The short answer
Roughly 2.5 L a day for men and 2 L for women from all sources, including food and other drinks. Use urine colour as the real gauge: pale straw is right, dark amber means drink more.
The "eight glasses" number is folklore. Total fluid intake and body signals are what matter.
Step by step
- Start from about 2–2.5 L total fluid per day, food included.
- Add roughly 400–800 ml per hour of hard exercise or heat.
- Drink with meals — that alone covers most of the requirement.
- Check urine colour mid-morning and mid-afternoon.
- Keep a bottle within reach if you forget; convenience beats willpower.
- Increase during illness with fever, vomiting or diarrhoea.
Common questions
Does coffee dehydrate you?
No — at normal intakes it counts toward your fluid total.
Can I drink too much?
Yes, rarely. Litres per hour of plain water can dangerously dilute sodium.
Do I need electrolyte drinks?
Only after heavy sweating over an hour, or illness with fluid loss.