How a Calorie Deficit Works for Weight Loss
Weight loss is fundamentally about energy balance: calories in vs calories out. When you consistently eat fewer calories than your TDEE (Total Daily Energy Expenditure), your body uses stored fat for energy. One pound of body fat contains approximately 3,500 calories. A sustained deficit of 500 calories per day = roughly 1 pound of fat loss per week — and that is the mathematical foundation behind every successful weight loss plan.
The Mifflin-St Jeor Formula (How We Calculate Your TDEE)
Our calculator uses the Mifflin-St Jeor equation — the gold standard for BMR estimation, validated by the American Dietetic Association and preferred by registered dietitians over older formulas like Harris-Benedict.
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
TDEE = BMR × Activity Multiplier (1.2 to 1.9)
Safe Calorie Deficit by Goal
| Weight Loss Goal | Daily Deficit | Weekly Loss | Safety |
|---|---|---|---|
| Slow & sustainable | 250 cal | 0.5 lbs / 0.23 kg | ✅ Ideal |
| Standard (recommended) | 500 cal | 1 lb / 0.45 kg | ✅ Safe |
| Aggressive | 750 cal | 1.5 lbs / 0.68 kg | ⚠️ Monitor |
| Maximum safe | 1,000 cal | 2 lbs / 0.9 kg | ⚠️ Short-term only |
| Unsafe | 1,200+ cal | 2.5+ lbs/week | ❌ Avoid |
Why Protein Matters in a Calorie Deficit
When you eat in a calorie deficit, your body can lose both fat and muscle. High protein intake (0.7–1g per pound of bodyweight) combined with resistance training preserves lean muscle mass during weight loss. Our calculator provides a macro breakdown that emphasizes protein to help you lose fat, not muscle — a critical distinction for long-term body composition goals.