Fasting timer

How to use the fasting timer

Pick your goal first: 12:12, 14:10, 16:8, 18:6, 20:4 or OMAD (a 23-hour fast). The goal is locked while a fast runs, so choose it before you press Start fast. Press End fast when you eat again.

If your fast began before you opened the page, type the time your last meal ended and press Start from this time. The timer takes the most recent time that matches, so 8:00 PM typed at 9:00 AM means last night. You can go back up to 24 hours this way. While a fast is running, the same box lets you correct the start time.

What the timer shows

The ring fills as you get closer to your goal, with the hours, minutes and seconds in the middle. Under the ring, the timer names the stage you are in by the hours since your last meal.

Hours since your last meal	Stage name on the timer
0 to 4	Fed
4 to 12	Early fasting
12 to 24	Metabolic switch
24 and over	Longer fast

The hours are a guide. In human studies, the body’s switch from sugar to fat and ketones came somewhere between 12 and 36 hours after the last meal. When you pass your goal, the ring changes colour and says you can eat.

A worked example

Say you finished dinner at 8:00 PM and open the timer at 9:00 AM on a 16:8 goal. Type 20:00 and press Start from this time. The timer reads 13:00:00 and 81% there, because 13 of your 16 hours are done. It shows the Metabolic switch stage, since you are past 12 hours. Your goal comes at 12:00 PM.

Streaks, history and CSV

When you end a fast, it goes into your history with its start, end, hours and goal. Fasts shorter than 12 hours are still saved, but only fasts of 12 hours or more count towards your streak and your total. The 12-hour mark and the 48-hour streak gap are our rules, so a 12:12 day is enough to keep a streak going.

Say one counted fast ends at noon on Monday and the next ends at 10:00 AM on Wednesday. They are 46 hours apart, so the streak goes up. If the second fast ended at 1:00 PM instead, the gap would be 49 hours and the streak would restart at 1.

The page lists your 10 most recent fasts. The CSV download has your saved history, up to your last 365 fasts. Times are written in UTC so the file reads the same in any time zone.

Offline, private and on your device only

The timer runs in your browser, and once the page has loaded it needs no connection. Your browser also keeps a copy of the page after you visit, so it can open later with no signal. Everything is stored in this browser on this device. It does not sync to your other devices, and clearing your browser data wipes your streak and history. Download the CSV first if you want a copy.

Tick Tell me when I reach my goal to get a notification at your goal. It only works while the page is open in a tab, and your browser has to allow notifications for the site. If the browser blocks them, the timer tells you so.

The start of a fast is saved as a moment in time, so flying across time zones doesn’t change how long you have been fasting.

Who should be careful

Before you time a fast, get your doctor’s advice if you are pregnant, under 18, or treat diabetes with insulin or a sulfonylurea. After an eating disorder, our advice is to leave fasting alone and talk to a doctor. The Longer fast stage starts at 24 hours, and if you plan to keep the timer running past that, talk to a doctor first.

Related tools and guides

Your eating window for each day, with a calendar file, comes from the fasting calculator. To see what happens in your body hour by hour, read the fasting timeline. Water, black coffee and plain tea are covered, with other drinks, in what to drink while the timer runs.