How a Calorie Deficit Works
Weight loss is fundamentally a math problem described by the laws of thermodynamics. To lose weight, you must consume fewer calories than your body burns each day (TDEE).
- Energy In: Food and drink.
- Energy Out: BMR + Activity (TDEE).
- The Deficit: The difference, which your body covers by burning stored fat.
The 3500 Calorie Rule
It is widely accepted that 1 pound of fat contains approximately 3,500 calories. This gives us a simple formula for planning:
= 1 lb of fat loss per week
Is it Safe? The Limits
While a larger deficit means faster weight loss, there is a limit. Eating too little can trigger "metabolic adaptation," where your body slows down its calorie burn to survive.
- Women: Should generally not eat below 1,200 calories.
- Men: Should generally not eat below 1,500 calories.
*This calculator will alert you if your plan crosses these dangerous thresholds.
Tips for Success
Protein
Keep protein high (1g per lb of lean mass) to protect muscle while in a deficit.
Sleep
Poor sleep increases hunger hormones (ghrelin) and kills willpower.
Activity
Don't just tackle "Energy In". Increasing steps is the easiest way to boost "Energy Out".
Tools & Data Verified by the EverydayCalculators Medical Research Team.
Last updated: December 2026.