Why your calorie calculator might be wrong — and why that's still okay

Put your height, weight, age, and sex into five different calorie calculators and you will get five slightly different answers. That is not because four of them are broken. It is because "how many calories does this person burn" is not a number you can look up — it is a number you have to estimate, and every estimate is a compromise.

The difference between BMR and TDEE

Your basal metabolic rate (BMR) is the energy your body uses just to keep you alive: heart pumping, lungs breathing, cells repairing, temperature stable. It is the floor — the calories you would burn lying motionless in a warm room after a full night's sleep.

Your total daily energy expenditure (TDEE) is that floor plus everything you do on top of it: walking to the kitchen, typing, exercising, even digesting food. Since most people are not lying motionless all day, TDEE is the number that matters for weight management, and it is always higher than BMR.

Mifflin-St Jeor: the current standard, with a known error bar

In 1990, researchers Mifflin and colleagues published an equation derived from measurements of nearly 500 people, and it has since become the most widely recommended predictive formula. It is also a reminder of how rough these estimates are: the equation predicts BMR accurately to within about 10% for most people — but "most people" leaves a meaningful minority outside that range.

Older formulas still float around the internet. The Harris-Benedict equation (from 1919) was based on measurements from a century ago and tends to run a bit high. The Katch-McArdle formula tries to account for body composition but requires a body-fat percentage most people do not know. If two calculators disagree, the difference is usually which equation and which activity multiplier they used — not a bug in either.

The activity multiplier is where the real guesswork happens

Estimating BMR is the comparatively reliable part. Multiplying it to reach TDEE is the guess. A "moderately active" person might burn 1.55 times their BMR — or, depending on what they actually do all day, noticeably more or less. Most people overestimate their activity level, which is a quieter source of error than the equation itself.

How to use the number honestly

The right way to use a TDEE estimate is as a starting point to be corrected by results, not a target to trust blindly. Eat at your estimated maintenance for two weeks while weighing daily. If the trend holds flat, the estimate is close. If your weight climbs, the true number is lower; if it drops, higher. A two-week real-world calibration tells you more than any formula ever will.

This is also why "eat 500 calories less to lose a pound a week" is a guideline, not a law. A 500-calorie deficit is a reasonable, commonly cited target — but the actual deficit depends on how accurate your TDEE was to begin with.

Calorie estimates are general planning tools, not prescriptions. People with medical conditions, a history of disordered eating, or who are pregnant should work with a professional rather than self-managing based on a calculator.