What Is My BMR? Basal Metabolic Rate, Explained With Numbers

Your basal metabolic rate is the energy your body burns doing absolutely nothing — lying still, awake, not digesting a meal, not moving a muscle on purpose. Over a full twenty-four hours that "nothing" adds up to a startling amount: a 30-year-old male at 70 kg and 175 cm burns about 1,649 kcal a day just staying alive. Heart pumping, lungs breathing, kidneys filtering, cells repairing and replacing themselves — that is where most of your daily calories actually go, before a single step is taken.

The number comes from four facts about your body — sex, age, weight, and height — fed through one equation. Which means you can see exactly where every kilocalorie comes from, and exactly what moves the number when your body changes.

The formula: Mifflin-St Jeor

The modern standard for estimating BMR is the Mifflin-St Jeor equation. It takes weight in kilograms, height in centimetres, and age in years, then finishes with a constant set by sex:

• male: BMR = 10 × kg + 6.25 × cm − 5 × age + 5
• female: BMR = 10 × kg + 6.25 × cm − 5 × age − 161

The first three terms are identical; only the final constant differs. Run the defaults — a 70 kg, 175 cm, 30-year-old male:

• 10 × 70 = 700
• 6.25 × 175 = 1,093.75
• −5 × 30 = −150
• male constant = +5

Total: 700 + 1,093.75 − 150 + 5 = 1,648.75, which rounds to about 1,649 kcal/day. The same female body swaps +5 for −161 and lands at 1,482.75, about 1,483 kcal/day.

The BMR calculator runs this for you, and the weight and height fields take kilograms or pounds, centimetres or inches — it converts to metric internally, because the equation is defined in kg and cm. Type your numbers in whichever units your scale and tape measure use.

What each input is worth

Because the equation is linear, every input has a fixed price per unit, and knowing the prices makes the whole thing intuitive:

• Weight: +10 kcal/day per kg. Go from 70 to 80 kg and the default male result climbs from 1,649 to about 1,749 kcal/day. Weight is the heaviest lever, and the only one you meaningfully control.
• Height: +6.25 kcal/day per cm. A 165 cm version of the same body sits at about 1,586 kcal/day; the 175 cm version at 1,649. More body simply costs more to run.
• Age: −5 kcal/day per year. The same 70 kg, 175 cm male at 50 comes out at about 1,549 kcal/day — a hundred kilocalories lower than at 30, from the age term alone.
• Sex: +5 or −161. Not a per-unit lever but a single constant — and the biggest one-step swing in the equation.

Male vs female: one constant, 166 kcal

Take the default body and change nothing but the sex toggle. The first three terms — 700 + 1,093.75 − 150 = 1,643.75 — are identical either way. The male form adds 5; the female form subtracts 161.

That is the entire 166 kcal/day difference between about 1,649 and about 1,483. No other input changed. It is why the sex toggle in the calculator matters as much as the weight field: one click shifts the result by well over a hundred kilocalories.

BMR is a floor, not a budget

The single most common mistake with this number: reading BMR as "the calories I should eat." It is not. BMR assumes a day spent lying completely still, and almost nobody has that day. Standing, walking, working, and exercising all burn energy on top of the baseline, so your real daily needs are always higher than your BMR.

To turn the floor into a realistic daily figure, multiply by an activity factor. That product is your total daily energy expenditure, or TDEE, and on the default 1,649 kcal baseline it runs from just under 2,000 to more than 3,100 kcal a day:

• Sedentary (×1.2): about 1,979 kcal/day
• Moderate (×1.55): about 2,556 kcal/day
• Very active (×1.9): about 3,133 kcal/day

The TDEE calculator does this step for you — it builds the same Mifflin-St Jeor baseline and applies the activity multiplier, so the natural workflow is BMR first to understand the floor, TDEE second to get a number you can plan around.

What BMR actually pays for

It helps to know what that 1,649 kcal buys. BMR covers everything your body does without being asked: the heart pumping, the lungs breathing, the kidneys filtering, the constant repair and replacement of cells, the maintenance of body temperature and brain function. The definition is strict — awake but lying still, not digesting a recent meal, not moving on purpose. It is the floor of your energy needs: the amount you would burn if you did nothing at all that day.

That strictness is also why the number feels surprisingly large. Most people assume exercise dominates their calorie burn; for most bodies, the resting baseline does.

How accurate is the estimate?

Mifflin-St Jeor is generally the most accurate of the common BMR equations for today's population, which is why it largely replaced the older Harris-Benedict formula. But it remains an estimate, not a measurement. The equation was fitted to averages across a study population, so it predicts the typical metabolism for a given sex, age, weight, and height — not yours specifically. Two people who match on all four inputs can still have meaningfully different real metabolisms, because muscle mass, body composition, genetics, and hormones all move the true number, and none of them appear in the inputs.

Treat the result as a well-grounded starting point rather than a verdict. If you are planning around a target weight, the ideal weight calculator gives you a reference range to pair with it — and once you have a maintenance figure, the protein intake calculator covers the next planning question. For anything tied to your specific health — a medical condition, a significant weight change, a clinical diet — this is general information only, and a registered dietitian or your doctor is the right place to go.

Common mistakes

• Eating at your BMR and calling it maintenance. BMR is the floor; maintenance is BMR × activity factor. Eating at the floor is an unplanned deficit.
• Ignoring the sex constant when comparing numbers. Two identical bodies differ by 166 kcal/day on the constant alone.
• Mixing units. The equation is defined in kg and cm — set the toggles to match what you type, and let the tool convert.
• Treating the estimate as a measurement. It describes a typical body with your four inputs, not your body. Use it as a starting point and adjust from real-world results.
• Stopping at BMR. The useful planning number is TDEE; BMR is the ingredient, not the answer.

Four inputs, one equation, one honest number: the energy your body spends before you do anything at all. Get the baseline, then scale it by how you actually live.