Free BMR Calculator Online

BMR Calculator: Estimate the Calories Your Body Burns at Complete Rest

Basal Metabolic Rate is the number of calories your body needs just to stay alive if you did nothing all day but lie still — no walking to the kitchen, no climbing stairs, no workout, not even digesting a meal. It covers the energy cost of keeping your heart beating, your lungs cycling air, your organs functioning, and your cells repairing themselves. This BMR calculator estimates that resting figure from four inputs — sex, age, weight, and height — using the Mifflin-St Jeor equation by default, with the older Harris-Benedict equation shown alongside it for comparison. Every calculation happens the instant you type, right there in the input fields, because the underlying arithmetic is a handful of multiplications and additions that a browser finishes long before you'd notice any delay.

It's worth being precise about what this number is not: it is not the number of calories you should eat today, and it is not how many calories you burn in a normal day. BMR intentionally strips out movement, exercise, work, and even the energy your body spends digesting food. It's the floor, not the total. If you want the full daily number that adds activity back in, that's a separate calculation — this tool's job is narrower and more specific: give you the resting baseline that everything else gets built on top of.

How the BMR Calculator Works

The interface asks for a small set of details and updates the result live as you adjust them:

1. Pick a unit system — metric (kilograms and centimeters) or imperial (pounds and inches). Imperial entries are converted internally to kilograms and centimeters before the formulas run, so the math itself always operates in metric regardless of which units you typed.
2. Select biological sex. Both supported equations branch on sex because, on average, men carry a higher proportion of metabolically active lean mass than women at the same weight and height, which the formulas account for with a different constant term.
3. Enter age, body weight, and height.
4. Choose an activity level from the dropdown. This doesn't change the BMR figure itself — BMR is activity-independent by definition — but the calculator uses it to show a preview TDEE (Total Daily Energy Expenditure) figure alongside the resting number, so you can see at a glance what a realistic full-day estimate would look like without leaving the page.
5. Three results appear side by side: the Mifflin-St Jeor estimate, the Harris-Benedict estimate, and the activity-adjusted preview. All three recalculate instantly whenever you change an input.

There's no submit button, no account, and no saved history — you get a number, you can change an input and watch it update, and that's the entire interaction.

Why People Look Up Their Resting Metabolic Rate

A BMR figure on its own is rarely the end goal — it's almost always step one of something larger:

• Building a calorie target from scratch. Before you can figure out how much to eat for weight loss, maintenance, or gain, you need a resting baseline to multiply by an activity factor. BMR is that starting number.
• Understanding why metabolism seems to slow with age. Because age carries a negative coefficient in both formulas, plugging in the same weight and height at 25 versus 55 produces a visibly lower resting number for the older age — a concrete illustration of a pattern people often hear about only in the abstract.
• Comparing scenarios. Swapping the weight or height field to see how a different body size changes the resting number is a quick way to understand the direction and rough size of the effect, without needing to memorize or re-derive the formula by hand.
• Cross-checking a number from elsewhere. If a trainer, app, or article gave you a BMR figure and you want a second, transparent calculation using a named, standard equation to compare it against.
• Curiosity about metabolic rate in general. Plenty of people simply want to know the number — no diet or training plan attached.

The Two Equations, Side by Side

Both formulas take the same four inputs and return an estimate in calories per day, but they were derived from different populations at different times, and they don't always agree.

Equation	Origin	Formula (metric)	Worked example: 80 kg, 180 cm, age 30, male
Mifflin-St Jeor	1990 (default equation used here)	Men: 10×kg + 6.25×cm − 5×age + 5. Women: same, minus 161 instead of plus 5.	1780 calories/day
Harris-Benedict (revised)	1984 revision (Roza & Shizgal) of the original 1919 equation	Men: 88.362 + 13.397×kg + 4.799×cm − 5.677×age. Women: 447.593 + 9.247×kg + 3.098×cm − 4.330×age.	~1854 calories/day

Using the same 80 kg / 180 cm / 30-year-old inputs for a woman instead of a man, the Mifflin-St Jeor result drops to 1614 calories/day — the same base calculation, just closing with −161 instead of +5, which is the formula's way of accounting for typical differences in body composition between the two groups it was calibrated on. Mifflin-St Jeor is generally regarded by dietitians and researchers as the better fit for the modern general population, which is why it's the default equation in this calculator, but Harris-Benedict is included because it's still widely referenced and you may want to compare a number you found somewhere else against the equation that actually produced it.

What Actually Moves the Number

Each of the four inputs pulls the result in a specific, predictable direction, which is worth understanding even before you look at the exact formula. Weight has the largest single coefficient in both equations — heavier bodies generally require more resting energy, largely because more total tissue, including organs and muscle, needs to be maintained. Height contributes a smaller positive effect, reflecting the additional structural tissue that comes with a taller frame. Age works in the opposite direction: both equations subtract a multiple of age from the total, capturing the well-documented tendency for resting metabolic rate to decline gradually across adulthood, largely tied to a gradual loss of lean muscle mass over the decades if nothing is done to counteract it through resistance training. Sex changes the constant term added or subtracted at the end, a rough stand-in for average differences in body composition — namely, that men typically carry a higher proportion of muscle relative to fat than women at a matching weight and height, and muscle tissue burns more resting energy than fat tissue does.

None of the four inputs captures body composition directly, though — two people at an identical weight, height, age, and sex could have quite different actual muscle-to-fat ratios and therefore quite different true resting metabolisms, even though both would receive the exact same output from this calculator. That's the single biggest reason the result is described throughout this page as an estimate rather than a measurement.

How Accurate Is This Estimate, and Is It Medical Advice?

Neither equation was built by measuring your individual metabolism — both are regression formulas fit to data from groups of study participants, so what you get back is a population-level estimate, not a personal lab reading. Real resting metabolic rate varies with muscle mass, genetics, hormone levels (including thyroid function), certain medications, and other individual factors that a formula using only sex, age, weight, and height simply can't capture. The only way to measure your actual resting energy expenditure directly is indirect calorimetry, a supervised clinical or lab test that measures the oxygen you consume and carbon dioxide you produce while resting.

With that limitation in mind: this calculator is an informational and educational tool. It produces an estimate, not a diagnosis, and nothing it outputs should be treated as medical or nutrition advice. If you have a thyroid condition, are managing a chronic illness, are pregnant, or have any specific health concern that affects metabolism, talk to a doctor or a registered dietitian before making decisions based on a number from any BMR calculator, including this one.

Mistakes Worth Avoiding

• Treating BMR as a daily eating target. Eating at your resting number alone, with zero allowance for activity, would put almost anyone in a fairly steep deficit — BMR is a component of a calorie plan, not the plan itself.
• Skipping the unit toggle. Entering pounds into a field expecting kilograms (or the reverse) produces a wildly wrong number without any error message, since the field just accepts whatever numeric value you type.
• Confusing the "activity level" dropdown with the BMR result. That dropdown only affects the preview TDEE figure shown next to the resting numbers — the Mifflin-St Jeor and Harris-Benedict values themselves never change based on activity, because resting metabolism is activity-independent by definition.
• Assuming a single number is exact. Given the population-estimate nature described above, treat the output as a reasonable starting range rather than a figure accurate to the calorie.

Getting a BMR Number Without This Calculator

There's more than one route to a resting-metabolism figure:

• Doing the arithmetic by hand. Both equations are simple enough to compute with a calculator app once you have the constants memorized or written down — the Mifflin-St Jeor version in particular is just a few multiplications and one addition or subtraction.
• A clinical indirect calorimetry test. The most accurate option, but it requires specialized equipment and a supervised setting, so it's typically reserved for athletes, clinical patients, or research contexts rather than everyday use.
• Fitness trackers and smart scales. Some wearables and body-composition scales display a BMR-style estimate, though many of these use proprietary formulas that incorporate additional data like body-fat percentage or heart-rate patterns, so the number won't necessarily match a standard equation.
• This calculator's API. If you're building an app, spreadsheet macro, or automation that needs a Mifflin-St Jeor or Harris-Benedict figure programmatically instead of through a browser form, the same calculation is exposed as a metered API endpoint that accepts weight, height, age, sex, and an optional formula choice, and returns the estimate as JSON — see the API documentation for the exact request shape and credit cost.

Why doesn't changing the activity level dropdown change my BMR number?

Because BMR is defined as the energy cost of being completely at rest — activity is deliberately excluded. The dropdown only feeds a separate preview figure (BMR multiplied by an activity factor) shown next to the resting numbers so you can see roughly what a full-day estimate would look like; the Mifflin-St Jeor and Harris-Benedict figures themselves stay fixed regardless of what you pick there.

Which formula should I trust more, Mifflin-St Jeor or Harris-Benedict?

Mifflin-St Jeor (1990) is the more recent of the two and is generally considered the better default for a modern general population, which is why it's the primary figure this calculator leads with. Harris-Benedict, even in its 1984 revised form, tends to run a bit higher for many body types. Neither is "correct" in an absolute sense — both are estimates — but Mifflin-St Jeor is the more commonly recommended starting point.

Does this tool save or upload my age, weight, or height?

No. The calculation runs entirely as JavaScript in your own browser tab. Nothing you type into the form is transmitted anywhere to produce the on-screen result.

Can I enter my height in feet and inches instead of just inches?

The imperial mode expects a single numeric height field in inches, so a height like 5'11" needs to be entered as 71 (5 feet × 12 + 11 inches) rather than split across two fields.

Is my BMR the number of calories I should eat every day?

No — that would leave no calorie allowance for walking, working, exercising, or even digesting food. BMR is the resting floor beneath a full daily estimate; if you want a number that accounts for how active you actually are, that's what an activity-adjusted daily energy calculation is for.

Why do the two formulas give me slightly different numbers for the same inputs?

They were derived from different study populations using different statistical methods decades apart, so some divergence is expected rather than a sign that either one is broken. Neither figure should be read as more "exact" than the other — both are estimates, and the gap between them is a useful visual reminder of how much uncertainty is baked into any formula-based approach.

Related Tools

Once you have a resting number, the natural next step is turning it into a real-world daily figure with the TDEE calculator, which multiplies this same BMR by an activity factor. From there, the calorie calculator takes that daily figure and applies a deficit or surplus for weight loss, maintenance, or gain goals. If body composition rather than metabolism is what you're curious about, the body fat calculator estimates fat percentage from circumference measurements, and the ideal weight calculator offers a set of classic reference-weight formulas based on height alone.