Skip to main content
BeHabitual

Body

Intermittent fasting calculator

Pick a schedule and a first meal. The window, and the time you stop eating, follow from those two.

By Nabeel Mustafa. Updated .

Runs in your browser. Nothing is sent, saved, or shared.

Used to show how long before bed you stop eating.

Your 8-hour eating window

12:00 to 20:00

Fast from 20:00 until 12:00 the next day

00:0006:0012:0018:0024:00

Eating windowFasting

Fasting
16 hr
Eating
8 hr
Last bite before bed
3 hr 0 min

Hours of the fast you sleep through start here

Last bite
20:00

Choose the window around your evenings

Most people set the first meal and let the last one fall where it falls. That is backwards. The first meal is easy to move. The last one is dinner with other people, and a schedule that ends at 4pm will break the first night someone cooks for you.

So start from the evening. If dinner is at 7.30, the window has to include 7.30, and the first meal follows. On 16:8 that means eating from about 11.30. The bedtime field is there to show the other side: how many hours before sleep the kitchen closes.

What the ratios mean

The first number is hours fasting and the second is hours eating, adding to 24. 16:8 is the usual starting point, partly because about half the fast is sleep. 18:6 and 20:4 are the same idea with a narrower window. One meal a day is 23:1, which leaves no room for a late meeting or a delayed train.

This page only does the clock arithmetic. It does not say whether fasting suits you, and it cannot. People with diabetes, a history of eating disorders, or who are pregnant should not start a fasting schedule from a web page.

Where fasting schedules fail

Weekends. A schedule built around a weekday office routine meets a Saturday brunch and loses. Decide in advance whether weekends run the same window, a shifted one, or none, and write that down. A rule decided on Friday holds better than one decided at 10am on Saturday.

Find your eating window

  1. Pick a fasting schedule

    16:8 if you are starting out.

  2. Set your first meal

    Work backwards from the dinner time you do not control.

  3. Set your usual bedtime

    This shows how long before sleep your last bite falls.

  4. Read the window and the bar

    The bar shows the whole day, with the eating window filled.

Questions people ask

What is the 16:8 eating window?

Eight hours of eating and sixteen of fasting in each 24. A common choice is noon to 8pm, which puts the fast across the night and the morning.

What is the best time for a 16:8 eating window?

The one that includes the meals you cannot move. For most people that is dinner, so the window ends after dinner and starts eight hours before.

Does black coffee break a fast?

That depends on which rules you follow, and the answers online disagree. This calculator only works out times. It has no view on what counts as breaking a fast.

Can I use this for one meal a day?

Yes. Pick 23:1 and set the meal time. The window is one hour.

Other tools

Read next

Numbers are easy. Doing it every day is the hard part.

A calculator tells you where you are once. If the thing you actually want is to keep something up, that takes a record rather than a result.

Start trackingSee how it works