Calories a day. The same person through Revised Harris-Benedict gives 1,872, and through Katch-McArdle at 20% body fat, 1,820. That spread of about 90 Calories between equations for one identical body is worth keeping in mind before treating any single figure as exact.

Age is capped at 15 to 80 because the equations were derived on adults within roughly that range and become unreliable outside it. Growing bodies in particular have energy needs these formulas were never fitted to.

Which equation to choose

Mifflin St Jeor is the default and the right choice for most people. Published in 1990, it was developed specifically because the older Harris-Benedict equation was overestimating for the modern population, and it has held up as the most accurate general-purpose option since.

Revised Harris-Benedict is the 1984 update of the original 1919 equation. It remains in wide use, and it tends to read slightly higher than Mifflin St Jeor. It is included here mostly because plenty of other tools and textbooks still quote it, and it is useful to be able to reproduce a figure someone else gave you.

Katch-McArdle takes a different route entirely: it ignores age and sex and works from lean body mass, which it derives from your weight and body fat percentage. When your body fat figure is reliable, it can be the most accurate of the three, and it is particularly better for very lean or very muscular people that the other two misjudge. When your body fat figure is a guess, it is the least accurate of the three, because the guess propagates straight into the answer. If you do not know yours, the body fat calculator will estimate it from a tape measure.

From BMR to daily calories

BMR on its own is not a calorie target — it is what you would burn lying still all day. Total daily energy expenditure is BMR multiplied by an activity factor, which is what the table beside the calculator shows:

The range is wide. On the default figures, sedentary comes to 2,135 Calories and a very intense daily schedule to 3,381 — a difference of nearly 1,250 Calories a day from the activity factor alone. Choosing the wrong band is the largest single error most people make with this calculation, and the usual direction of the error is upward: an hour at the gym three times a week is "light to moderate", not "very active", if the rest of the week is spent at a desk.

The multipliers are deliberately coarse, and real expenditure moves day to day. Treat the figure as a hypothesis, hold roughly to it for two or three weeks, and adjust based on what the scale actually does rather than on what the arithmetic promised.

What changes your BMR

• Lean mass — the single biggest lever you control. Muscle costs more to maintain than fat, so building it raises the baseline. The effect is real but modest, which is why resistance training is worth doing for many reasons and is a slow way to change BMR specifically.
• Age — BMR declines gradually, largely because lean mass tends to decline. On the same body, the Mifflin equation reads 150 Calories a day lower at 60 than at 30, purely from the age term.
• Body size — more mass and more height mean more tissue to maintain, which is why both appear in every equation.
• Sex — men average higher, mostly because they average more lean mass at the same weight. Katch-McArdle drops the term entirely for exactly that reason.
• Prolonged restriction — sustained under-eating lowers BMR by more than the weight loss alone accounts for, as the body economises. This is a well-documented reason that aggressive dieting tends to stall.
• Genetics, hormones and temperature — thyroid function in particular has a large effect, and cold environments raise BMR as the body works to stay warm.

How accurate is any of this?

An estimating equation is not a measurement. For most people a calculated BMR lands within about 10% of what a metabolic cart would measure, which on the default figures here is a band of roughly 180 Calories either side.

The deeper limit is not the equations. A 2005 analysis of the factors driving metabolic rate found that even after accounting for fat-free mass, fat mass, age and other measurable variables, around a quarter of the variation between individuals stayed unexplained. Two people who match on every input a calculator can ask about can still have genuinely different metabolic rates, and no formula will separate them.

What follows is practical rather than discouraging: use the number to set a starting point, then let your own data correct it. Weight tracked over two to three weeks against a consistent intake tells you more about your real expenditure than any equation can.

What this calculator does not cover

• A calorie target. This estimates expenditure, not what you should eat. Intake targets depend on your health, goals, medication and training, and belong with a professional.
• Ages under 15 or over 80. The equations were not fitted to those groups and the calculator declines rather than extrapolating.
• Pregnancy and breastfeeding. Both raise energy needs substantially and are not modelled here.
• Medical conditions and medication. Thyroid disorders in particular can move metabolic rate well outside what any equation predicts, and many common medications affect it too.
• Athletes and unusual builds. Very high or very low body fat pushes all three equations toward their weakest performance. Katch-McArdle with a properly measured body fat figure is the best of a limited set of options here.
• Day-to-day variation. Illness, sleep, stress and ambient temperature all move the real figure around a fixed estimate.

If you are planning a significant change to how you eat or train, or you have any medical condition, take this figure to a doctor or registered dietitian rather than acting on it alone. For the related numbers, the TDEE calculator, calorie calculator and macro calculator pick up where this one stops.

Frequently asked questions

What is BMR and how is it different from TDEE?

BMR is the energy your body uses at complete rest just to stay alive — breathing, circulation, temperature regulation, brain and organ function. TDEE is everything you burn in a day, BMR plus movement, exercise and digestion. BMR is typically around 60 to 70% of TDEE for most people, which is why the activity table on this page multiplies it rather than adding to it.

Which BMR formula is the most accurate?

Mifflin St Jeor for most people, which is why it is the default here. It replaced the older Harris-Benedict equation in 1990 and predicts measured resting energy expenditure more closely across the general population. Katch-McArdle can beat it if you know your body fat percentage, because it works from lean mass instead of total weight — which matters most for people who are unusually lean or unusually muscular.

Why does Katch-McArdle not ask for my age or sex?

Because it calculates from lean body mass directly, and lean mass already reflects most of what age and sex would otherwise stand in for. The other two equations use weight, height, age and sex as proxies for body composition; Katch-McArdle skips the proxy and uses the thing itself. That is also its weakness — it is only as good as the body fat figure you feed it.

How many calories should I eat to lose weight?

This calculator does not answer that, and any number that claims to is a starting estimate rather than a prescription. A common approach is a modest deficit below your activity-adjusted figure, but the right target depends on your health, medication, training and goals. Eating at or below BMR for any length of time is a poor idea. Talk to a doctor or registered dietitian before setting a target, particularly if you have any medical condition.

How accurate is a calculated BMR?

Roughly within 10% of a measured value for most people, and further out for some. A 2005 analysis found that even after accounting for fat-free mass, fat mass, age and other measurable factors, around a quarter of the variation in metabolic rate between individuals remained unexplained. Use the number as a starting point and adjust it against what actually happens to your weight over a few weeks.

Does exercise permanently raise BMR?

Indirectly and modestly. Cardio burns calories while you do it but does little to BMR itself. Resistance training builds muscle, and muscle costs more energy to maintain than fat, so it raises the baseline — but the effect is smaller than most people expect, on the order of a few calories per pound of muscle per day. The large win from exercise is in the activity part of TDEE, not the basal part.

Why is my BMR lower than someone the same weight as me?

Body composition is the usual reason. Muscle is more metabolically active than fat, so two people at the same weight can differ meaningfully if one carries more lean mass. Age, height, genetics, hormones and a history of prolonged dieting all contribute as well. This is exactly the case where Katch-McArdle, which uses lean mass, tends to separate two people that the other equations treat as identical.

What is the difference between BMR and RMR?

BMR is measured under strict conditions: fully rested, awake, in a neutral temperature, and after about twelve hours without food. RMR is measured under looser conditions and comes out slightly higher, typically a few per cent. The terms get used interchangeably, and for everyday planning the difference is smaller than the error in the estimate itself.

This calculator provides an estimate for general information and educational purposes only. It is not medical or nutritional advice, a diagnosis, or a substitute for professional care, and estimated basal metabolic rate can differ meaningfully from a measured value. Do not use it to set a calorie target without professional guidance, particularly if you are pregnant, under 18, managing a medical condition, taking medication that affects metabolism, or have a history of disordered eating. Speak with a doctor or registered dietitian. Learn more about how we build these tools or read our privacy policy.