How much water you really need
Hydration needs scale with your body size and how much you sweat. A widely used, research-backed guideline is:
Base = weight(kg) × 35 ml
+ 350 ml for every 30 min of exercise
+ extra for warm or humid climates
+ 350 ml for every 30 min of exercise
+ extra for warm or humid climates
For most adults this works out to somewhere between 2 and 3.5 litres a day — noticeably more than the one-size-fits-all "8 glasses" (which is about 2 litres).
What counts toward it
- All fluids count — water, tea, coffee, milk. The idea that caffeine dehydrates you is a myth at normal intakes.
- Food counts too. Fruit, vegetables, soups and yoghurt can provide 20%+ of your daily water.
- Thirst is a lagging signal. By the time you feel thirsty you're already mildly dehydrated — sip steadily rather than waiting.
Signs you're well hydrated: pale-yellow urine, steady energy, and no mid-afternoon headache. Dark urine is the simplest cue to drink more.
Easy ways to actually hit your target
- Keep a marked bottle on your desk — visible cues beat willpower.
- Anchor a glass to existing habits: one when you wake, one before each meal.
- Front-load earlier in the day so you're not chugging (and waking up) at night.