Water makes up about 60% of your body weight and plays a role in virtually every physiological process relevant to exercise performance. Even mild dehydration — as little as 1-2% of body weight — can impair strength, endurance, and cognitive function.
How Much Water Do You Actually Need?
The "8 glasses a day" rule is a simplification. Your actual needs depend on body size, activity level, climate, and diet. A reasonable starting point: take your bodyweight in pounds, divide by two — that's roughly your ounce target per day. An active 180-pound person needs about 90 ounces minimum. During exercise, add another 8-16 ounces per hour of moderate activity.
Sweat Rate Calculation
Weigh yourself before and after a training session (without clothes, after toweling off). The difference in pounds equals fluid loss — approximately 1 pound lost equals 16 ounces of fluid. To replace it, drink 1.5 times that amount over the next few hours. If you lost 1 pound, drink 24 ounces over the next few hours to fully rehydrate.
Electrolytes During Long Sessions
For sessions under 60 minutes, plain water is fine. Beyond 60-90 minutes, especially in heat, adding electrolytes (sodium, potassium, magnesium) becomes important. Sodium is the most critical — it drives water absorption and prevents hyponatremia. Sports drinks or electrolyte tablets are useful for sessions longer than 90 minutes.