Two litres a day is not a number that fits everyone. What actually decides how much you need.
The two-litre rule is widely repeated, but there is no reason a person weighing 50kg and one weighing 90kg, or a desk day and a summer run, would need the same amount. Guidance generally scales with body weight and activity, and rises with heat and sweat. Knowing your own baseline is more useful than memorising a single figure.
A large share of daily fluid comes from food. Soups and stews, fruit, vegetables and milk all carry a lot of water, and coffee and tea add fluid on balance even after their diuretic effect. If your meals include soup often, you may reach enough without drinking much water separately. On the other hand, it is natural to feel thirstier after salty food, so drinking a little more than usual on those days is fine.
The simplest signal is urine colour. Pale yellow usually means you are fine; consistently dark means you are behind. Thirst arrives only after you are already short, so relying on thirst alone is late. Thirst also fades with age, which is why drinking on a schedule is safer for older adults. Weighing yourself before and after exercise shows roughly how much you lost through sweat, a useful guide for heavy sweaters on how much to replace.
Drinking a very large volume in a short time can dilute the sodium in your blood enough to cause harm. It is rare but real, and it shows up mainly when people drink only water during long exercise. For prolonged sweating, replace salt along with fluid. If nausea, headache or confusion appear despite drinking plenty, it may be too much water rather than too little; seek medical help promptly.
There are times when you need more fluid than usual. In those cases, drink ahead of thirst and in portions. Small, frequent amounts are easier for the body to absorb than a lot at once, and you will not need the bathroom right away.
Water is the default. Caffeine-free teas such as barley or corn tea hydrate just as well. Sugary drinks and juices add plenty of sugar along with the water, so if they become your go-to for thirst the calories add up. Sports drinks are useful after long sweaty sessions because they replace salt and sugar together, but they are more than you need as an office substitute for water. With severe diarrhoea or vomiting, water alone struggles to replace electrolytes, so an oral rehydration solution from a pharmacy is the better choice.
A common mistake is drinking mostly in the morning and evening. Drinking very little all day and then a lot in the evening wakes you up at night to use the bathroom and disrupts your sleep. Spread drinks evenly over the day and taper off one or two hours before bed. Forcing yourself to hit a fixed daily target is not ideal either. If your urine is pale and you got through the day without headaches or dizziness, you are usually drinking enough. Older adults and children may not express thirst well, so people around them should keep an eye out.
Attaching drinking to something you already do lasts longer than trying to hit a target. Keep a cup in sight, take one with each meal, take a sip whenever you stand up. If you have kidney disease or a fluid restriction, general advice does not apply to you as written. Marking lines on a bottle or deciding how many bottles to finish in a day means you do not have to count separately.