BMR and TDEE: How Many Calories You Burn

Every diet plan, calorie tracker, and fitness app eventually points back to one question: how many calories does your body actually burn in a day? The answer starts with two acronyms that sound technical but describe something very ordinary. BMR is the energy your body spends just staying alive, and TDEE is that figure plus everything else you do. Get a handle on both and the whole business of eating to lose, gain, or hold your weight stops being guesswork. This guide explains what each number means, the three formulas this tool can use to estimate them, how the activity factors work, and how much trust the final figure deserves. You can run your own numbers at any point with the BMR Calculator.

What BMR is

BMR stands for basal metabolic rate. It is the number of calories your body burns at complete rest simply to keep its vital functions going: breathing, circulating blood, keeping your brain ticking over, maintaining body temperature, and repairing cells around the clock. Imagine lying still all day, awake but doing absolutely nothing. The energy you would still spend to stay alive is your BMR.

For most people, BMR is the single largest part of daily energy use. That surprises people who assume exercise dominates the calorie budget, but the quiet, constant work of keeping a body running usually outweighs anything you do at the gym. That is also why BMR is the foundation every other calorie figure is built on. Once you know it, everything else is a matter of adding the energy cost of movement on top.

The three formulas

There is no practical way to measure BMR exactly outside a laboratory, so researchers have built equations that estimate it from things you already know, like your height, weight, age, and sex. This tool can use three of the best known. Each has its own history and its own best use.

Mifflin-St Jeor (1990)

The Mifflin-St Jeor equation is the one most dietary guidelines now recommend for healthy adults. Published in 1990, it was designed to reflect modern populations more accurately than the older formulas, and it has held up well in comparison studies. If you are not sure which to pick, this is the sensible default. In metric units it looks like this:

Men:   BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5
Women: BMR = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161

The only difference between the two versions is the constant at the end: men add 5, women subtract 161. Everything before that is identical.

Revised Harris-Benedict (1984)

The Harris-Benedict equation is the long-standing classic, first devised in the early 20th century and revised by Roza and Shizgal in 1984 to improve its accuracy. For decades it was the standard, and it still gives reasonable results. If you have seen a BMR figure quoted in an older book or app, there is a good chance this is where it came from. Like Mifflin-St Jeor, it works from height, weight, age, and sex.

Katch-McArdle

The Katch-McArdle formula takes a different route. Instead of estimating from height and weight, it works from your lean body mass, which means it needs your body fat percentage as an input. Because it accounts for how much of your weight is muscle rather than fat, it is the best choice for lean or muscular people who actually know their body fat figure. If you do not know yours, one of the other two equations will serve you better. To understand why body composition matters so much here, see the guide on BMI versus body fat.

Do not be alarmed if the three formulas disagree. Differences of 100 to 200 kcal between them are perfectly normal, and no single one is the true answer. They are three reasonable estimates of the same thing.

BMR versus TDEE and the activity factors

BMR only covers the calories you burn at rest. In real life you also stand, walk, work, cook, fidget, and exercise, and all of that costs energy. TDEE, or total daily energy expenditure, captures the whole picture. It is your BMR multiplied by an activity factor that reflects how much you move on a typical day:

So if your BMR is 1,600 kcal and you are moderately active, your TDEE is roughly 1,600 x 1.55, or about 2,480 kcal a day. TDEE is the number that matters for day to day decisions. It is the figure you compare against what you eat: eat below it to lose weight, above it to gain, and around it to hold steady. For a fuller walk through of turning TDEE into a calorie target for a specific goal, read the companion guide on calorie needs and TDEE.

How accurate is it

Here is the honest part. Any of these equations is an estimate, and for an individual it can be off by 10 percent or more. That is not a flaw in the maths; it is the reality of human bodies. Your metabolism depends on genetics, hormones, body composition, and overall health, and no formula built from height, weight, age, and sex can see all of that. Two people with identical stats can genuinely burn different amounts.

That does not make the numbers useless. It just means you should treat your calculated TDEE as a well informed starting point rather than a precise target. The practical fix is simple: track your weight over a few weeks while eating around your estimated number, then adjust. If you are steadily gaining when you meant to hold steady, your real TDEE is lower than the estimate, and you nudge your intake down. If you are losing, it is higher. Your own results are the most accurate calculator you own.

Using your number

Put together, the workflow is straightforward. Estimate your BMR with whichever formula fits you best. Pick the activity factor that honestly matches your week to get your TDEE. Use that TDEE as the baseline you eat around, aiming below it to lose weight or above it to gain. Then check your actual weight trend over a few weeks and fine tune the figure to your body, not the other way round.

1. Calculate BMR. Use Mifflin-St Jeor by default, or Katch-McArdle if you know your body fat percentage.
2. Multiply by your activity factor to get TDEE, choosing the level conservatively.
3. Set a calorie target relative to TDEE for your goal, and eat around it.
4. Track your weight for a few weeks and adjust the number up or down to match reality.

The bottom line

BMR is the energy your body spends just to stay alive, and TDEE adds everything else you do on top. Between them they turn the vague question of how many calories you burn into a concrete number you can plan around. Remember that it is an estimate, not a verdict: expect the formulas to disagree by a hundred kilocalories or so, expect your own metabolism to differ from any equation by up to 10 percent, and let a few weeks of real weight data have the final say. Start by running your figures with the BMR Calculator, then read them with the context this guide has laid out. For deeper background on how these energy numbers are derived and applied, see the NIH Body Weight Planner and the USDA Dietary Reference Intakes.