Fasting calculator
Enter when you finished your last meal and how long you want to fast. The calculator shows when the fast ends and the eating window that follows.
Loading the calculator…
How the calculator works
A fast starts when you finish eating and ends a set number of hours later. Finish at 7:00 PM, fast for 18 hours, and the fast ends at 1:00 PM the next day.
For a fast of less than 24 hours the calculator assumes it repeats daily, so the eating window is whatever is left of the day: an 18-hour fast leaves 6 hours. For 24 hours or more it assumes a two-day or longer cycle that brings you back to the same clock time.
Any whole number of hours from 1 to 72 can be entered. Lengths over 36 hours carry more risk, and the tracker asks you to confirm before it saves one.
Questions
How do I work out when my fast ends?
Add the length of the fast to the time you finished your last meal. The calculator does this for any start time and length, and shows the result in your own time zone.
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.
What does Start this fast do?
It opens the ADF Chronicles tracker with this schedule filled in and shows you what it will do. Nothing is saved until you confirm there. The tracker then counts down the fast, keeps a calendar and records how each fast went, on your own device with no account.
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.