To lose weight, you must burn more calories than you consume. Extreme low-calorie diets tend to preserve fat and waste muscle. Since muscle is your calorie-burning furnace, losing muscle slows your metabolism, hindering weight loss.
You burn the majority of your daily calories through basic life functions such as circulation and digestion. This number is called your basal metabolic rate, or BMR. To find your BMR, multiply your weight in kilograms by 24 – one kilogram is 2.2 pounds – times 1 for men and 0.9 for women. Multiply the result by 0.85 if you are obese, 0.9 if you are overweight, 0.95 if you are a healthy weight and 1 if you are very lean.
Your daily activities also burn calories. Multiply your BMR by 1.3 if you are sedentary, 1.55 if you are lightly active but don't work out and 1.65 if you exercise daily. This is roughly the number of calories you burn in a day.
To lose 20 pounds, you have to burn 70,000 more calories than you consume. Subtract the 1,000 calories eaten from your daily calorie expenditure and divide that number into 70,000 to calculate how many days it will take you to lose 20 pounds.