A calorie deficit — eating less than your body burns — is the mechanism behind weight loss, but how large a deficit and how fast to pursue it matters more than most quick-fix advice suggests.
The basic math
One pound of body fat represents roughly 3,500 calories. A daily deficit of 500 calories, sustained over a week, works out to roughly 3,500 calories — approximately one pound of weight loss per week. This is why 500 calories/day is the most commonly cited starting point for a moderate, sustainable pace.
Why moderate beats aggressive
A much larger deficit can produce faster short-term results, but tends to come with real costs: greater muscle loss alongside fat loss, a larger drop in energy and workout performance, and a metabolic slowdown that can make the deficit harder to sustain and easier to rebound from once normal eating resumes. A moderate 500-750 calorie deficit is generally easier to maintain consistently, which matters more for long-term results than short-term speed.
Your starting point: TDEE
Total Daily Energy Expenditure (TDEE) is the number of calories your body burns in a day at your current weight and activity level. A deficit is calculated relative to this number, not an arbitrary "cut to X calories" target — someone who's very active has a meaningfully higher TDEE than someone sedentary at the same height and weight, so the same calorie target could be a large deficit for one person and roughly maintenance for another.
Recalculating as you go
TDEE isn't fixed — as you lose weight, your body burns somewhat fewer calories at rest and during activity, since a smaller body requires less energy. This is why many people hit a plateau after initial progress: their deficit shrinks even though their intake hasn't changed. Recalculating TDEE every few weeks and adjusting keeps the deficit accurate.
A note on sustainability
The most effective approach for most people is the one they can actually sustain long enough to see results and keep them. Extremely low-calorie approaches are harder to maintain and more likely to be followed by regaining the weight. If you're planning a significant change to your eating, it's worth discussing with a healthcare provider, particularly if you have any underlying health conditions.
Calculate your own TDEE and see deficit/maintenance/surplus targets side by side with the Calorie Calculator.