The 8x8 rule (eight 8 oz glasses) is a heuristic, not science. The European Food Safety Authority recommends 2.0 L total fluid a day for women and 2.5 L for men, including water in food. About 20% of typical intake comes from food, so a 70 kg adult usually needs 1.6 to 2.0 L of actual drinking water at baseline.
Adjust for your training and weather
Add roughly 500 to 750 ml per hour of moderate training, more in heat. Caffeine up to 400 mg per day is not meaningfully dehydrating in habitual users, so coffee and tea count toward the total. Alcohol does not. If you weigh yourself before and after a long session and drop more than 2% of body weight, you under-drank: replace it slowly across the next two to three hours rather than chugging it all at once.
What to do with this number
Use urine colour as the daily check. Pale straw is hydrated. Dark amber means drink more. Clear, copious urine means you are over-drinking and washing out electrolytes. Spread intake across the day rather than backloading it. If you sweat heavily or train over 90 minutes, add 300 to 500 mg of sodium per litre during the session: plain water on long efforts hurts performance more than it helps.
