Pre-hydrating before a workout, especially in hot or long sessions, by consuming approximately half a pound of water per pound of body weight is recommended.

“you want to preh hydrate if you know you're going to do a workout session and it's going to be hot and long or one of those things um you want to look for that half a pound uh per body weight of ounces so that's the number we're looking for to start our hydration session”