Do you really need eight glasses of water a day?
No fixed number fits everyone. Drink enough to avoid starting thirsty and replace what hard, hot training costs you — without forcing water long after thirst is gone.
Eight glasses is memorable because it is simple, not because the body works in eight-glass increments. Your fluid requirement moves with body size, food, weather, altitude, pregnancy, illness, and especially how much you sweat. Water in coffee, tea, milk, fruit, and meals counts too.
For an ordinary day, thirst and pale-to-light-yellow urine are useful rough signals. During training, the stakes depend on the session. A short gym workout in air conditioning is not the same problem as a two-hour run in July. Dehydration makes prolonged exercise in the heat harder, and drinking after a meaningful fluid loss can improve the next bout. But the famous rule that any 2% body-mass loss ruins performance is less universal than it sounds; self-paced, real-world trials often find a smaller penalty.
Make the plan match the sweat
If a session is long, hot, or repeatedly leaves you several pounds lighter, weigh yourself before and after a representative workout. Each kilogram lost is roughly a litre of net fluid deficit. Use that as a starting point for the next similar session, not as a command to replace every drop while moving.
Thirst still matters. So do sodium and food during very long events. Chugging plain water beyond thirst can dilute blood sodium, which is a real medical risk. More hydrated is not endlessly better.
The takeaway
Skip the universal glass count. Arrive normally hydrated, keep water available, and drink more deliberately when heat, duration, or a high sweat rate demands it. Your goal is enough fluid to feel and perform well — not winning a contest against your water bottle.