Three numbers, one calculator
Type "calorie calculator" into a search bar and three different questions come back bundled into one: how many calories do you burn just existing (BMR), how many do you burn on an average day once activity is added (TDEE, also called maintenance calories), and how many should you eat to hit a specific goal. This tool answers all three in sequence: BMR feeds into TDEE, and TDEE feeds into the goal target, so changing your activity level or your goal updates the whole chain instead of one isolated number.
The Lose, Maintain, and Gain buttons don't touch the math that produces BMR or TDEE. They only decide what happens after TDEE is calculated. Maintenance is the same number no matter which goal you pick; it describes your body at rest and in motion, and the three buttons just decide what you do with it.
Why Mifflin-St Jeor and not Harris-Benedict
Harris-Benedict, published in 1919 and revised in 1984, was the default resting-metabolism formula for most of the 20th century, and plenty of older calculators still run on it. Mifflin-St Jeor came out of a 1990 study of 498 people measured by indirect calorimetry, the lab method of measuring actual gas exchange, and it was built specifically because Harris-Benedict tends to overestimate resting energy needs in a more sedentary modern population. Since then, comparison studies have repeatedly found Mifflin-St Jeor lands within about 10% of measured resting energy expenditure more often than the alternatives, which is why dietitians and most current calculators default to it.
The imperial inputs here (pounds, feet, inches) don't run through a separate imperial formula. They convert to kilograms and centimeters first, using the exact international conversion factors, then go through the same equation as the metric path. That keeps the two unit systems in agreement to the decimal instead of drifting apart the way some rounded-constant shortcuts do.
The activity multiplier is a starting point, not a wearable
BMR measures your body at complete rest. TDEE adds everything else: walking, working, exercising, fidgeting. No government body sets an official conversion from a description like "moderately active" to an exact number; the 1.2 to 1.9 range used here is a long-running convention in sports-nutrition references, nothing more official than that. Two people who both call themselves lightly active can burn very different amounts of real-world energy, so treat this number as a starting point to adjust after a few weeks of tracking.
If your actual weight trend after two or three weeks doesn't match what the target predicted, check the activity level first. It's usually the input that's off; the BMR formula is standard math and rarely the problem.
Why a 500-calorie deficit doesn't produce exactly a pound a week, every week
The "3,500 kcal equals one pound of fat" figure goes back to a 1958 calculation by Max Wishnofsky that treats calorie deficit and weight loss as a straight line: cut 500 calories a day, lose one pound a week, indefinitely. Kevin Hall's group at the NIH's National Institute of Diabetes and Digestive and Kidney Diseases, the team behind the NIH Body Weight Planner, has since shown that relationship isn't actually linear. As you lose weight, your body needs fewer calories to maintain the smaller version of itself, so the same 500-calorie deficit produces less and less loss the longer you stay on it.
This calculator still uses the 3,500-kcal rule because it's simple, transparent, and close enough for a short-term estimate. Just don't treat week 1's math as a promise about week 12.
What a stalled plateau usually turns out to be
Annie Barry, a dietitian specializing in weight management and perimenopause with 20 years in practice at All Round Wellness, says a true plateau, the kind this calculator's math explains, is the rarest of three explanations she sees in clients who think they've stalled. About 8 in 10 cases trace back to habit creep: the extra walk or the food tracking that quietly stopped, without feeling like cheating at the time. Another 1 in 10 is impatience, since a client's definition of a plateau (one to two weeks without a change) is shorter than what actually counts as one (three to four weeks or more).
The remaining 1 in 10, in her experience, is genuine, and even then her first move isn't cutting calories further. Most of her clients already have tight calorie budgets from desk jobs, so she looks first at whether there's room to add incidental movement before subtracting food.