20:4 fasting calculator
Enter when you finished eating. The calculator shows when your 20-hour fast ends and your 4-hour eating window.
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How a 20:4 schedule works
20:4 is 20 hours of fasting and a 4-hour eating window each day. With a window that short, the time it opens decides the shape of the whole day.
Finish eating at 7:00 PM and the fast ends at 3:00 PM; the window is 3:00 PM to 7:00 PM. That is an evening window. A 2:00 PM finish gives a 10:00 AM to 2:00 PM window instead.
Questions
What time does a 20-hour fast end if I stop eating at 7:00 PM?
At 3:00 PM the next day. The 4-hour eating window then runs from 3:00 PM to 7:00 PM. Enter your own time above to see your window.
What if I start later than planned?
In the tracker, each fast is timed from when you actually start it. Start a 20-hour fast an hour late and its end moves an hour later, while the rest of your schedule stays where it was.
What happens when the clocks change?
A fast is measured in real hours. On the night the clocks go back, a 16-hour fast that starts at 8:00 PM ends at 11:00 AM by the clock, not at noon. The calculator and the tracker both count real hours, and the calculator marks any fast that has a clock change inside it.
Is this medical advice?
No. The calculator does arithmetic on times. It knows nothing about your health and cannot tell you whether fasting, or a fast of this length, is suitable for you. Talk to a healthcare professional before you start, and read the safety information.