Intermittent Fasting Calculator
Pick a fasting method and when your first or last meal is. See your eating window, your fast, meal times and how long you've been fasting right now.
The most common pattern, often by skipping breakfast or an evening snack.
Between 10 and 23 hours. Half hours work too, e.g. 15.5.
Enter today's times from your local mosque or prayer timetable. They change a little every day.
Pick 2 days that are not next to each other.
After this, every other day is a fast day.
How your fasting schedule is worked out
A day has 24 hours: your eating window, then your fast.
- 1
Pick your split
16:8 = 24 h− 8 h eating= 16 h fast
- 2
Window opens
First meal at12:00
- 3
Window closes
12:00 + 8 h= 20:00
- 4
Fast until
20:00 + 16 h= 12:00 next day
Read more: the formula, a worked example, the methods and who should take care
Intermittent fasting is about when you eat, not what you eat. A day has 24 hours: you eat within one part of it (the eating window) and don't eat for the rest (the fast). This calculator only plans the times. It doesn't set calorie targets or promise any result. If you'd like to understand how much energy you use, see the calorie calculator or the TDEE calculator.
The formula
fasting hours = 24 − eating hours
window closes = first meal time + eating hours
window opens = last meal time − eating hours
fast ends = window closes + fasting hours
Worked example
On 16:8 you fast for 16 hours and eat within 8, because 24 − 8 = 16. With your first meal at 12:00, the window closes at 12:00 + 8 h = 20:00. The fast then runs from 20:00 for 16 hours, to 12:00 the next day. Three meals spread through the window, with the last one starting 30 minutes before it closes, are 12:00, 15:45 and 19:30: the gap is (8 h − 30 min) ÷ 2 = 3 h 45 min.
If you'd rather set it by your last meal, finishing by 20:00 gives the same window: 20:00 − 8 h = 12:00. Sleeping from 23:00 to 07:00 covers 8 of the 16 fasting hours.
The methods
| Method | Fast | Eat | What it means |
|---|---|---|---|
| 12:12 | 12 h | 12 h | An overnight fast. A gentle place to start. |
| 14:10 | 14 h | 10 h | A slightly longer overnight fast. |
| 16:8 | 16 h | 8 h | The most common pattern. |
| 18:6 | 18 h | 6 h | A shorter window, often two meals. |
| 20:4 | 20 h | 4 h | Sometimes called the Warrior diet. |
| OMAD | 23 h | 1 h | One meal a day. |
| 5:2 | 2 low-intake days a week | Eat as usual on 5 days and much less on 2 days that are not next to each other. | |
| Alternate-day | Every other day | Fast days and eating days take turns. With a 12-hour window on eating days, the longest stretch is 12 + 24 = 36 hours. | |
During the fast
Most intermittent fasting plans allow water, plain tea and black coffee during the fast, with nothing added. Drink plenty of water through the day. If you're new to fasting, starting with 12:12 and adding about 2 hours a week is often easier than jumping straight to a long fast; the Advanced view shows a week-by-week build-up.
Suhoor and iftar (Ramadan)
In the Advanced view you can enter the time suhoor ends (dawn, Fajr) and the iftar time (sunset, Maghrib). The fast is the time between them, with no food or drink. For example, suhoor ending at 04:30 and iftar at 18:15 is a fast of 18:15 − 04:30 = 13 h 45 min, leaving 24 h − 13 h 45 min = 10 h 15 min to eat and drink. The calculator suggests starting suhoor 45 minutes before it ends. Times change each day and depend on where you are, so use your local timetable. Islamic guidance exempts people who are ill, travelling, pregnant or breastfeeding; if you have a health condition, talk to your doctor and a religious scholar you trust.
Who should take care
Talk to a doctor before you start if you:
- are pregnant, trying to get pregnant or breastfeeding
- are under 18
- have had an eating disorder
- have diabetes or blood sugar problems, especially if you take insulin or other medicines that lower blood sugar
- take regular medicines, have a long-term health condition or are underweight
General information only, not medical advice. Sources: Johns Hopkins Medicine, Intermittent fasting: what is it, and how does it work? and NHS guidance on fasting. The live status uses your device's clock.
Common questions
What is 16:8 intermittent fasting?
You eat within an 8-hour window each day and fast for the other 16 hours. If your first meal is at 12:00, your eating window closes at 20:00 and you fast until 12:00 the next day. Many people do it by skipping breakfast or a late-evening snack, and much of the fast happens while asleep.
What can I have during the fasting window?
Most intermittent fasting plans allow water, plain tea and black coffee during the fast, with no sugar, milk or food. Drink plenty of water across the day. A Ramadan fast is different: no food or drink at all from dawn to sunset, so drink well between iftar and suhoor.
What is the 5:2 diet?
On 5:2 you eat as usual on five days a week and eat much less on two days that are not next to each other, for example Monday and Thursday. Many people keep their usual meal times on those days and just eat a lot less. The calculator shows your week and warns if you pick two days in a row.
How long is the fast on alternate-day fasting?
You fast every other day. With a 12-hour eating window on eating days (08:00 to 20:00), the longest stretch runs from 20:00 the evening before a fast day to 08:00 the morning after it, which is 36 hours. Some people use a gentler version with one small meal on fast days.
Who should not try intermittent fasting?
Talk to a doctor first if you are pregnant, trying to get pregnant or breastfeeding, under 18, have had an eating disorder, have diabetes or blood sugar problems (especially on insulin or other medicines that lower blood sugar), take regular medicines or have a long-term health condition. This calculator is a schedule planner, not medical advice.
Does sleep count as fasting time?
Yes. Any time you are not eating counts, so sleep makes up a big part of most fasts. On a 16:8 schedule with the window from 12:00 to 20:00, sleeping from 23:00 to 07:00 covers 8 of your 16 fasting hours, leaving 8 hours of the fast while you are awake.
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