Intermittent Fasting Calculator
Pick a protocol and the hour you want to start eating -- get the exact clock times for your eating and fasting windows, including the weekly fasting-hours total.
The most commonly cited protocol -- an 8-hour eating window, often achieved by skipping breakfast.
Eating window
12:00 PM – 8:00 PM
8h/day
Fasting window
8:00 PM – 12:00 PM
16h/day
That's 112 hours of fasting per week on this schedule.
The standard protocols
Every protocol below describes the same thing -- a fixed daily eating window -- at a different width. The tighter the window, the longer the daily fast and the higher the weekly fasting-hours total.
| Protocol | Eating window | Daily fast | Weekly fasting hours |
|---|---|---|---|
| 12:12 | 12h | 12h | 84h |
| 14:10 | 10h | 14h | 98h |
| 16:8 | 8h | 16h | 112h |
| 18:6 | 6h | 18h | 126h |
| 20:4 | 4h | 20h | 140h |
| OMAD (23:1) | 1h | 23h | 161h |
Worked example
A 16:8 schedule starting your eating window at 12:00 PM (noon).
Eating window: 12:00 PM – 8:00 PM (8 hours).
Fasting window: 8:00 PM – 12:00 PM the next day (16 hours) — 112 hours of fasting per week.
Keep going
- A fasting schedule sets when you eat; the macro calculator sets how much protein, carbs, and fat to eat inside that same window. Macro Calculator
Frequently Asked Questions
What do the numbers like 16:8 or 20:4 mean?
The first number is hours fasting, the second is hours eating, so 16:8 is a 16-hour fast with an 8-hour eating window, and 20:4 is a 20-hour fast with a 4-hour window -- standard shorthand used across intermittent fasting guides, not this site's own naming.
What's the difference between OMAD and 20:4?
OMAD (One Meal A Day) compresses eating into roughly one hour, effectively a 23:1 split; 20:4 allows a wider 4-hour window, often split into two smaller meals. Both are far tighter than 16:8, the most commonly cited starting protocol.
Where does the 5:2 method fit in?
5:2 isn't a daily eating-window schedule like the others -- it means eating normally 5 days a week and sharply reducing calories (not skipping food entirely) on 2 non-consecutive days. Because it's calorie-based rather than clock-time-based, it isn't part of this calculator's eating-window math.
Does a fasting window change automatically overnight if it crosses midnight?
Yes -- this calculator handles that automatically. Starting an 8-hour eating window at 10 PM, for example, correctly ends it at 6 AM the next day and shows the 16-hour fast that follows, rather than breaking at midnight.
Is a longer fasting window always "better"?
Not automatically -- this calculator only shows the schedule math, not health outcomes; how a given protocol works for you is individual, and starting with a shorter window like 12:12 or 14:10 before moving to something tighter is a common, low-risk approach.