BMR Calculator

Free BMR Calculator - Find Your Basal Metabolic Rate

Your BMR is the number of calories your body needs just to stay alive — no walking, no workouts, not even sitting up. This BMR calculator works it out in seconds using your age, gender, weight, and height, and gives you the Mifflin-St Jeor result by default while also showing the Harris-Benedict and Katch-McArdle numbers side by side, so you can see how they compare instead of blindly trusting one formula.

Once you have your BMR, the calculator goes a step further and shows your TDEE (Total Daily Energy Expenditure) based on your activity level, so you're not stuck with just one number that means little on its own. Whether you're starting a diet, planning a bulk, or just curious how many calories your body burns doing nothing, this tool gives you a real, usable answer in one place — no email, no app download, no login wall.

What Is BMR and Why It Actually Matters

BMR stands for Basal Metabolic Rate. It's the energy your body burns over 24 hours just to keep the lights on — breathing, pumping blood, repairing cells, keeping your brain and organs running. If you stayed in bed all day and did nothing at all, your BMR is roughly what you'd still burn.

This number matters because it's the floor. Every calorie calculator, every diet plan, every 'how much should I eat' answer starts from BMR and builds on top of it. Get your BMR wrong and everything downstream — your deficit for weight loss, your surplus for muscle gain, your daily calorie target — ends up wrong too. That's why this calculator shows the full math, not just a final number you have to trust blindly.

The Formula This Calculator Uses

By default, this calculator uses the Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate BMR formula for the general adult population. It's what most dietitians, personal trainers, and hospital nutrition teams use today.

For men: BMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5. For women, the setup is identical except you subtract 161 instead of adding 5. That's the entire formula — plain arithmetic, no hidden variables, and you can check the exact numbers in the step-by-step section above.

Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

This calculator doesn't just give you one number — it runs all three major formulas so you can see how close (or far apart) they land. Harris-Benedict is the oldest of the three, first published in 1919 and revised in 1984. It's still used today, but research comparing it against measured metabolic rate generally finds it runs a bit high compared to newer data, which is part of why Mifflin-St Jeor has replaced it as the modern default.

Katch-McArdle takes a completely different approach: instead of using your total body weight, it only uses lean body mass, calculated from your body fat percentage. This makes it the most accurate option for people who are lean, muscular, or who already know their body fat % from a scan or caliper test — because two people at the same weight but very different body fat can have noticeably different metabolisms, and Katch-McArdle is the only one of the three formulas that accounts for that.

• Use Mifflin-St Jeor if you don't know your body fat % — it's the best general-purpose option.
• Use Harris-Benedict mainly for comparison or if a doctor's chart specifically references it.
• Use Katch-McArdle if you know your body fat % and are lean or muscular — it tends to be the most personalized.

BMR vs RMR vs TDEE: Clearing Up the Confusion

These three terms get mixed up constantly, so here's the short version. BMR (Basal Metabolic Rate) is measured under strict lab conditions — fasted, fully rested, first thing in the morning. RMR (Resting Metabolic Rate) is the more practical, real-world version of the same idea, measured without those strict lab conditions, and it usually comes out slightly higher than BMR because true lab conditions are hard to replicate at home. For everyday use, most calculators — including this one — treat BMR and RMR as close enough to interchangeable.

TDEE (Total Daily Energy Expenditure) is the number that actually decides your calorie intake. It takes your BMR and adds everything else: walking around, workouts, digestion, fidgeting, even talking. This calculator shows TDEE right next to BMR so you're never left wondering which number to actually eat around — spoiler: it's TDEE, not BMR.

How Age, Gender, and Body Composition Change Your BMR

BMR isn't fixed — it shifts with a few key factors, and this calculator's 'By Age' chart shows one of them directly. Muscle tissue burns more calories at rest than fat tissue does, which is why men typically have a higher BMR than women at the same weight and height — men generally carry more muscle mass on average. It's also why BMR tends to decline gradually with age: after your 20s, most people lose a small amount of muscle mass each decade unless they actively train to keep it, and less muscle means a lower resting burn rate.

Genetics, thyroid function, and even how much you've dieted in the past all nudge BMR up or down slightly too. None of that shows up in a formula based on weight, height, age, and gender — which is exactly why this calculator treats its output as a very solid starting estimate, not a lab-measured fact.

What To Do With Your BMR Number

On its own, BMR isn't something you eat at — it's too restrictive to use as an actual daily target, since it doesn't include any activity at all. The real move is to take your BMR, multiply it by an activity factor (this calculator does that automatically once you pick your activity level), and use the resulting TDEE as your maintenance calories.

From there, a calorie deficit below TDEE drives weight loss, and a calorie surplus above TDEE drives weight gain — BMR is just the starting point that makes those numbers accurate to your actual body instead of a generic guess.

Common Mistakes People Make Reading Their BMR

A few mix-ups come up again and again with BMR calculators, and knowing them upfront saves confusion later.

• Eating at BMR-level calories thinking it's a normal daily target — it's not, it's the bare minimum with zero activity included.
• Ignoring the activity multiplier entirely and assuming BMR is 'how many calories I need per day.'
• Not updating the numbers after a big weight change — BMR shifts as your weight shifts, so recheck it every 5-10 lb (2-4 kg).
• Trusting one formula blindly instead of checking how close the three formulas land, especially if body fat % is available.
• Forgetting that BMR is an estimate, not a lab measurement — real metabolism varies by roughly 5-10% between people with identical stats.

Can You Increase Your BMR?

To a real but modest degree, yes. Building muscle is the most reliable lever — muscle tissue burns more calories at rest than fat does, so adding lean mass through resistance training gradually nudges BMR upward over months, not days. Eating enough protein supports that muscle, which indirectly supports BMR too.

What doesn't meaningfully move BMR: 'metabolism-boosting' teas, extreme low-calorie diets (which often lower BMR by triggering muscle loss and a slowdown response), or specific 'fat-burning' foods. There's no shortcut here — consistent strength training and adequate protein over time is what actually shifts the number, and even then the change is gradual, not dramatic.

How Accurate Is This BMR Calculator?

Mifflin-St Jeor, the default formula here, has been validated against indirect calorimetry (the gold-standard lab measurement) in multiple studies and generally lands within about 10% of measured BMR for most people — which is genuinely good for a free, instant formula. Katch-McArdle can edge closer for lean or muscular individuals specifically because it accounts for body composition rather than treating all weight the same.

No formula-based calculator, including this one, can match an actual lab test using indirect calorimetry, which measures the oxygen you consume and carbon dioxide you produce at rest. For almost everyone, though, that level of precision isn't necessary — the formula estimate is close enough to build a real nutrition plan around, especially if you treat it as a starting point and adjust based on how your weight actually changes over a few weeks.

Who This BMR Calculator Is For

This tool is built for anyone who wants a real, formula-based number instead of a rough guess — someone starting their first diet, a lifter checking their numbers before a bulk or cut, a coach setting a baseline for a client, or someone just curious what their body burns doing nothing at all. It's not a substitute for medical advice: people with diagnosed metabolic or thyroid conditions, pregnant or breastfeeding individuals, and anyone under 18 should get individualized numbers from a doctor or registered dietitian rather than relying on standard adult formulas like the ones used here.