Fasting Window Calculator
| Schedule | Last bite 8pm → | Eat again |
|---|---|---|
| 16:8 | fast to noon | 12:00 pm |
| 18:6 | fast to 2pm | 2:00 pm |
| 20:4 | fast to 4pm | 4:00 pm |
| 24 h OMAD | next dinner | 8:00 pm next day |
Intermittent fasting is not one schedule - it is a boundary, and the boundary is a clock question: the minute your last bite landed decides the minute your fast is over. This calculator fixes both edges and keeps a live countdown between them.
Bottom line: enter when you last ate and your fast length - the 16:8 staple, the tighter 18:6, or a 24-hour OMAD break - and read the exact stop-eating time and the exact time you can eat again. The countdown ticks live, so the 9pm kitchen question answers itself.
The honest part: the clock measures abstinence, not benefit. Research on time-restricted eating (the de Cabo-era reviews collect it) suggests benefits track the fasted window's length and consistency, not the exact hour you chose to start - which means the schedule you can repeat every day beats the mathematically optimal one you abandon on Thursday.
How to use
- Enter the time of your last bite - honestly, including the drink that had calories - and pick your fast length: 16, 18, 20 or 24 hours.
- Read the two edges: the minute your fast started (the countdown anchor) and the minute you can eat again tomorrow. The page keeps counting live while it is open.
- Pin the schedule: the same last-bite time daily produces the same eating window - and a window that starts earlier in the day aligns better with the circadian findings in the feeding-timing research than a late-night window of identical length.
Frequently asked questions
What counts as breaking a fast?
Calories do. Water, black coffee and plain tea keep the fast; milk in the coffee, a diet soda you count as free, or a handful of nuts end it by the practical definition most schedules use. Strict versions count artificial sweeteners and even the creamer's two calories; the working rule is simple - if it has energy, the clock restarts from that bite.
Is 16:8 the same as skipping breakfast?
Not necessarily - 16:8 means an 8-hour eating window wherever you place it. Skipping breakfast with lunch at noon and dinner at 8pm is one 16:8 (window noon-8pm); skipping dinner with noon-8pm collapsed into an earlier window is another. The calculator works from your actual last bite, so the label matters less than the two edges it produces.
When should my eating window start?
Earlier than most people expect: feeding-timing research favors windows that close in the evening - eating earlier in the day aligns better with circadian rhythms for glucose handling than an identical window shifted late. A practical target: last bite around 7-8pm, which this calculator converts into whatever fast length you are running.
Does the fast have to be exact to the minute?
No - the boundaries are conventions, not chemistry. Finishing at 7:52 instead of 8:00 changes nothing physiologically; the schedule's power comes from repetition, which is why this page shows edges you can actually repeat. If you miss the edge by an hour, shift the next day's window rather than extending a fast into misery you will abandon.