Warm weather means exercising outdoors and sweating more. Sweating more translates directly into losing fluids. Your body needs to stay properly hydrated to function and perform well. Through all that sweating, it is important to know how much you should be drinking to replenish your body.

The method of determining this is pretty simple. Weigh yourself before you work out and again when you are finished. The difference approximates the amount of body fluid you lost while exercising. If you weigh more after your run or ride, you drank too much water. You never want to lose more than two percent of your weight during any workout; a 130-pound woman should lose no more than 2.6 pounds. While working out your goal is to replenish at least 50 percent of the fluids you are losing, so you would want to drink a little over 16 ounces.

I know this is a lot of numbers and percentages, and if you don't like weighing yourself, then you should drink when you are thirsty.

Source