Free metabolism calculator
BMR Calculator
The number every diet starts from. Get your basal metabolic rate — what your body burns at rest — from four validated equations, compared side by side, and carry it straight into your recomp plan.
Sample numbers are shown as placeholders. Fill in your own and the result appears as soon as you edit anything.
The published equations use sex-specific coefficients. Not a gender identity field.
Enter it and we headline the lean-mass equations (Katch-McArdle / Cunningham), which don't under-read a muscular lifter. Leave it blank and we use Mifflin-St Jeor.
Enter your age, weight, and height above to see your resting calorie burn from all four validated equations, compared side by side.
Free. No signup. The tool's own usage events never include your exact numbers; a share link encodes them into its URL.
These are population estimates with a real error band, not medical advice. Don't use them for clinical decisions, under-18 nutrition, pregnancy, or eating-disorder recovery.
What BMR actually is
Your basal metabolic rate is the energy your body spends doing nothing. Keeping your heart beating, your brain running, your kidneys filtering, and your cells maintained costs calories even when you're completely still. For most people, BMR is the single biggest slice of daily energy expenditure — often 60–70% of the total — which is why it's the base of every calorie plan.
A quick honesty note on BMR versus RMR. True BMR is measured under strict conditions: an overnight fast, complete rest, a temperature-controlled room, first thing in the morning. RMR (resting metabolic rate) relaxes those conditions and comes out a little higher. The equations on this page were fit to real-world data and, in practice, estimate a resting burn that sits between the two — closer to RMR than to a chamber-measured BMR. We call it BMR because that's the term people search for, but we won't pretend the number is lab-exact.
What BMR is not is your calorie target. You burn more than your BMR the moment you get out of bed, and more still when you train. That's covered in the loud section further down — the short version is: never eat at your BMR.
The four equations
This calculator runs every equation it can and shows them together, because they genuinely disagree and a single number would hide that. Here's what each one is and where it's strongest.
| Equation | Formula | Best for | Source |
|---|---|---|---|
| Mifflin–St Jeor | 10·kg + 6.25·cm − 5·age + (male +5 / female −161) | The best all-round default — validated most widely for the general population and endorsed by the Academy of Nutrition and Dietetics. | Mifflin & St Jeor, 1990 |
| Revised Harris–Benedict | male: 88.362 + 13.397·kg + 4.799·cm − 5.677·age · female: 447.593 + 9.247·kg + 3.098·cm − 4.330·age | The modernised form of the 1919 classic. Tends to read a little higher than Mifflin for most builds. | Roza & Shizgal, 1984 |
| Katch–McArdle | 370 + 21.6·lean-mass-kg | Best when you know your body fat — it works from lean mass, so it doesn't under-read a muscular lifter the way weight-only equations do. | Katch & McArdle |
| Cunningham | 500 + 22·lean-mass-kg | Also lean-mass-driven; runs higher than Katch and is often cited for athletic populations. | Cunningham, 1980 |
Which equation should you trust?
If you don't know your body fat, use Mifflin–St Jeor. It's the most widely validated equation for the general population and the one the Academy of Nutrition and Dietetics recommends as a default. That's why it's the headline number here when body fat is blank.
If you do know your body fat, use Katch–McArdle. It works from lean mass, so it accounts for how muscular you are — exactly what the weight-only equations can't see. Two people at the same scale weight with very different lean mass have genuinely different resting burns, and Katch is the one that reflects that. Cunningham is a close cousin that reads a bit higher and is often cited for athletes.
Expect roughly a ±10% error band either way. These are population regressions, not a measurement of you specifically. The systematic failure case worth knowing: the weight-based equations tend to under-estimate very muscular lifters, because they read a heavy, lean bodyweight as if some of it were fat. If that's you, trust the lean-mass equations and, ideally, feed in a real body-fat number.
BMR is not your calorie target
This is the mistake to avoid, so we'll say it plainly: you do not eat at your BMR. BMR is what you'd burn if you stayed in bed all day. The moment you stand up, walk around, digest food, and train, your real expenditure climbs well above it — that total is your TDEE.
Eating at your BMR means eating far below what you actually burn. That's an aggressive deficit that's miserable to hold, cratering your energy and training and dragging muscle down with the fat. To lose fat, set a modest deficit below your TDEE, not your BMR.
Turn this number into an actual plan: the Body Recomp Calculator builds calories, macros, and a timeline from your BMR, and the Body Fat Calculator gets you the body-fat number that sharpens the lean-mass equations. The handoff buttons under your result carry your inputs across so you don't re-enter anything.
Common mistakes with BMR
- Eating at your BMR to lose weight. The big one. BMR is a resting floor, not a target — dieting at it is far too aggressive and costs you muscle. Set a deficit below your TDEE instead.
- Treating one equation's number as fact. Every equation has a real error band. Use the spread across all four as your honest range, not a single decimal.
- Using a weight-only equation as a muscular lifter. Mifflin and Harris can't see your muscle, so they under-read you. Enter a body-fat % and lean on Katch-McArdle.
- Confusing BMR with the calories you burn training. Your workout burn is separate and already lives in the activity side of TDEE. Don't stack it on top of a BMR figure.
- Never recalculating. Weight is the biggest driver of BMR. If yours has moved a few kilos, your old number is stale — re-check it.
Methodology
The four equations are implemented exactly as published: Mifflin–St Jeor (1990) and the revised Harris–Benedict (Roza & Shizgal 1984) from weight, height, age, and sex; Katch–McArdle and Cunningham (1980) from lean body mass, which we derive as bodyweight × (1 − body-fat % ÷ 100). All math runs in kilograms and centimetres; imperial inputs are converted first, and we round only for display. When you enter a body-fat %, Katch–McArdle becomes the headline because it accounts for how muscular you are; otherwise Mifflin–St Jeor headlines as the best-validated general default.
These estimate a resting burn adjacent to RMR, not a chamber-measured BMR, and carry a real error band — roughly ±10% for an individual, wider at the extremes of body composition. This is a fitness utility, not medical advice: don't use it for clinical decisions, medication dosing, pregnancy, eating-disorder recovery, or under-18 nutrition.
The calculation runs entirely in your browser. The tool's own usage events only ever send coarse buckets — a 100-kcal bucket for the result, and bucketed age and weight — never your exact numbers. If you use the share button, your inputs are encoded into that link so the page can rebuild your result; that link then sits in your address bar like any other URL.
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Exercise Library
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Browse exercisesFrequently asked questions
What's the difference between BMR, RMR, and TDEE?
BMR is the energy you burn at complete rest, measured in a lab after an overnight fast. RMR is the same idea under looser conditions, so it reads a touch higher — the equations here estimate an RMR-adjacent resting burn. TDEE is the whole picture: BMR plus digestion, daily movement, and training. Your real calorie needs are your TDEE, not your BMR.
Why do the four equations give different numbers?
Each equation was fit to a different population. Mifflin and Harris work from weight, height, age, and sex, so they can't tell muscle from fat. Katch-McArdle and Cunningham work from lean mass, which is why they need a body-fat %. For an average build they agree within a few percent; for a very lean or muscular person they diverge, because the weight-based ones under-read real muscle. The spread is an honest error bar, not a bug.
Is a BMR of 1,500 low?
Not necessarily — it depends on your size, sex, and age. A smaller or older person, or a smaller woman, can have a perfectly normal BMR around 1,300–1,500; a larger or more muscular person reads higher. BMR scales mostly with lean mass and body size, so there's no single “normal” number. Your own trend matters more than the comparison.
Does building muscle raise your BMR?
Yes, but less than gym folklore claims. At rest, a kilogram of muscle burns only about 13 kcal per day — so a few kilos of muscle adds a few dozen resting calories, not hundreds. The bigger payoff is indirect: more muscle lets you train harder and move more, and the training itself burns calories. Build muscle for strength and composition, not as a resting furnace.
Can dieting permanently wreck my metabolism?
Not permanently. Prolonged dieting causes real but temporary adaptation — you burn a little less than predicted as your body defends a deficit, and spontaneous movement drops. Research shows this largely reverses at maintenance with time to recover. It's a reason to diet sensibly and take breaks, not proof you're broken. Be skeptical of anyone selling a cure for “permanent” metabolic damage.
How often should I recalculate my BMR?
Whenever your inputs move — mainly after a bodyweight change of a few kilos, since weight is the biggest driver. If you're cutting or bulking, re-check every few weeks. Age and height barely move, so they're not worth recalculating for. It's an estimate with a real margin, so follow the trend, not the decimal.
Should I eat at my BMR to lose weight?
No — and it's a risky mistake. BMR is what you'd burn lying in bed all day; you burn more than that just living, and more still if you train. Eating at BMR is an aggressive deficit that's hard to sustain and costs you muscle. Set a modest deficit below your TDEE, not your BMR. The recomp handoff on this page builds that target for you.
Which equation should I use for a cut?
With a decent body-fat estimate, use Katch-McArdle — lean-mass based, so it won't lowball a muscular lifter into too deep a cut. Without one, Mifflin-St Jeor is the safest default. Either way, treat BMR as the base of a TDEE estimate, set a small deficit, and adjust from real results over 2–3 weeks — the scale and the mirror beat any formula.
Training drives the other half of the equation
A BMR is a number. Stronger logs every set, scores your strength, and builds the muscle that moves your metabolism — the part a calculator can't do for you.
