What TDEE and BMR actually measure
Your basal metabolic rate (BMR) is the energy your body spends doing nothing at all. Lying still, fully rested, you still breathe, pump blood, repair cells, and run your organs. That baseline is BMR, and for most adults it accounts for 60 to 70 percent of the calories burned in a day. Total daily energy expenditure (TDEE) is the full picture: BMR plus the energy of digesting food, fidgeting, walking, standing, and any deliberate exercise. TDEE is always the larger number, and it is the one that matters when you plan how much to eat.
The relationship is simple: TDEE equals BMR multiplied by an activity factor. That factor ranges from 1.2 for a sedentary desk worker to 1.9 for someone with a physical job who also trains hard daily. A 1,700-calorie BMR becomes a 2,040-calorie TDEE at the sedentary end and a 3,230-calorie TDEE at the most active end. Same body, very different daily burn, driven entirely by movement.
Mifflin-St Jeor versus Katch-McArdle
This calculator estimates BMR with one of two peer-reviewed equations. The Mifflin-St Jeor equation, published in 1990, uses your weight, height, age, and sex. For men it is 10 times weight in kilograms, plus 6.25 times height in centimeters, minus 5 times age, plus 5. For women the final constant is minus 161 instead of plus 5. Mifflin-St Jeor is the default here because it is the most accurate general formula for the broad population and is the one most dietitians reach for first.
The Katch-McArdle formula takes a different route. Instead of weight and height, it uses lean body mass: 370 plus 21.6 times your lean mass in kilograms. Because it is built on the tissue that actually drives metabolism, it is more accurate for people at the edges, the very lean and the very muscular, where weight-based formulas tend to misjudge. The catch is that it needs a body fat percentage. If you enter one, this calculator switches to Katch-McArdle automatically; if you leave it blank, it stays with Mifflin-St Jeor. Neither is a perfect measurement of your real metabolism, but both give a starting estimate within a few percent for most people.
Picking an activity multiplier honestly
The activity multiplier is where most TDEE estimates go wrong, and almost always in the same direction: people pick a level that is too high. The multiplier is meant to capture your entire week of movement, not just your best training days. Three gym sessions does not make you very active if the other four days are spent at a desk and on a couch.
- ▸Sedentary (1.2): A desk job with little intentional movement and no regular exercise. This fits more people than they think.
- ▸Lightly active (1.375): Light exercise or sport one to three days a week, or a job with some walking.
- ▸Moderately active (1.55): Moderate exercise three to five days a week. A solid, consistent routine.
- ▸Very active (1.725): Hard training six to seven days a week, or a physically demanding routine.
- ▸Extremely active (1.9): A physical job plus daily training, or two-a-day sessions. Reserved for genuine athletes and laborers.
When you are between two levels, choose the lower one. A TDEE that is slightly too low will still produce fat loss if you eat at it; a TDEE that is too high quietly erases the deficit you thought you had. The table in your results shows all five levels at once so you can see exactly what each choice costs or buys you, then sanity-check it against how your weight actually moves over two to three weeks.
Why TDEE matters on keto (and what it does not change)
Keto changes a lot about how you eat, but it does not change the physics of TDEE. Your body burns the same number of calories at rest and in motion whether those calories arrive as fat, protein, or carbohydrate. There is no hidden keto multiplier and no metabolic bonus that lets you ignore energy balance. Fat loss on keto still comes from eating below your TDEE over time.
What keto genuinely changes is the demand side. Ketones blunt the hunger hormone ghrelin, and meals built on fat and protein keep you full far longer than carbohydrate-heavy ones. The practical result is that most people eat several hundred calories below their TDEE without consciously restricting, which is why keto feels effortless for some dieters even though the underlying math is identical to any other approach. Knowing your TDEE turns that effect from luck into a plan: you can see your maintenance number, choose a deficit you can sustain, and check whether your appetite-driven intake is actually landing where you want it.
The water-weight caveat, and when to recalculate
A TDEE-based deficit predicts fat loss, and fat loss is steady and slow. The scale in your first keto week is neither. Carbohydrate is stored in muscle and liver as glycogen, and every gram of glycogen holds three to four grams of water. When you cut carbs, you burn through 400 to 500 grams of glycogen and release the water bound to it, which is why the scale can drop three to seven pounds in days one through seven. That is real and motivating, but it is water, not fat, and a pure-deficit projection will understate it. Expect the scale to slow once water loss finishes, then track the gentler fat-loss trend the math actually predicts.
Finally, treat your TDEE as a moving number. As you lose weight, you carry less tissue and burn fewer calories, so the TDEE you calculated at the start slowly becomes too high. Recalculate after every 10 to 15 pounds, or whenever your activity level shifts for more than a week or two. An outdated maintenance number is one of the most common reasons a deficit that once worked quietly stops working. When you are ready to turn this TDEE into a daily eating target, the Keto Calorie Calculator applies your goal adjustment and the Keto Macro Calculator splits the result into grams.