Intermittent fasting window
Enter when fasting starts and how many hours you fast to get the exact fasting and eating windows, compare 16:8, 18:6, 20:4 and other common protocols, and see how each splits the 24-hour day.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Fasting settings
Calculated on a 24-hour wall clock, wrapping past midnight; fasting plus eating always adds up to 24 hours.
This window
20:0012:0020:0016 h8 h66.7 %33.3 %Common protocols
| Protocol | Fasting ends | Eating window | Fasting share |
|---|---|---|---|
| 12:12 | 08:00 | 08:00 ~ 20:00 | 50.0 % |
| 14:10 | 10:00 | 10:00 ~ 20:00 | 58.3 % |
| 16:8 | 12:00 | 12:00 ~ 20:00 | 66.7 % |
| 18:6 | 14:00 | 14:00 ~ 20:00 | 75.0 % |
| 20:4 | 16:00 | 16:00 ~ 20:00 | 83.3 % |
| 23:1 | 19:00 | 19:00 ~ 20:00 | 95.8 % |
What this tool does
- Before starting 16:8, pin down the window: finish eating at 20:00 and the next meal opens at 12:00 the following day.
- Irregular schedule? Move the start to 21:30 or 22:00 and see where the eating window lands before committing.
- Compare 14:10, 16:8, 18:6 and 20:4 and pick the one you can actually keep up.
- Booking lunch with colleagues? Check the eating window first so the meal does not land inside your fast.
Example
Input
Fasting starts at 20:00, fasting hours 16 (the 16:8 protocol)
Output
Fast 20:00 to 12:00 the next day, eating window 12:00–20:00; 16 hours fasting (66.7%), 8 hours eating (33.3%)
All times are 24-hour wall-clock values that wrap past midnight, with no dates or time zones involved.
Frequently asked questions
How is the window calculated?
The fast ends at start + fasting hours, and the eating window runs from there for 24 − fasting hours. Starting at 20:00 with a 16-hour fast means fasting until 12:00 the next day and an eating window of 12:00–20:00. Everything is computed in minutes from midnight modulo 1440, so crossing midnight needs no special case.
What is the difference between 16:8, 18:6 and 20:4?
The numbers are fasting hours : eating hours and always add up to 24. Longer fasts leave a narrower eating window: 16:8 gives you 8 hours, 20:4 gives 4, and 23:1 (OMAD) leaves just one. Narrower windows make it harder to hit protein and micronutrient needs, so starting at 14:10 or 16:8 is usually more sustainable.
Can I drink water, coffee or tea while fasting?
This tool only computes the clock window, not what counts as fasting. The common convention is calorie-free drinks only: water, black coffee and unsweetened tea. Coffee with milk or sugar, juice and soft drinks break the fast. What is allowed depends on the protocol you follow, so check with a dietitian if it matters to you.
Why does the tool not tell me how much of today’s fast is left?
Because progress depends on the current time. To stay independent of the system clock and time zone, this tool does pure clock arithmetic and never reads the clock. You can read the window boundaries directly, or subtract the window start from the current time yourself.
Is intermittent fasting suitable for everyone?
No. It is not appropriate during pregnancy or breastfeeding, for children and adolescents, for people with diabetes or a risk of hypoglycaemia, for anyone with a history of eating disorders, or for people taking medication that must be taken with food. Talk to a clinician or registered dietitian first, and stop immediately if you feel dizzy, faint or unable to concentrate.
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