How much water do you really need?
The "8 glasses a day" rule has no scientific basis. Real needs depend on your body weight, activity level, climate, and diet. The most defensible starting point is about 35 ml of water per kilogram of body weight per day — that comes to roughly 2.5 L for a 70 kg adult.
That figure includes all fluids: water, tea, coffee, soups, even watery foods like cucumber and watermelon. You don't have to drink it all as plain water.
When to drink more
Add 350-700 ml on training days, hot or humid environments, high-altitude travel, or when you're sick with a fever. Pregnancy and breastfeeding also increase needs significantly.
Hydration and weight loss
Mild dehydration can be misread as hunger. Drinking water before meals reduces calorie intake in studies. For people on GLP-1 medications, hydration is especially important — the drugs slow gastric emptying, which can mask thirst signals and lead to constipation if water intake is low.
Signs you're well hydrated
Pale-yellow urine, peeing every 2-4 hours during the day, no persistent dry mouth, and no afternoon energy crashes. If your urine is consistently dark, you need more water.