What BMR is, and what this page actually estimates
Basal metabolic rate (BMR) is the energy a body uses at complete rest, under strict laboratory conditions. Most online tools, including this one, are closer to a resting energy estimate from a regression equation than to a true bedside BMR test. The number is still useful: it is the floor of a typical day’s burn, before you add walking around, work, and training.
Mifflin–St Jeor (1990) is the default on FuelCount because it is simple and widely used for general adults. Revised Harris–Benedict (Roza and Shizegal, 1984) often comes out a little higher. Seeing both on one card is a reminder that “your metabolism” on a website is a model, not a measurement. A spread of one or two hundred calories between equations is ordinary, not a sign that one of the scripts is broken.
Katch–McArdle takes a different route. It estimates resting calories from lean body mass: 370 + 21.6 × lean mass in kilograms. Lean mass here is body weight minus the fat mass implied by your body-fat percentage. If that percentage comes from a careful tape method or a clinical scan, Katch can be a better fit for someone who is quite muscular or who already tracks composition. If the percentage is a guess, the formula just carries the guess forward. The Navy body fat calculator can provide a tape estimate, with the usual caveats about circumference methods.
Once you have a resting figure, total daily expenditure is usually estimated by multiplying by an activity factor. The TDEE shown on this page always starts from Mifflin–St Jeor so the activity step stays consistent with the TDEE calculator. If you prefer Harris–Benedict or Katch as the base, switch formulas there.
None of these equations can see thyroid status, recent crash dieting, fever, or a high-muscle physique that the weight-and-height model was not built on. Use the result as a planning baseline, then judge it against a few weeks of consistent intake and body-weight trend.
Frequently asked questions
Is BMR the same as RMR?
They are close cousins, not identical tests. BMR is measured under more rigid lab conditions; resting metabolic rate (RMR) is a related measurement that is a bit easier to take. Consumer formulas are often trained on mixed resting data and then labeled loosely as BMR. For meal planning, the distinction matters less than whether you treat the output as an estimate.
Why is Katch–McArdle hidden until I enter body fat?
Without a body-fat percentage the formula does not have lean mass, so a number would be invented rather than calculated. Enter a percentage only if you have a reasonable source for it. A wild guess makes Katch look precise without being precise.
Which formula should I live by?
Pick one and keep it for a while. Mifflin–St Jeor is a solid default for a general adult. If you have a trusted composition number, compare Katch as a second opinion. Changing formulas every few days will look like progress or backsliding when nothing in your routine has changed.
Does a higher BMR mean I can eat whatever I want?
No. BMR is only the resting piece. Total intake still has to line up with your goal, food quality still matters, and a high resting estimate can be wrong. The useful habit is to compare a stable calorie average with a stable weight trend.
Can I raise my BMR?
You cannot reliably “hack” it with a supplement advertised on a banner. Building or keeping lean mass, moving more, sleeping enough, and avoiding extreme prolonged restriction are the boring levers that actually show up in research and in practice. This calculator will not detect those changes until your inputs or your real-world trend change.