BMR Calculator

Basal metabolic rate is the energy your body uses at complete rest to stay alive: breathing, circulating blood, maintaining temperature, running the brain, kidneys, liver and heart. It is what you would burn lying still in a warm room, awake, having not eaten for twelve hours.

For most people it is 60% to 70% of total daily energy expenditure, which makes it the largest single component and the natural starting point for any calorie target.

The three formulas

Mifflin-St Jeor, published in 1990 and the current default:

Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5 Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161

Harris-Benedict, from 1919 and revised in 1984. It was derived from a population that ate and moved differently from a modern one, and it tends to read slightly high.

Katch-McArdle, which ignores total weight and uses lean body mass:

BMR = 370 + (21.6 × lean mass in kg)

It requires a body fat measurement, which most people do not have, but it is the most accurate of the three when they do.

How much they disagree

For a 30-year-old man, 75 kg and 175 cm:

For a 30-year-old woman, 60 kg and 162 cm:

The interesting pair is the two Katch-McArdle rows. Same weight, same height, same age — and 130 calories a day between them, because one person carries more muscle. Neither Mifflin nor Harris-Benedict can see that difference at all, since neither has an input for it.

That is the case for measuring body fat if you train seriously. For everyone else, Mifflin-St Jeor is close enough and needs nothing you cannot measure at home.

Why no formula can be exactly right

These are regression equations fitted to populations. They describe the average person of your height, weight, age and sex, and you are not the average one.

Measured individual variation is roughly ±200 calories around any formula's estimate at typical body sizes. Contributing factors include thyroid function, the proportion of organ mass to muscle mass, genetics, sleep, ambient temperature and recent dieting history.

The consequence is practical rather than discouraging: use the number as a starting point, then adjust from observed results. Eat at the estimate for two to three weeks, weigh yourself consistently, and move the target by 100 to 200 calories based on what actually happened. Three weeks of your own data beats any formula's estimate of you.

What lowers BMR

Age. Roughly 1% to 2% per decade after 30, and most of it is muscle loss rather than metabolic change. Resistance training slows the decline substantially.

Losing weight. A smaller body needs less energy, which is why a deficit that worked at the start of a diet stops working — the target has to be recalculated as the weight falls.

Prolonged severe restriction. Adaptive thermogenesis reduces expenditure by more than the weight loss alone predicts, by around 10% to 15% in sustained aggressive dieting. It is real and it is reversible, and it is the strongest argument for a moderate deficit rather than an extreme one.

Loss of muscle. Muscle is metabolically more active than fat. Losing it through crash dieting without resistance training lowers BMR permanently until it is rebuilt.

What raises it, and by how much

More muscle. Each kilogram of muscle burns roughly 13 calories a day at rest, against about 4.5 for fat. Adding 5 kg of muscle — a serious year of training — is worth about 45 calories a day. This is real and it is much smaller than fitness marketing implies.

Being larger generally. A bigger body costs more to run, which is why BMR rises with weight in every formula.

Illness, fever and pregnancy, all temporarily.

Notably absent from this list: eating small frequent meals, drinking cold water, green tea, and most things sold to "boost metabolism". The thermic effect of food is real — about 10% of intake — but it depends on how much you eat, not on how often.

The metabolism excuse, examined

Metabolic rate does vary between people, and the variation is far smaller than commonly assumed.

Studies using doubly-labelled water — the reference method for measuring real-world energy expenditure — consistently find that people who describe themselves as having a slow metabolism have normal measured expenditure and substantially under-report what they eat, typically by 30% to 50%. The gap is not deliberate deception; estimating portions is genuinely difficult, and drinks, oils and snacks are the items most reliably forgotten.

This matters because the belief changes behaviour. Someone convinced their metabolism is the problem has no reason to weigh their food, and weighing food for two weeks is the single most informative thing an unsuccessful dieter can do.

Genuine metabolic conditions — hypothyroidism most commonly — do exist and are diagnosable with a blood test. If weight is changing without a change in habits, that test is worth having rather than guessing.

From BMR to a daily target

BMR alone is not a calorie target. Multiply it by an activity factor to get total daily energy expenditure, and set the deficit or surplus against that figure. A deficit taken from BMR rather than TDEE is roughly a third too aggressive and is how people end up eating 1,100 calories a day and feeling terrible.

As a floor, most guidance suggests not eating below BMR for extended periods, and not below about 1,200 calories for women or 1,500 for men without medical supervision — below those levels, meeting micronutrient requirements becomes genuinely difficult regardless of the energy arithmetic.

Which formula to use, in practice

The choice is simpler than the number of options suggests.

Use Katch-McArdle if you have a recent body fat measurement from a DEXA scan or a careful tape measurement. It is the only formula that distinguishes body composition, and at the same weight it can differ from the others by over a hundred calories in either direction.

Otherwise use Mifflin-St Jeor. It was derived from a modern population, it is the current default in clinical practice, and it is more accurate than Harris-Benedict for the majority of people.

Harris-Benedict is worth keeping only for comparison. Its 1919 derivation used a sample that was leaner and more active than a contemporary one, and its revised 1984 version still tends to read slightly high.

At high body fat, all the weight-based formulas overestimate, because they treat every kilogram as metabolically similar and fat tissue is not. That is exactly the situation where Katch-McArdle earns its extra input.

What BMR does not cover

It is worth being clear about the boundary, because the number gets misapplied.

BMR excludes digestion, all movement, temperature regulation beyond a comfortable room, and any recovery from exercise. It is measured after twelve hours of fasting, at complete rest, in a thermally neutral environment — conditions you are essentially never in.

The related term resting metabolic rate is measured under less strict conditions and comes out 10% to 20% higher. Many calculators and devices use the two words interchangeably, which is a source of unexplained disagreement between sources.

Neither is a calorie target. Both are the base that a target is built on, and the step people skip — applying an activity factor — is the step that makes the number usable.

What this calculator assumes

• Mifflin-St Jeor by default, with Harris-Benedict and Katch-McArdle available for comparison.
• Katch-McArdle needs a body fat percentage; without one it cannot be computed.
• The result is energy at complete rest, not a daily calorie target — apply an activity factor for that.
• Formulas are fitted to adult populations and are not appropriate for children, pregnancy or clinical conditions.
• Individual variation of roughly ±200 calories applies to every estimate here.