# When does my fast end?

Works out when a fast ends from its start time and length, and for daily intermittent fasting such as 16:8 the eating window that follows.

- Page: https://www.acalculator.org/health/fasting-calculator
- JSON spec: https://www.acalculator.org/health/fasting-calculator.json
- Version: 25f6cd75fffc

## Default answer

Example with the default inputs (Fast starts on September 30, 2026, At 20:00, Fast length (hours) 16) on the example date Wednesday, September 30, 2026: A 16-hour fast that starts at 8:00 PM on September 30, 2026 ends on Thursday, October 1, 2026, 12:00 PM.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| date | Fast starts on | The date of your last meal before the fast. Today, unless you change it. |
| time | At | The time you finish your last meal, when the fast starts. |
| hours | Fast length (hours) | How many hours to fast: 16 for 16:8, 18 for 18:6, 24 for a whole day. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| end | Your fast ends | The date and time the fast ends: the start plus the fast length, to the nearest minute. |
| endDate | End date | The date the fast ends. |
| endTime | End time | The time the fast ends. |
| window | Eating window | For a daily fast under 24 hours: the hours left in the day to eat, 24 − the fast length. |
| windowEnds | Eating window closes | For a daily fast under 24 hours: when the next fast starts, 24 hours after this one started. |
| minutes | Fast length in minutes | The fast length in whole minutes, halves rounded up. |

## Method

End = start date and time + fast length (rounded to the nearest minute); for a fast under 24 hours, eating window = 24 − fast length, closing 24 hours after the fast started.

## Assumptions

- Times are clock times on the calendar with no time zone or daylight saving change during the fast.
- A daily plan such as 16:8 repeats every 24 hours: fast for 16 hours, then eat within 8.
- This is a timing tool, not medical advice. Ask a health care provider before fasting if you are pregnant, have diabetes, take medicines, or have had an eating disorder.

## Worked examples

1. date = 2026-09-29, time = 20:00, hours = 16 gives end = Wednesday, September 30, 2026, 12:00 PM, endDate = 2026-09-30, endTime = 12:00, window = 8 hours, windowEnds = Wednesday, September 30, 2026, 8:00 PM, minutes = 960. Source: Clock arithmetic with 1 h = 60 min and 1 d = 24 h (NIST SP 811): 8:00 PM + 16 hours = 12:00 PM the next day.
2. date = 2026-12-31, time = 18:30, hours = 18.5 gives endDate = 2027-01-01, endTime = 13:00, window = 5 h 30 min, minutes = 1,110.
3. date = 2026-10-05, time = 19:00, hours = 36 gives end = Wednesday, October 7, 2026, 7:00 AM, minutes = 2,160.

## FAQ

### How do I work out when my fast ends?

Add the fast length to the time of your last meal. A 16-hour fast that starts at 8:00 PM ends at 12:00 PM the next day. The calculator carries the hours over midnight and across months and years for you.

### What does 16:8 mean?

Fasting for 16 hours and eating within the other 8 hours of each day. 18:6 is an 18-hour fast with a 6-hour eating window, and 20:4 a 20-hour fast with a 4-hour window. The two numbers always add up to 24.

### When does my eating window close?

For a daily plan, the eating window closes 24 hours after your fast started, when the next fast begins. With a fast from 8:00 PM to 12:00 PM, you eat between 12:00 PM and 8:00 PM, then start again.

### Can I enter a fast longer than a day?

Yes, up to 168 hours (7 days). The calculator shows when it ends. There is no daily eating window for fasts of 24 hours or more.

### Can I use half hours?

Yes. Enter 16.5 for 16 hours 30 minutes. The end time is rounded to the nearest minute.

### Is intermittent fasting safe for everyone?

No. People who are pregnant or breastfeeding, have diabetes or take medicines that affect blood sugar, or have had an eating disorder should talk to a health care provider before fasting. This page only does the clock arithmetic.

## Sources

- de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease. N Engl J Med. 2019;381(26):2541-2551 (describes daily time-restricted feeding such as 16:8). https://doi.org/10.1056/NEJMra1905136
- National Institute of Standards and Technology. Guide for the Use of the International System of Units (SI), NIST Special Publication 811 (1 h = 60 min, 1 d = 24 h). https://www.nist.gov/pml/special-publication-811