The best time to hydrate is before your workout. This isn’t to say you shouldn’t be drinking during the workout; rather, you should be heading into your workout fully hydrated on a cellular level. Drinking during the workout should be for the purpose of keeping yourself hydrated, not to actually hydrate. If you’re following this rule, then you won’t need a gallon jug of water; a regular 24- to 32-ounce bottle should be fine.
The American College of Sports Medicine states: “The goal of prehydrating is to start the activity euhydrated and with normal plasma electrolyte levels [aka fully hydrated]. Prehydrating…should be initiated when needed at least several hours before the activity to enable fluid absorption.” This means that if you want to fully benefit from your workout without dehydrating, then you have to focus on getting in your fluids a few hours before hitting the gym.
And the amount of water you need to sustain adequate hydration during a workout depends on the workout and the conditions in which you are working out—for example, if you are exercising outside in 90% humidity or it’s 100°F, then you’re going to need a lot more water. In order to determine how much water is needed to stay hydrated during a workout, it’s helpful to calculate your “sweat rate,” which doesn’t have to be overly precise for a non-athlete; it’s more of a gauge. You can do this by measuring your weight before and after a workout to determine the difference. Convert the difference into a percentage of your total body weight. Ideally you should lose less than 2% of total body weight—every pound of weight lost is equal to 16 ounces of sweat. If you lose a lot more, then you started your workout dehydrated or need to amp up fluid intake during your workout.
