BMR (Basal Metabolic Rate) is the energy your body burns at complete rest — running your heart, brain, liver, and lungs, keeping your temperature stable, repairing cells. For most people it is the single biggest slice of daily calorie burn, around 60–70% of the total — more than exercise and daily movement combined.

There is no 'good' or 'bad' BMR — it simply scales with body size, muscle mass, age, and sex. Typical adult values run roughly 1,200–1,600 kcal/day for women and 1,500–2,000 kcal/day for men. A bigger or more muscular body has a higher BMR; that is arithmetic, not virtue.

How do I calculate my BMR?

Use the Mifflin-St Jeor equation: 10 × weight (kg) + 6.25 × height (cm) − 5 × age, then +5 for men or −161 for women. It is the formula this calculator leads with because it validates best against lab measurements. If you know your body fat percentage, Katch-McArdle (370 + 21.6 × lean mass in kg) can be even more accurate.

Should I eat below my BMR to lose weight?

Usually not, and it is rarely necessary. Weight loss comes from eating below your TDEE — your BMR times an activity factor — which sits comfortably above BMR for almost everyone. Chronic very-low-calorie dieting below BMR tends to cost muscle, wreck adherence, and trigger metabolic adaptation that slows further loss.

Which BMR formula is the most accurate?

For most people, Mifflin-St Jeor — studies find it lands within 10% of lab-measured BMR more often than any other general equation. If you have a genuinely measured body fat percentage (DEXA, not a mirror guess), Katch-McArdle often edges it out, especially for very lean or very muscular people. Harris-Benedict, first published in 1919, typically runs a few percent high.

BMR Calculator — Basal Metabolic Rate (3 Formulas)

Enter your age, sex, height, and weight to get your Basal Metabolic Rate — the calories your body burns at complete rest. Add your body fat percentage to unlock the Katch-McArdle estimate and compare three formulas side by side.

How the BMR calculator works

Your Basal Metabolic Rate is the energy your body spends doing absolutely nothing: pumping blood, powering your brain (a ~20% shareholder all by itself), filtering things through your liver and kidneys, holding 37 °C. For most people it is the biggest single line item in the daily calorie budget — typically 60–70% of everything you burn, which means your organs out-burn your workouts on almost every day of your life.

Nobody measures true BMR outside a lab (it requires waking up in a metabolic chamber and lying very still), so this calculator estimates it three ways: the Mifflin-St Jeor equation (the headline number, and the one most dietitians use), the revised Harris-Benedict equation (the 1984 update of the 1919 classic), and — if you enter a body fat percentage — Katch-McArdle, which estimates from lean mass alone.

The formula

Mifflin-St Jeor: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5 (men) / − 161 (women)

Katch-McArdle: BMR = 370 + 21.6 × lean mass (kg), where lean mass = weight × (1 − body fat %)

Revised Harris-Benedict (men): BMR = 88.362 + 13.397w + 4.799h − 5.677a • (women): 447.593 + 9.247w + 3.098h − 4.330a

In all three, w is weight in kilograms, h is height in centimeters, and a is age in years (US units are converted for you). Notice what Katch-McArdle does differently: it ignores age, sex, and height entirely and bets everything on lean mass — because muscle and organs are what actually burn the calories.

Worked example

A 30-year-old man, 178 cm (about 5′10″), 75 kg (about 165 lbs), 20% body fat:

Mifflin-St Jeor: 10 × 75 + 6.25 × 178 − 5 × 30 + 5 = 750 + 1,112.5 − 150 + 5 = 1,718 kcal/day

Katch-McArdle: lean mass = 75 × 0.80 = 60 kg → 370 + 21.6 × 60 = 1,666 kcal/day

Revised Harris-Benedict: 88.362 + 13.397 × 75 + 4.799 × 178 − 5.677 × 30 = 1,777 kcal/day

Three formulas, a 111-kcal spread — about a tablespoon and a half of peanut butter. That spread is the honest error bar on any BMR estimate.

BMR vs TDEE: which number do you actually use?

BMR is a component, not a target. You don't spend the day in a coma, so the number you plan meals around is your TDEE — BMR multiplied by an activity factor of 1.2–1.9 to cover walking, working, fidgeting, digesting, and training. Our TDEE calculator does exactly that multiplication and adds cutting and bulking targets. The practical division of labor: use this page to understand your resting burn and compare formulas; use the TDEE page to set calories. Eating at your BMR is already a fairly aggressive deficit for most people, which surprises almost everyone the first time they do the math.

Why formulas disagree, and can dieting really lower your BMR?

Every equation here is a regression fitted to a sample of measured humans, and you are not the average of any sample. Real BMRs scatter about ±10% around the predictions — muscle mass, genetics, thyroid function, and even ambient temperature all move the needle. That is why the formulas disagree with each other by 5–7% and why none of them is "wrong": treat the Mifflin number as your best single estimate and the spread as the confidence interval.

And yes — aggressive dieting genuinely suppresses BMR, a phenomenon called adaptive thermogenesis. Under a large, prolonged deficit your body downshifts: thyroid output drops, spontaneous movement quietly evaporates, and each lost kilogram of tissue takes its running costs with it. Studies of crash dieters find resting burn falling meaningfully below what their new, lighter body would predict. The dieting moral: moderate deficits, plenty of protein, and lifting to keep lean mass are how you lose fat without teaching your metabolism to hibernate.

BMR Calculator