How Much Water Should You Drink Each Day?
The 8-glasses-a-day rule has no real scientific basis. Here's what does.
The bodyweight rule of thumb
A reasonable starting point is 35 ml per kg of bodyweight per day. For a 70 kg adult, that's around 2.45 litres of total fluid intake. Importantly, this is total fluid — including tea, coffee, soft drinks, and the ~20% that typically comes from food (especially fruits, vegetables, and soups).
Use the water intake calculator to see the target for any bodyweight.
What actually changes the target
- Exercise: add roughly 500 ml per hour of moderate activity, more if heavy sweating.
- Hot weather: 20–50% more, depending on temperature and humidity.
- Pregnancy / breastfeeding: add 300–700 ml/day.
- Illness: fever and diarrhoea both raise the target significantly.
The colour test
Urine colour is a reliable real-time indicator: pale straw = well-hydrated; darker than apple juice = behind. Very pale / clear can actually indicate over-hydration, which dilutes blood sodium and is unpleasant at best and dangerous at worst.
Caffeine and alcohol
Tea and coffee count toward fluid intake despite the mild diuretic effect — the net contribution is positive at typical UK consumption (3–4 cups/day). Alcohol is genuinely dehydrating; rule of thumb is to match each alcoholic drink with roughly 250 ml of water alongside, especially before bed.
Common mistakes
- Front-loading the day: drinking most of the daily total before noon is uncomfortable and pulls overnight sleep apart with bathroom trips. Spread intake evenly through the day.
- Counting only “water”: milk, herbal tea, soup, fruit and salad all count toward total fluid.
- Hard targets in cool weather: 3+ litre targets that are reasonable in summer are excessive in a UK winter at moderate activity.