Just wanted to drop this here if you guys never tried it, so far it has been pretty accurate !
https://www.thisiswhyimfit.com/water-fasting-weight-loss-calculator/
It seems to be focused on scale numbers,. The scale number goes down quickly the first couple of days because the food being digested in your guts is pooped out and no new food comes in.
But that’s not fat loss.
Fat loss starts around day 3, after all the short-term food and glucose/glycogen reserves are burned off. Most people burn about half a pound of fat from day 3.
Anyway, don’t be fooled by the scale numbers.