There’s 3500 calories in a pound of fat. So you do the math based on your basal metabolic rate (which you know with a high degree of accuracy, right?) Everything else you lose is water, and you will lose a lot of water. Rule of thumb I’ve heard is you will lose half a pound of fat per day, and then gain about half of it back once you start eating again. So by that rule, you’d lose 7lbs and gain back 3.5lbs after you started eating again. Of fat.