Calculate your sweat rate.
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To know how much you should hydrate during a specific athletic activity, you first need to know how much you sweat during that activity. Calculating sweat rate is a fairly simple process. First, do some brief warm-up exercise to begin perspiring, then weigh yourself naked on a scale. Perform the exercise activity at the normal intensity for an hour, drinking a measured amount of fluids when you feel thirsty. After the hour, weigh yourself again. Subtract your post-workout weight from your pre-workout weight, and convert the lost weight to grams (one pound is about 454 grams). Add the amount of fluids consumed (in milliliters; 1 ounce is about 30 ml) to that figure, and this final amount is the number of milliliters you should drink per hour to remain adequately hydrated.