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A 30-year-old at 80 kg and 180 cm burns 1,780 kcal doing absolutely nothing — and 2,448 lightly active. Every diet starts from these two numbers.
Every diet succeeds or fails on energy balance, and energy balance starts with two numbers: BMR (what you burn at complete rest) and TDEE (what you actually burn living your life). This calculator uses the Mifflin-St Jeor equation — the one the Academy of Nutrition and Dietetics rates most accurate — then layers your activity level to produce maintenance calories plus honest cut and bulk targets.
Dieters use it before any plan: Mifflin-St Jeor says a 30-year-old at 80 kg / 180 cm burns 1,780 kcal at rest, 2,448 lightly active — cut from that number, not a guess.
Within about ±10% for most adults — the best of the prediction equations, which is why dietetics associations prefer it over the older Harris-Benedict. The 10% band matters: a calculated 2,448 TDEE could really be 2,200 or 2,700, and no equation can see your individual NEAT (fidgeting, walking, standing — which varies by up to 2,000 kcal/day between people). The professional protocol: eat at the calculated target for 2–3 weeks, track morning weight weekly, then adjust by the observed trend — losing faster than 0.75% of body weight per week means the estimate ran low; not losing on a "500 deficit" means it ran high. The equation gives the starting hypothesis; your scale is the measuring instrument.
Usually one of four causes, in order of likelihood. First, tracking error: studies consistently show people underreport intake by 20–40% — cooking oil, condiments, bites, weekend drift. Second, activity overestimation: three gym sessions a week is "light" (×1.375), not "moderate," and exercise machines overstate burn by 15–30%. Third, metabolic adaptation: after weeks of dieting, TDEE drops 5–15% beyond what weight loss alone predicts — your body defends its weight. Fourth, water masking fat loss: cortisol, sodium, and new exercise inflammation can hide 2–3 weeks of real fat loss on the scale. The fix sequence: weigh everything for one week (yes, the oil), drop the activity multiplier one notch, and if the trend still fails after three honest weeks, cut another 100–200 kcal.
Only half of them, at most — and only if your activity multiplier didn't already count them. The double-counting trap: choosing "moderate — 3–5 days/week" already prices in those workouts, so eating back the 400 kcal your watch reported spends the same calories twice. Wearables compound it: studies on fitness trackers find calorie-burn errors of 20–90%, with cardio machines at the gym nearly as optimistic. The clean systems: either pick a sedentary/light multiplier and deliberately eat back half of tracked exercise, or pick the honest activity multiplier and eat back nothing. For muscle gain, protein is the lever that matters more than the surplus size — ~2 g per kg of body weight on a cut, ~1.6 g/kg on a bulk, spread across 3–5 meals to maximize muscle protein synthesis.