What is the Intermittent Fasting Window Calculator?
This tool turns a popular intermittent fasting (IF) protocol into exact clock times. Choose a schedule such as 16:8, 18:6, 20:4 or OMAD (23:1), tell it when you want to start eating, and it instantly shows when your eating window closes, how long you'll fast, and when the next eating window opens.
How to use it
Pick your protocol from the dropdown. The first number is the fasting hours, the second is the eating hours (they always sum to 24). Then enter the hour and minute you plan to take your first bite. The calculator does the rest using a 24-hour clock, wrapping past midnight automatically.
The formula explained
If your protocol allows \(h_{eat}\) eating hours, the eating window ends at \(\text{start} + h_{eat}\). The fast then lasts the remaining \(24 - h_{eat}\) hours, so the next eating window opens at \(\text{eating end} + (24 - h_{eat})\). Both end times are taken modulo 24 so an 8 PM finish correctly rolls into the next day.
$$t_{eatEnd} = (t_{start} + h_{eat}) \bmod 24$$
$$t_{fastEnd} = (t_{eatEnd} + (24 - h_{eat})) \bmod 24$$
Worked example
On a 16:8 plan starting at 12:00, the eating window runs 8 hours and ends at 20:00. The 16-hour fast then runs from 20:00 to 04:00... wait — adding 16 hours to 20:00 gives 12:00 the next day, confirming an 8-hour eating window opening again at noon. The fast period is reported as 16 hours.
$$\text{eating end} = 12{:}00 + 8 = 20{:}00$$
$$\text{fast end} = (20{:}00 + 16) \bmod 24 = 12{:}00$$
FAQ
Does the next eating window always equal my start time? Yes — because eat hours plus fast hours always equal 24, the cycle repeats at the same clock time each day.
What is OMAD? One Meal A Day, modeled here as a 23-hour fast with a 1-hour eating window.
Is this medical advice? No. Intermittent fasting may not suit everyone; consult a healthcare professional before starting.
