BMR or Basal Metabolic Rate can calculte how many calories you need to eat a day to maintain your current weight.  If you want to lose weight then you need to eat less that this amount

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If you want to lose weight and reduce body fat, you need to use more calories than you consume, creating what's called a calorie deficit. This is often accomplished by either reducing the calories you take in, increasing the calories you burn, or both. To lose a pound, you need to burn off the equivalent number of calories found in that pound.

The common advice has long been that you need to burn 3,500 more calories than you eat to drop one pound. To do this in one week, then, you need to create a calorie deficit of 500 calories each day