Eating-window calculator
See when a chosen eating window opens and closes.
Choose a start and length
How the times are calculated
The window closes after the number of hours you choose. The remaining hours in a 24-hour day fall outside that window. This is clock arithmetic, not a recommendation for how long to fast.
Want to track it? Start the timer when your window closes, or plan a different start for each day.
How to use the eating-window calculator
- Enter the time of your first meal, such as 12:00 for noon.
- Choose how many hours you plan to eat within: 4, 6, 8, 10 or 12.
- Read the result: the opening and closing times, the hours outside the window and whether it closes after midnight.
Example windows
The table shows common pairings, written as hours outside the window and hours inside it. The times are worked out with the same arithmetic the calculator uses.
| Pattern | First meal | Window closes | Hours outside the window |
|---|---|---|---|
| 12:12 | 8:00 | 20:00 | 12 |
| 14:10 | 9:00 | 19:00 | 14 |
| 16:8 | 12:00 | 20:00 | 16 |
| 16:8 (early) | 8:00 | 16:00 | 16 |
| 18:6 | 13:00 | 19:00 | 18 |
These are examples of clock arithmetic, not recommendations. Many people start with a longer window, such as 12 hours, which mostly covers the overnight gap between dinner and breakfast. Intermittent fasting schedules explained describes each pattern and what research does and does not show.
When the window crosses midnight
If your first meal is late, the window can close the next day. A first meal at 19:00 with an 8-hour window closes at 03:00, and the calculator labels it “closes the next day”. This is common for night-shift workers. Our guide to fasting and shift work has worked examples.
Choosing a window that fits your life
A window that lines up with your work, sleep and family meals is easier to keep than one that fights them. If you usually eat dinner with others at 18:30, a window that closes at 19:00 or later avoids eating alone. If you exercise in the morning, an earlier window may suit you better. Change the first-meal time and watch how the closing time moves.
Leave room for enough food. A window is about timing, not about eating less than you need, and people who take medication with food should keep to their prescribed timing. Read understanding eating windows for more detail.
Questions
Is an 8-hour window the same as a 16-hour fast?
On paper, yes: 8 hours inside the window leaves 16 hours outside it in a 24-hour day. In practice, the time between your last meal and your next first meal depends on when you actually eat.
Can I choose a window shorter than 4 hours?
The calculator offers 4 to 12 hours. Very short windows make it hard to eat enough and are best discussed with a healthcare professional.
Reviewed October 2026 by Lee Palmer · How we write our guides · Report an error