BMI, BMR, and what energy expenditure actually means
Body Mass Index (BMI) is a population-level index: weight (kg) divided by height (m) squared. Its strength is simplicity and consistency across large studies; its weakness is that it treats all tissue the same. A lean powerlifter and a sedentary person with the same height and weight will share the same BMI, even though their body composition differs radically. For most adults in the general population, BMI tracks body fat reasonably well, which is why it remains useful as a first screening tool — not a diagnosis.
Basal Metabolic Rate (BMR) is the energy your body uses at complete rest: keeping your heart beating, breathing, maintaining temperature, and repairing cells. Two widely used equations are Mifflin–St Jeor (developed in 1990 and validated across many modern populations) and Katch–McArdle (which requires lean body mass and tends to be more accurate for lean or athletic people). Measured BMR in a metabolic chamber is precise; estimated BMR is a solid starting point with a typical margin of error of ~10%.
From there you scale up. Total Daily Energy Expenditure (TDEE) = BMR × Physical Activity Level (PAL). PAL ranges from ~1.2 (sedentary, little or no exercise) to ~1.9 (very active, hard training daily). NEAT (non-exercise activity thermogenesis) — walking, fidgeting, chores — often varies by several hundred kilocalories per day between people with identical exercise habits, which is why two people can eat the same and gain differently.
A practical rule: a calorie deficit of ~500 kcal/day over weeks tends to produce about 0.5 kg (1 lb) of weight change, while a surplus of ~250–400 kcal/day supports lean muscle gain when paired with resistance training. These are population averages, not guarantees — adherence and body composition shift the exact number.