You've probably heard you need eight glasses of water a day. It's a memorable rule — and it isn't based on strong evidence. Your real needs depend on several personal factors.
What the evidence says
Health authorities suggest general fluid targets, but these include water from all beverages and foods. Fruits, vegetables, soup, tea, and coffee all contribute.
Signs you're drinking enough
Two simple signals work for most people: you rarely feel thirsty, and your urine is pale yellow. Dark urine and persistent thirst suggest you need more.
When you need more
Hot weather, exercise, fever, pregnancy, and breastfeeding all raise fluid needs. Athletes and outdoor workers should drink proactively rather than waiting for thirst.