Water Intake: How It Works
The advice to drink eight glasses a day has no strong scientific origin and no adjustment for body size, activity or climate. Actual fluid needs scale with weight, how much you sweat and how hot it is — and a meaningful share of your intake comes from food rather than drinks.
A better starting estimate
A common practical guide is 30–35 ml per kg of body weight per day of total fluid.
| Body weight | Baseline total fluid |
|---|---|
| 50 kg | 1.5 – 1.8 L |
| 60 kg | 1.8 – 2.1 L |
| 70 kg | 2.1 – 2.5 L |
| 80 kg | 2.4 – 2.8 L |
| 90 kg | 2.7 – 3.2 L |
Then adjust upward:
| Circumstance | Additional |
|---|---|
| Each hour of moderate exercise | +400 – 800 ml |
| Hot or humid conditions | +500 – 1,000 ml |
| High altitude | +500 ml |
| Fever, vomiting or diarrhoea | Substantially more — seek advice if prolonged |
Food counts
Roughly 20–30% of typical fluid intake comes from food. Cucumber, watermelon, tomatoes, oranges, lettuce and soup are largely water; so are yoghurt, rice and most cooked dishes. Someone eating plenty of fruit, vegetables and soups genuinely needs fewer glasses than someone eating mostly dry, processed food — which is one reason a single universal target makes little sense.
The reliable indicator
Urine colour is a simple and reasonably accurate guide, more useful than counting glasses.
| Colour | Generally indicates |
|---|---|
| Clear / colourless | Possibly more than needed |
| Pale straw to light yellow | Well hydrated — the target |
| Darker yellow | Drink more |
| Amber or darker | Noticeably dehydrated |
B vitamins and some medications turn urine bright yellow regardless of hydration, so read the signal alongside thirst rather than on its own. Thirst is a real and generally reliable mechanism in healthy adults; it becomes less reliable with age, which is why older adults are advised to drink on a schedule rather than waiting for it.
Tea, coffee and other drinks
Caffeinated drinks have a mild diuretic effect, but at ordinary intakes they still deliver a clear net fluid gain. Tea and coffee count toward your total. Alcohol is the genuine exception — it suppresses the hormone that helps you retain water, so it produces a net loss, which is a substantial part of how a hangover works.
Drinking too much is possible
Consuming very large volumes rapidly can dilute blood sodium to dangerous levels — hyponatraemia. It is rare, but it has occurred in endurance athletes and in water-drinking challenges, and it can be serious. Healthy kidneys clear roughly 0.8–1.0 litres per hour at most. Spreading intake through the day rather than consuming litres in a short window avoids the problem entirely.
Practical habits
- Have a glass with each meal — three built-in servings without any tracking.
- Keep a filled bottle within reach; proximity does more than intention.
- Drink before, during and after exercise rather than only afterwards.
- Increase deliberately in hot weather, when thirst lags behind actual need.
- Front-load toward the earlier part of the day if night waking is a problem.
This page is general information, not medical advice. People with kidney, heart or liver conditions may be given specific fluid limits by their doctor, and those instructions take precedence over any general guide.