If you plan to compete in an endurance event lasting more than four hours, figure out your sweat rate so you'll know how much fluid you need to drink.
1 Weigh yourself naked before training, then again after you finish. If you exercise for an hour and lose a pound, your sweat rate is a pound an hour.
2 One pound equals one pint of liquid. So you would need to drink one pint, or 16 ounces, over an hour to replace the fluid lost.
3 Account for fluid intake, adding back the weight of what you drink to the weight you lose. If you drink half a pint and lose half a pound, you will have lost a pound without the liquid.