BMR — the calories you'd burn doing nothing
Basal Metabolic Rate (BMR) is the energy your body needs just to keep functioning — breathing, circulation, cell repair — if you stayed in bed all day. The Mifflin-St Jeor equation, the most widely validated formula in current clinical use, calculates it from weight, height, age, and sex: for men, BMR = 10×weight(kg) + 6.25×height(cm) − 5×age + 5; for women, subtract 161 instead of adding 5.
On a 70 kg, 170 cm, 30-year-old man: BMR = 10×70 + 6.25×170 − 5×30 + 5 = 700 + 1,062.5 − 150 + 5 = 1,617.5, rounding to about 1,618 calories/day at complete rest.
TDEE — BMR scaled by how active you actually are
TDEE = BMR × an activity multiplier reflecting your typical weekly activity: 1.2 for sedentary (little to no exercise), 1.375 for lightly active (light exercise 1-3 days/week), 1.55 for moderately active (moderate exercise 3-5 days/week), 1.725 for active (hard exercise 6-7 days/week), and 1.9 for very active (very hard exercise, or a physically demanding job).
For the same person above at a moderate activity level: TDEE = 1,618 × 1.55 ≈ 2,507 calories/day — this is roughly how much they'd need to eat to maintain their current weight. The same BMR at a sedentary activity level instead gives TDEE = 1,618 × 1.2 ≈ 1,941 calories/day — a difference of about 566 calories/day purely from activity level, with body weight, height, and age unchanged.
Using TDEE for weight goals
Eating consistently below your TDEE creates the calorie deficit needed for weight loss — a widely used rule of thumb is that roughly a 7,700-calorie deficit corresponds to about 1 kg of fat loss, though real-world results vary by individual. Eating above TDEE creates a surplus for weight or muscle gain (a muscle gain phase needs some surplus, but a smaller one than most people assume, since a large surplus mostly adds fat rather than muscle).
Be honest about your actual activity level — most people, especially those who exercise but otherwise sit most of the day, overestimate it, leading to an inflated TDEE estimate and a weight-loss plan that doesn't actually create a deficit.
TDEE is an estimate, not a precise measurement
The Mifflin-St Jeor equation is a population-average formula — individual metabolisms vary, and TDEE can only be measured precisely with specialized lab equipment (indirect calorimetry). Use it as a starting point, then adjust based on your actual weight trend over 2-3 weeks: if you're not seeing the expected change, adjust your calorie target up or down rather than assuming the formula is exactly right for you.