I know I won’t gain fat, but around how much in terms of water weight?
I don’t have experience with longer fasts, but I believe the average adult can store something like 500g of glycogen in the liver and muscles. Each gram of glycogen requires 3 grams of water. So somewhere in the ballpark of 3 lbs if your body decided to replenish its whole reserve.
Also maybe I’m reading too much into this question, but be sure you’re taking care of yourself OP. Fasting for 5 days, not even eating your full TDEE after, and being curious about the exact weight you’re going to regain is a bit troubling.
It’s not just water weight. It’s also the food left in your stomach. This is why day to day weight changes are terrible metrics. Only look at your weight as a trend. If you truly want measurements, go get a dexa body scan.