What the numbers actually mean
- BMR (Basal Metabolic Rate): Calories you'd burn lying perfectly still in a thermoneutral room for 24 hours. The body's "idle" cost.
- RMR (Resting Metabolic Rate): Nearly the same as BMR, measured with less restrictive conditions. The two are often used interchangeably; RMR is typically ~10% higher.
- TEF (Thermic Effect of Food): Calories burned digesting food. Roughly 10% of intake; higher for protein, lower for fat.
- NEAT (Non-Exercise Activity Thermogenesis): Walking, fidgeting, standing — everything you do that isn't deliberate exercise. The most variable component.
- EAT (Exercise Activity Thermogenesis): Deliberate exercise. Smaller than people think for most people.
- TDEE (Total Daily Energy Expenditure): BMR + TEF + NEAT + EAT.
How the formulas estimate BMR
The Mifflin–St Jeor equation (1990) is the current standard and replaced the older Harris–Benedict (1919). For an average adult, it's accurate to within 10% about 80% of the time. Where it goes wrong:
- Very muscular individuals: BMR underestimated, sometimes by 200+ calories/day.
- Very high body fat: BMR slightly overestimated (fat tissue burns less than lean mass).
- Older adults: BMR overestimated; muscle loss is steeper than the linear age term captures.
- Repeated dieters: BMR depressed below the formula's prediction by 5–15% — "metabolic adaptation".
Activity multipliers are the bigger problem
To go from BMR to TDEE, calculators multiply by an "activity factor" — typically 1.2 (sedentary) up to 1.9 (very active). These multipliers are old, derived from doubly-labeled water studies on small samples, and they leave very little room for someone in between categories.
What people pick wrong: "I work out 3x a week so I'm 'moderately active'". For most desk workers, even with three workouts a week, sedentary-to-light is closer to the truth. The hours of sitting dominate over the four hours of weekly exercise. Many overestimate their activity level by one full bucket — that's 200–400 calories/day of phantom expenditure.
The honest deficit math
A pound of fat is roughly 3,500 calories; a kilogram is about 7,700. To lose 0.5 kg/week of fat, you need a sustained deficit of ~550 calories/day. To lose 1 kg/week, ~1,100 calories/day. Anything above 1 kg/week sustainably is uncommon outside of obese starting weight or short timeframes.
This calculation assumes the body adapts perfectly. In reality:
- Some of the weight lost will be glycogen and water, especially in the first 2 weeks. Real fat loss is slower.
- BMR drops 5–15% over weeks of dieting (adaptive thermogenesis).
- NEAT silently drops too — people move less, fidget less, take elevators more. This can erase 100–300 calories/day from your deficit without you noticing.
Practical protocol
- Calculate TDEE with the formula. Subtract 10–20%, not 30–40%.
- Eat this for 2 full weeks. Weigh daily, average weekly, ignore single-day moves.
- If the weekly average dropped 0.3–0.7 kg, the number is right. Hold.
- If less, drop calories by another 100–150 or add deliberate steps. Re-evaluate weekly.
- Don't drop below 1,200 kcal (women) / 1,500 kcal (men) without medical supervision.
Macros, briefly
Protein has the strongest evidence base for maintaining muscle in a deficit. A reasonable target is 1.6–2.2 g/kg body weight per day. Beyond that, the protein vs carbs vs fat split is mostly individual preference and adherence. The diet you stick to for 6 months wins by a margin that dominates any macro nuance.
What the calculator can and can't tell you
The calculator gives you a starting estimate — better than guessing, worse than measured calorimetry. Treat the first 2 weeks as calibration. The real number for you is the one that makes your weekly average move at the rate you want; everything else is just where to begin.