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Understanding Calorie Deficit
Everything you need to know about calorie deficits, BMR, TDEE, and safe weight loss.
What is a Calorie Deficit?
A calorie deficit occurs when you consume fewer calories than your Total Daily Energy Expenditure (TDEE). When you are in a deficit, your body uses stored fat (and some muscle) for energy, resulting in weight loss. A deficit of 3,500 calories is roughly equivalent to losing 1 lb of body fat, though the actual amount varies based on body composition.
BMR vs TDEE — What's the difference?
BMR (Basal Metabolic Rate) — calories your body burns at complete rest to maintain vital functions (breathing, circulation, cell repair). It accounts for 60-75% of your TDEE.
TDEE (Total Daily Energy Expenditure) — your total daily calorie burn including all physical activity. This is your "maintenance calories" — eating at this level keeps your weight stable.
How low should your deficit be?
Recommended range: 250–750 calories/day deficit
Women: Never eat below 1,200 calories/day
Men: Never eat below 1,500 calories/day
Too large a deficit slows metabolism, causes muscle loss, and makes the diet unsustainable. A moderate deficit of 500 cal/day leads to approximately 0.5-1 lb loss per week.
The 3,500 Calorie Rule — Is it accurate?
The old rule states that a 3,500-calorie deficit = 1 lb of fat loss. Research by Dr. Kevin Hall (NIH) shows this overestimates weight loss because it ignores metabolic adaptation. As you lose weight, your BMR decreases, your deficit becomes smaller, and weight loss slows. This is why our calculator uses a more realistic non-linear model.
Activity Level Multipliers (PAL)
1.2 — Sedentary: desk job, no exercise
1.375 — Light: 1-3 days exercise/week
1.55 — Moderate: 3-5 days exercise/week
1.725 — Very active: 6-7 days hard exercise
1.9 — Extra active: physical job + hard exercise
2.3 — Professional athlete: twice-daily training
Most people overestimate their activity level. When in doubt, choose one level lower.