How Calorie Deficits Work
Weight loss fundamentally comes down to energy balance. When you consume fewer calories than your body burns, you create a calorie deficit. Your body then taps into stored energy — primarily body fat — to make up the difference.
A widely used estimate is that 3,500 calories equals roughly one pound of body fat. While this is a simplification (actual fat loss depends on metabolic adaptation, body composition, and other factors), it provides a practical framework for planning:
- 250 cal/day deficit ≈ 0.5 lb lost per week
- 500 cal/day deficit ≈ 1 lb lost per week
- 750 cal/day deficit ≈ 1.5 lb lost per week
- 1,000 cal/day deficit ≈ 2 lb lost per week
This calculator uses the Mifflin-St Jeor equation — considered the most accurate formula for estimating Basal Metabolic Rate (BMR) — and then multiplies by an activity factor to find your Total Daily Energy Expenditure (TDEE). Your deficit is subtracted from this TDEE.
What is a safe deficit? Most dietitians recommend a deficit of 500 to 1,000 calories per day, resulting in 1–2 pounds of loss per week. Deficits larger than this increase the risk of muscle loss, nutrient deficiencies, gallstones, and metabolic slowdown. Women should not eat below 1,200 calories/day and men should not eat below 1,500 calories/day without medical supervision.
As you lose weight, your TDEE decreases because your body requires less energy to maintain a smaller mass. This is why progress can stall — recalculating every 4–6 weeks helps you stay on track.
Frequently Asked Questions
What is a calorie deficit?
A calorie deficit occurs when you consume fewer calories than your body burns (your TDEE). This forces your body to use stored energy — primarily fat — for fuel, resulting in weight loss over time. For example, if your TDEE is 2,500 calories and you eat 2,000 calories, you have a 500-calorie deficit.
How many calories should I cut to lose weight safely?
A safe calorie deficit is typically 500 to 1,000 calories per day, which translates to about 1 to 2 pounds of weight loss per week. Cutting more than this can lead to muscle loss, nutritional deficiencies, and metabolic slowdown. For most people, a 500 cal/day deficit (1 lb/week) is the best balance of results and sustainability.
What is the minimum number of calories I should eat per day?
Most health organizations recommend that women eat no fewer than 1,200 calories per day and men no fewer than 1,500 calories per day without medical supervision. Eating below these thresholds can impair metabolic function, reduce thyroid output, and lead to serious nutrient deficiencies.
Will a calorie deficit cause muscle loss?
A moderate calorie deficit (around 500 cal/day) combined with adequate protein intake (0.7–1 gram per pound of body weight) and regular resistance training minimizes muscle loss. Very aggressive deficits above 1,000 cal/day are far more likely to cause muscle loss, especially without strength training.
How accurate is the 3,500 calories per pound rule?
The 3,500-calorie rule is a useful starting estimate, but real-world weight loss is not perfectly linear. As you lose weight, your metabolism adapts and your TDEE decreases, which can slow progress over time. Hormonal changes, water retention, and body composition also play a role. Recalculating every few weeks helps keep your plan accurate.
Should I eat back the calories I burn through exercise?
It depends on how your activity level is accounted for. If your activity level is already factored into your TDEE calculation (as this calculator does), you generally do not need to eat back exercise calories. However, if you perform additional exercise beyond your selected activity level, eating back a portion (about 50–75%) of those extra calories can prevent an excessively large deficit and support recovery.