Alternate-day fasting calculator
Enter when you finished eating. The calculator lays out each 36-hour fast and the 12-hour eating window after it.
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How alternate-day fasting is timed
Alternate-day fasting, as timed here, repeats every two days: a 36-hour fast, then a 12-hour eating window. Finish dinner at 7:00 PM on Monday, do not eat on Tuesday, and eat from 7:00 AM to 7:00 PM on Wednesday. The next fast starts at 7:00 PM on Wednesday.
There are other versions. Some people eat a small amount on fasting days, which is often called modified alternate-day fasting. This calculator times the version with a full fast, because that is the one with clear start and end times.
A 36-hour fast is a long fast. It is not suitable for everyone, and it is at the limit of what the tracker saves without an extra confirmation.
Questions
Why 36 hours and not 24?
Skipping a whole day of eating means fasting from dinner on one day to breakfast two days later. Dinner at 7:00 PM to breakfast at 7:00 AM across a full day in between is 36 hours.
Which days are fasting days?
They alternate, so they move through the week: if the first full fasting day is a Tuesday, the next are Thursday, Saturday, Monday and so on. The calculator lists the dates.
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.