How-to · Updated October 2, 2026
How much water should you drink on a water fast?
Direct answer
The calculator budgets 2.5 liters a day. That is 7.5 L over 3 days, 12.5 L over 5 days, and 17.5 L over 7 days. After 48 hours the notes add electrolytes. Plain water alone can dilute sodium.
Liters by length of fast
| Fast | Per day | Total |
|---|---|---|
| 1-day (24 h) | 2.5 L | 2.5 L |
| 3-day (72 h) | 2.5 L | 7.5 L |
| 5-day (120 h) | 2.5 L | 12.5 L |
| 7-day (168 h) | 2.5 L | 17.5 L |
Many adult fluid ranges sit around 2–3 L/day. This tool picks the middle of that range and holds it fixed.
What the notes add
- Short-term fast: stay hydrated; stop if you feel unwell.
- Consider electrolytes after 48 hours — not just plain water.
- After day 3, metabolic adaptation (~10–15% lower burn) is common in models.
Limits
- Educational fluid budget, not medical advice. Thirst, heat, and kidney disease change what a safe amount is.
- Drinking far past this budget to “flush” a fast can lower sodium. Lightheadedness is a stop signal.
- Fasts longer than a few days, pregnancy, eating disorders, diabetes on medication, and age under 18 need a clinician. This page does not set a sodium, potassium, or magnesium dose.
Use the calculator
Timing is in when ketosis starts on a water fast. The scale estimate is in how much weight a water fast loses. Change the dates in the water fast calculator.