While fasting, aim for roughly 2.5–3.5 liters of water per day — slightly more than usual. Food normally supplies 20–30% of your fluid, so skipping meals means drinks have to cover the whole requirement.
On a normal day, 20–30% of your total water intake arrives in food: fruit, vegetables, soups, yoghurt, even bread. Remove the meals and that share disappears. Your body's requirement has not changed — but the amount you have to drink has.
For most adults that means moving from around 2.0–2.5 L of drinks to roughly 2.5–3.5 L during fasting days, scaled to body weight and activity as usual.
Milk, juice, smoothies and anything sweetened contain calories and end the fasted state, regardless of how small the amount is.
This is the part most fasting guides get wrong. Water alone is fine for a 16:8 eating-window fast, but during fasts longer than about 24 hours you take in no sodium at all while continuing to lose it in urine and sweat. Drinking large volumes of plain water on top of that dilutes blood sodium and produces exactly the symptoms people blame on fasting: headache, light-headedness, weakness and cramps. A pinch of salt in water, or a sugar-free electrolyte tablet, resolves most of it.
With a 16:8 or similar pattern, spread intake evenly rather than saving it for the eating window:
Thirst and hunger signals overlap, and during fasting that overlap becomes obvious. Many people find that a large glass of water removes what felt like hunger for 20–30 minutes. That is not a weight-loss trick so much as a signal-clarity one — it makes it easier to tell real hunger from routine.
Fasting with diabetes, kidney disease, low blood pressure, an eating-disorder history, or during pregnancy needs individual medical guidance, including on fluid. If a clinician has given you a fluid limit or target, that overrides general advice.
Plain water contains no calories and does not break a fast in any common protocol — intermittent fasting, extended fasting or pre-surgical fasting typically allow it (follow medical instructions where given). Drinks with calories, including milk and juice, do break a fast.
Yes, both are essentially calorie-free and count toward fluid intake. Their diuretic effect is mild and the net hydration balance is still positive, though very large amounts of coffee on an empty stomach can cause discomfort.
Yes, and it matters more during fasting. Without food you take in almost no sodium, so drinking very large volumes of plain water can dilute blood sodium. Keep intake spread out, and include electrolytes during fasts longer than 24 hours.
When drinking is not allowed during daylight, front-load fluid before dawn and rebuild steadily after sunset — roughly 500 ml at each of several points rather than large volumes at once. Salty and very sweet foods increase thirst the next day.