BMR Calculator

What your body spends doing nothing at all

A 30-year-old man, 180 cm, 80 kg burns about 1,780 calories a day if he does nothing whatsoever — no walking, no work, no exercise. That is the energy required to keep his heart beating, lungs moving, kidneys filtering, liver processing and brain running, plus maintaining body temperature. It accounts for roughly 60 to 70% of everything he burns in a typical day.

Basal metabolic rate is that floor. It is the single largest component of energy expenditure for almost everyone, which is the reason it matters: the difference between people's daily calorie needs is driven far more by body size and composition than by how much exercise they do. An hour of moderate running burns perhaps 600 calories. The body burns nearly three times that simply existing.

BMR is not the same as TDEE, total daily energy expenditure, which is BMR multiplied by an activity factor and is the number you actually eat against. BMR is the input; TDEE is the answer. Confusing the two is the most common error on this topic, and it leads people to eat at their BMR — a substantial and usually unsustainable deficit.

The equation, and why this one

This calculator uses the Mifflin-St Jeor equation, published in 1990:

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

Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161

Each term reflects something real. Weight carries the largest coefficient because more tissue costs more energy to maintain. Height contributes because a taller frame has more surface area and more lean mass at a given weight. Age subtracts, because metabolically active muscle declines and cellular activity slows from roughly the third decade onward. The sex constant — +5 against −161 — is a crude adjustment for the fact that men typically carry a higher proportion of lean mass at the same height and weight.

Mifflin-St Jeor replaced the older Harris-Benedict equations of 1919 because it proved more accurate against modern populations, typically landing within about 10% of measured resting expenditure for most people. The Katch-McArdle formula can be more accurate still, but it requires a known body fat percentage, which most people do not have.

That 10% band is the honest accuracy of any of these. Individual metabolic rates vary by several hundred calories at identical height, weight, age and sex, driven by lean mass, thyroid function, genetics and non-exercise movement.

Working the example

Male, 80 kg, 180 cm, 30 years old.

• 10 × 80 = 800
• 6.25 × 180 = 1,125
• 5 × 30 = 150, subtracted
• Male constant: +5

800 + 1,125 − 150 + 5 = 1,780 calories a day.

The same person at 50 instead of 30: the age term becomes 5 × 50 = 250, giving 1,680 — a drop of 100 calories a day across two decades. Real metabolic decline with age is larger than that, but most of the additional fall comes from *losing muscle*, which shows up through the weight and composition terms rather than the age term. This is why resistance training matters so much for maintaining energy needs later in life.

A woman of the same size and age: 800 + 1,125 − 150 − 161 = 1,614 — about 166 calories lower, reflecting typical differences in lean mass.

Now turn BMR into TDEE by applying an activity multiplier:

• Sedentary (desk job, little exercise) × 1.2 = 2,136
• Lightly active (1–3 days a week) × 1.375 = 2,448
• Moderately active (3–5 days) × 1.55 = 2,759
• Very active (6–7 days) × 1.725 = 3,071
• Extremely active (physical job or twice-daily training) × 1.9 = 3,382

The spread between sedentary and extremely active is over 1,200 calories — which is why choosing an activity level honestly matters far more than the precision of the BMR itself.

When this is worth calculating

Before setting a calorie target. A deficit or surplus is only meaningful relative to maintenance, and maintenance is TDEE, not BMR.

When weight loss stalls. As you lose weight, BMR falls, because there is less tissue to maintain. A deficit calculated at 90 kg is no longer a deficit at 78 kg — recalculating every 5 kg or so is the usual fix for a plateau.

When planning a return to training, or after a long break, to understand how much of your intake is baseline versus activity.

To set realistic expectations. Seeing that exercise adds a few hundred calories while existence costs 1,780 tends to reframe where dietary change actually comes from.

Reading your result

Never eat at your BMR. It is the energy required for organ function at complete rest, and eating at that level while living a normal life creates a deficit of 400–1,000 calories a day. Sustained, that drives muscle loss, fatigue, hormonal disruption and metabolic adaptation, and it is a well-documented route to regaining the weight afterwards.

Treat the figure as a starting estimate with a ±10% band. For the example above, true resting expenditure plausibly sits anywhere between roughly 1,600 and 1,960. The practical approach is to compute TDEE, eat at that level for two to three weeks while tracking weight honestly, and adjust based on what actually happens. Your own trend data beats any equation.

Note also that the equation cannot see body composition. Two people at 80 kg and 180 cm — one at 12% body fat, one at 32% — get the same answer here, though the leaner person's real BMR is meaningfully higher. If you know your body fat percentage, the Katch-McArdle formula will serve you better.

This is a general-information estimate, not medical or dietary advice. Thyroid disorders, PCOS, medications and several other conditions materially affect metabolic rate, and anyone with a diagnosed condition, a history of disordered eating, or who is pregnant or breastfeeding should work from clinical guidance rather than a formula.

Common mistakes

Eating at BMR instead of TDEE. The single most consequential error here.

Overstating activity level. Most people who describe themselves as moderately active are closer to lightly active. Structured exercise three times a week does not offset a desk job and a car.

Never recalculating. BMR falls as weight falls. A stalled plateau is usually an out-of-date maintenance figure.

Expecting precision. These are population equations. A 10% error on 1,780 is nearly 180 calories.

Assuming metabolism is broken. Very large unexplained gaps between predicted and actual are uncommon; under-recording intake is far more common, and is usually the explanation.

Ignoring non-exercise movement. Fidgeting, standing, walking to the shops — NEAT varies by hundreds of calories between individuals and is invisible to activity multipliers. ## BMR, RMR and the numbers on your watch

Three related terms circulate, and they are not interchangeable.

BMR is measured under strict laboratory conditions: after an overnight fast, following eight hours of sleep, lying still in a thermally neutral room, having done no exercise. Almost nobody ever measures a true BMR outside a research setting.

RMR, resting metabolic rate, is what clinics actually measure — the same idea under realistic conditions, without the overnight stay. RMR typically comes out 10 to 20% higher than true BMR, and in practice the two terms are used interchangeably in consumer tools, including this one.

What a smartwatch reports is neither. Wearables estimate total daily expenditure from heart rate, movement and your entered profile, and their accuracy for calorie burn is poor — validation studies routinely find errors of 20 to 40%, usually in the direction of overestimating. They are useful for tracking *relative* change in your own activity from week to week, and unreliable as an absolute figure to eat against.

If you want a genuine measurement rather than an estimate, indirect calorimetry — breathing into a metabolic cart for 10 to 20 minutes while the device measures oxygen consumption and carbon dioxide production — is the accessible clinical standard, and is offered by some sports medicine clinics and hospital nutrition departments. For most people the formula plus two weeks of honest weight tracking gets close enough that the measurement is not worth the cost.