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Generic hydration advice fails because sweat rates vary 5×.
Sweat rates range from under 0.5 L/hr to over 2.5 L/hr between athletes — so "drink eight glasses" is useless for training. The field test is simple: weigh before and after, account for what you drank, divide by hours. This calculator gives your rate, your dehydration percentage against the 2% performance threshold, and exactly how much to drink during and after similar sessions.
Runners and field-sport athletes use it once per season per climate: weigh before and after, get your personal L/hr, and hydration stops being guesswork. It also answers the "why do I cramp and you don't" question between training partners.
Enough to keep body-mass loss under 2% — beyond that, endurance performance and cognition measurably decline. Your sweat rate from this calculator sets the target: someone at 1.5 L/hr needs roughly double the fluids of someone at 0.75 L/hr in the same workout. In practice, gut absorption caps intake around 0.8–1.2 L/hr for most people.
You keep losing fluid to urine after the session ends, and rapid drinking triggers diuresis before full rehydration. ACSM guidance is to replace 125–150% of the deficit over the following hours, ideally with sodium included — plain water alone dilutes blood sodium and blunts the thirst drive before you are actually rehydrated.
Under an hour: water is fine. Over an hour, or heavy/salty sweaters (white streaks on clothes, stinging eyes): include sodium — sweat carries roughly 500–1,000 mg per liter, and replacing large volumes with plain water risks hyponatremia, which has hospitalized marathoners.