A common general guideline is about 2 litres a day for women and 2.5 litres for men, counting all fluids including water, tea, milk and the water in food. You usually do not need to track this precisely. A simple practical signal is your urine: pale straw-coloured urine and rarely feeling very thirsty usually means you are well hydrated.
You need more in hot weather, during exercise, when you have a fever, or when losing fluid through vomiting or diarrhoea. Pregnancy and breastfeeding also raise needs. Older adults sometimes feel thirst less reliably, so sipping regularly through the day helps.
Some people should not simply drink as much as possible. If you have heart failure, advanced kidney disease, or are on certain medicines, your doctor may set a daily fluid limit. Seek urgent care if you have confusion, very dark urine with little output, dizziness on standing, or signs of severe dehydration. Discuss your personal target with a doctor or pharmacist if you have a chronic condition.