How to Calculate Your BMR, TDEE, and Macros for Weight Loss or Muscle Gain

Basal metabolic rate is the energy your body uses at complete rest to keep you alive — circulation, breathing, temperature, cell repair. It is what you would burn lying still all day, and for most adults it accounts for 60 to 70 percent of daily energy use. Total daily energy expenditure is BMR plus everything else: the thermic effect of digesting food, roughly 10 percent of intake; deliberate exercise; and non-exercise activity thermogenesis, which is all the walking, standing and fidgeting you do without calling it exercise and which varies enormously between people. TDEE is the number that matters for setting intake. Eating at TDEE holds weight steady; below it you lose; above it you gain. BMR is only an input, and eating at BMR is not a diet plan — it is an aggressive deficit dressed up as a floor.

Mifflin-St Jeor is the current default and validated as more accurate than the older Harris-Benedict equation for the general population. For men: BMR = 10 x weight in kg + 6.25 x height in cm - 5 x age in years + 5. For women, the same expression ending in -161 instead of +5. A worked example: a 35-year-old woman, 165 cm, 68 kg gives 10 x 68 + 6.25 x 165 - 5 x 35 - 161 = 680 + 1031 - 175 - 161 = 1375 kcal. If you know your body-fat percentage from a reliable measurement, the Katch-McArdle formula — 370 + 21.6 x lean body mass in kg — is more accurate for lean or muscular people, because it does not care how the weight is distributed. Every one of these is a population estimate with an individual error margin of roughly 10 percent, which is exactly why the calibration step below is not optional.

Multiply BMR by an activity factor: 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active, 1.9 for extremely active. The trap is that these labels describe your entire day, not your gym habit, and almost everyone picks one level too high. A desk job with three gym sessions a week is light to moderate, not very active — the training is a few hundred calories spread across a week, while the other 23 hours are sedentary. Choosing 1.725 instead of 1.375 for our example adds about 480 kcal a day, which erases a typical deficit entirely and produces the familiar experience of eating carefully and losing nothing. When in doubt, take the lower factor. Being pleasantly surprised in two weeks is better than the alternative.

The sustainable range is 15 to 20 percent below TDEE, or roughly 300 to 500 kcal a day, which corresponds to about 0.25 to 0.5 kg of loss per week. That pace is well supported and, more importantly, it preserves lean mass and does not require heroics. Aggressive deficits above 25 percent lose more scale weight quickly, and a meaningful fraction of it is muscle; they also drive hunger, fatigue and adherence failure, and there is evidence that non-exercise movement drops in compensation, quietly shrinking the deficit you thought you had. A reasonable floor is not to go below roughly your BMR, and never below about 1200 kcal for women or 1500 for men without medical supervision. For lean gain, invert it and keep the surplus small — 5 to 10 percent above TDEE, about 200 to 300 kcal — because muscle accrues slowly regardless of how much you eat and the excess simply becomes fat.

Set protein first. For an adult in a deficit or training regularly, 1.6 to 2.2 g per kg of bodyweight is the well-supported range, with the higher end more useful when the deficit is larger, because protein is what protects muscle while you are underfed. It is also the most satiating macronutrient, which makes the deficit easier to hold. Set fat second, with a floor around 0.8 g per kg to support hormone production and absorption of fat-soluble vitamins — going much lower for extended periods is a common mistake in low-fat dieting. Carbohydrate takes whatever calories remain, and it is the lever you adjust rather than a target to hit: more when you are training hard, fewer on rest days if you prefer. For our 68 kg example at 1375 BMR and a 1.375 factor, TDEE is about 1890, a 20 percent deficit gives 1510 kcal, and 120 g protein plus 55 g fat leaves roughly 155 g carbohydrate.

Everything above is an estimate, and the estimate is where most people stop — which is why it fails. Treat the calculated number as a starting hypothesis and test it. Eat at that intake for two full weeks, weighing food rather than eyeballing portions, and weigh yourself each morning after using the bathroom and before eating. Take the average of each week rather than comparing single days, because daily weight swings by a kilogram or more on water, sodium and gut contents alone. If the two weekly averages are unchanged, your true TDEE is lower than calculated: drop intake by 150 to 200 kcal and repeat. If you lost more than about 0.7 kg a week, the deficit is too steep and worth easing. After one or two of these cycles you have a number derived from your own body rather than a population average, and that number is worth far more than a more elaborate formula. Aura's free BMR and macro calculator runs the arithmetic, but the two-week check is the part that actually makes it yours.