FastWiselyUnderstanding water fasting
A water fast means consuming nothing but water (and sometimes plain black coffee or tea) for a set period. It is a stricter form of fasting than daily time-restricted eating, and it deserves more care. This calculator helps you plan the practical details: when the fast ends, roughly how much water to aim for and how to approach your first meal.
How the calculator works
Enter your body weight, how long you plan to fast and when you're starting. The tool adds the duration to your start time to find the end time, estimates a total water target using a common rule of thumb of about half your body weight in ounces per 24 hours, and suggests a refeeding approach based on the length of the fast. These are planning aids, not medical instructions.
Short fasts vs. extended fasts
A 16- to 24-hour fast is something many healthy adults can do occasionally with little preparation. Beyond about 36 hours the picture changes: electrolyte shifts, low blood pressure, light-headedness and a condition called refeeding syndrome become real concerns. That's why we strongly recommend medical supervision for anything longer, and why the calculator flags those durations.
Preparing for a water fast
- Eat balanced meals in the days beforehand rather than a large feast right before.
- Choose a low-demand day; avoid intense workouts, long drives or operating machinery if you feel unwell.
- Keep water nearby and sip regularly instead of drinking large amounts at once.
- Tell someone you're fasting, especially for longer attempts.
Breaking the fast
The longer the fast, the more gently you should return to food. After a day, a small, easy-to-digest meal is usually fine. After several days, refeeding should be gradual and supervised. Listen to your digestion and avoid heavy, greasy or very sugary foods at first.
Who should not water fast
Water fasting is not suitable for people who are pregnant or breastfeeding, under 18, underweight, managing diabetes, taking blood pressure or blood sugar medication, or who have a history of disordered eating, heart, kidney or liver conditions. If in doubt, ask your doctor first.