Basically for every 3500 excess Calories consumed, your body stores about 1 lb of fat, and for every 3500 Calories burned, you lose about 1 lb of fat. If your intake is equal to your output, there’s nothing being stored.

You would have to get that burn rate 2300 Calories above the intake rate, so yeah if your daily output is 2300 Calories, you’d have to eat literally nothing for 30 days to lose 20 lbs.