The most accurate method for determining your personal fluid needs is to take a sweat-rate test during your training. Weigh yourself naked before and after a hard one-hour run. Convert the amount of weight lost to ounces to figure out your sweat rate per hour. A loss of one pound means you sweated about 16 ounces of fluid (assuming you didnt drink any fluids during the run). Going forward, you would try to replenish fluids at a rate of about 16 ounces per hour.