Understanding Calorie Deficits: The Real Math Behind Weight Loss
"Eat less than you burn" is true, but the numbers behind it are more specific — and more forgiving of imprecision — than most calorie-counting apps let on.
Two numbers you need first: BMR and TDEE
Your Basal Metabolic Rate (BMR) is roughly how many calories your body burns at complete rest, just keeping your organs and systems running. Your Total Daily Energy Expenditure (TDEE) adds in everything else — walking, exercise, fidgeting, digestion — to estimate your true daily calorie burn. TDEE, not BMR, is your real maintenance number: eat exactly that many calories and your weight should stay roughly stable.
Estimating BMR: the Mifflin-St Jeor equation
Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
This is one of the better-studied BMR formulas for the general population, generally outperforming older equations like Harris-Benedict in accuracy comparisons. It's still an estimate, though — individual metabolisms vary for reasons a formula based on just age, sex, height, and weight can't capture.
From BMR to TDEE: activity multipliers
| Activity level | Multiplier |
|---|---|
| Sedentary (little or no exercise) | 1.2 |
| Light (exercise 1-3 days/week) | 1.375 |
| Moderate (exercise 3-5 days/week) | 1.55 |
| Active (exercise 6-7 days/week) | 1.725 |
| Very active (hard exercise daily) | 1.9 |
Multiply BMR by the multiplier that best matches your typical week to get TDEE. Most people overestimate their own activity level, so if you're unsure, it's usually safer to round down.
The deficit math — and its limits
The commonly cited rule is that 1 kg of body fat holds roughly 7,700 calories (about 3,500 per pound), so a 500-calorie daily deficit should produce about 0.5 kg of weight loss per week. This is a useful starting estimate, but it's a simplification, not a law of physics for your specific body. As you lose weight, your BMR drops (a smaller body burns fewer calories at rest), and prolonged deficits can trigger metabolic adaptation — your body becomes somewhat more efficient at conserving energy. Both effects mean real-world weight loss tends to slow down over time even at a constant calorie deficit, which is normal and not a sign anything is "broken."
In practice, this means: treat the 500-calorie/week-loss relationship as a starting point to plan around, not a guarantee to hit exactly, and expect to recalculate your TDEE periodically as your weight changes.
Calculate your numbers
Use the Calorie Calculator to get your estimated BMR, TDEE, and calorie targets for mild or standard weight loss or gain.