According to the American College of Sports Medicine, dehydration is likely to start affecting exercise performance when sweating causes you to lose 2% or more of your normal (hydrated) body weight. That's more than 51 ounces, or a little over 3 pounds, for an "average" person of 160 pounds. At this level of mild dehydration, you’ll probably be a little thirsty (though many people don’t experience thirst until they’re already dehydrated), and you may start to feel as if you have to work significantly harder to maintain your performance level. As dehydration gets progressively more severe, you may start to feel lightheaded, uncoordinated, or have muscle cramps. If you continue, you may start experiencing the symptoms of heat exhaustion, and that can progress to heat stroke, which is potentially fatal and needs immediate medical attention.
The goals of fluid intake during exercise are to prevent dehydration from occurring and to not drink in excess of one’s sweating rate. One good way to figure out whether you need to drink something during your workout is to simply weigh yourself (without clothes) just before and after a typical workout. If your weight change is more than 2% of your starting weight, then in the future, you should plan to drink enough water during your workout to keep your post-workout weight within that 2% range. Typically, drinking a cup (8 oz) of water every 15-20 minutes will do the trick in all but the most extreme situations. While difficult to recommend a specific fluid schedule because of varying needs, this handy chart provides some basic guidelines:
*If possible, weigh yourself on the same scale, before and after exercise so you know how much to drink for rehydration.
How Much Should I Drink During Long Workouts, or While Exercising in Hot Weather?