About this tool
The Intermittent Fasting Window Calculator is a free, in-browser tool that turns a fasting protocol and the time of your first meal into concrete wall-clock windows: when your eating window opens and closes, and when your fast runs until the next day's first meal.
It covers the popular time-restricted-eating patterns — 16:8, 18:6, 20:4 and OMAD (one meal a day) — plus a custom eating-window length. Everything is simple clock math computed in your browser; nothing is tracked or stored.
Frequently asked questions
What do 16:8, 18:6 and 20:4 mean?
The first number is hours fasting, the second hours eating, totalling 24. On 16:8 you fast 16 hours and eat within an 8-hour window; OMAD compresses eating into roughly 1 hour.
Which time do I enter?
The time you want to start eating (your first meal). The eating window runs forward from there, and the fast covers the rest of the 24 hours until the same time tomorrow.
Is this a live countdown timer?
No — it is a schedule planner that shows the wall-clock start and end of each window. It computes times; it does not run in the background or send notifications.
Is intermittent fasting safe for everyone?
No. It is unsuitable for children, during pregnancy, with a history of eating disorders, and with some medications such as insulin. This tool only does the clock math — talk to a doctor before changing your eating pattern.