BMR
Calculator

What is BMR?

Your basal metabolic rate is the energy your body needs at complete rest just to keep you alive — powering your heart, lungs, brain, kidneys and the constant repair of cells. Even if you stayed in bed all day, you'd still burn roughly this many calories. For most people, BMR is the single biggest slice of daily energy use, often 60–70% of the total, which is why it's the foundation of any calorie plan.

The Mifflin-St Jeor equation

This calculator uses the Mifflin-St Jeor equation, the formula most dietitians reach for because a 2005 review found it predicts resting metabolism within 10% more often than the older Harris-Benedict, Owen or WHO equations. It is:

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s

where s = +5 for men and −161 for women. The sex term exists because, on average, men carry more lean muscle mass, which burns more energy at rest than fat tissue. Everything in the formula — weight, height, age and sex — is a proxy for how much metabolically active tissue you have.

BMR by weight (example)

To show how the inputs move the number, here's BMR for a 175 cm, 30-year-old at different weights:

Weight	Men	Women
55 kg	1,499 kcal	1,333 kcal
65 kg	1,599 kcal	1,433 kcal
75 kg	1,699 kcal	1,533 kcal
85 kg	1,799 kcal	1,633 kcal
95 kg	1,899 kcal	1,733 kcal

BMR, RMR and TDEE: what's the difference?

These three terms get muddled often. BMR is your resting burn under strict conditions (fasted, fully rested). RMR (resting metabolic rate) is almost the same thing measured under less strict conditions, and the two are usually close enough to use interchangeably. TDEE (total daily energy expenditure) is the big one for everyday planning: it takes your BMR and multiplies it by an activity factor to include walking, exercise and daily movement. If you want a calorie target to lose, maintain or gain weight, you want TDEE — see our TDEE calculator.

What affects your BMR

Body size and composition are the biggest factors: more tissue, and especially more lean muscle, raises your resting burn. Age lowers it gradually, partly because muscle mass tends to decline over the years. Sex plays a role through average differences in muscle and fat. Genetics, hormones (notably thyroid function) and body temperature contribute too. One practical takeaway: building and keeping muscle is one of the few levers you can actually pull to support a higher BMR over time.

Why BMR matters for weight management

You can't sensibly set a calorie target without knowing your baseline burn. BMR gives you that floor, and TDEE builds on it to tell you your maintenance calories — the amount that keeps your weight stable. To lose weight you eat below maintenance; to gain, above it. Skipping this step is why generic "eat 2,000 calories" advice works for some and not others: 2,000 might be a deficit for a tall, active man and a surplus for a smaller, sedentary person.

How accurate is a BMR estimate?

Equations like Mifflin-St Jeor are well-validated but still estimates — they can be off by around 10% for any individual, because they can't see your exact body composition or hormones. Two people with identical height, weight, age and sex but different muscle mass will have genuinely different BMRs that the formula reads as the same. Treat your number as a strong starting point, then adjust based on real-world results over a few weeks.

How to support a higher BMR

You can't change your age or height, but you can influence the parts of metabolism tied to body composition. Building and keeping muscle through resistance training is the most reliable lever — muscle is more metabolically active than fat, so more of it nudges your resting burn upward. Eating enough protein supports muscle maintenance, especially during weight loss, and has a higher thermic effect than carbs or fat. Avoiding very aggressive diets matters too, since prolonged severe restriction can suppress metabolism and cost you muscle. None of these turn a small metabolism into a furnace, but together they protect and modestly raise your baseline over time.

Does metabolism really slow with age?

The popular belief that metabolism nosedives in your 30s and 40s is largely a myth. Large research using precise measurement has found that, after accounting for body size and composition, resting metabolism is remarkably stable from about age 20 to 60, only declining meaningfully later in life. What most people experience as a "slowing metabolism" in midlife is usually a combination of lost muscle mass and reduced daily movement, not an inevitable drop in the metabolic rate itself. The encouraging implication: staying active and strong keeps your engine running closer to its younger self than the myth suggests.

Other BMR formulas

Mifflin-St Jeor is the modern default, but you may see others. The older Harris-Benedict equation tends to overestimate slightly. The Katch-McArdle formula is based on lean body mass instead of total weight, which can be more accurate if you know your body-fat percentage — useful for lean, muscular people whom weight-based formulas underrate. For the general population without a body-fat measurement, Mifflin-St Jeor remains the best balance of accuracy and simplicity, which is why this calculator uses it.

Can you really have a "slow metabolism"?

It's a common worry, but genuinely slow metabolisms are rarer than people think. When researchers measure resting metabolic rate, most individuals fall within about 10% of what the equations predict for their size, age and sex — there's far less variation between people than diet folklore suggests. The bigger driver of why two people of the same size seem to "burn differently" is usually NEAT: the unconscious daily movement (fidgeting, standing, walking) that can vary by hundreds of calories a day. So if weight loss feels hard, it's much more often about total intake and activity than a broken metabolism. Medical conditions like hypothyroidism can lower BMR, but they're the exception, not the rule — worth checking with a doctor if you suspect one, rather than assuming.

Don't eat below your BMR

A common temptation is to slash calories below BMR to speed things up. It usually backfires: extreme deficits are hard to stick to, tend to strip away muscle along with fat, and leave you tired and hungry. The more sustainable approach is a moderate deficit from your TDEE — large enough to make progress, small enough to live with. Slow and steady genuinely wins here.