Intermittent fasting calculator: your eating window, worked out
This intermittent fasting calculator turns a fasting plan into clock times. Pick 16:8, 18:6, 20:4 or one meal a day, enter when you want to eat your first meal — or when your last meal ends — and it shows your eating window on a 24-hour clock with the fasting hours for every day of the week. For the 5:2 approach it lays out your two low-energy days; for alternate-day fasting it marks the fast days.
A free fasting timer sits underneath: press Start fast when you finish eating and it counts the hours, shows how long is left and when your fast ends, and keeps a short history. Everything runs in your browser, with nothing to install.
How to use the Intermittent Fasting Calculator
- Choose a Fasting plan. 16:8 means 16 hours of fasting and an 8-hour eating window each day.
- For daily plans, choose whether you are setting your First meal or when your Last meal ends, and enter the time.
- For 5:2, tick your two fasting days; for alternate-day fasting, pick the first fast day.
- Read your window on the clock and the weekly table. Use Copy schedule to paste it into notes or a message.
- When you finish eating, press Start fast. Press End fast at your next meal to save the duration.
Intermittent Fasting Calculator features
Every common plan
12:12, 14:10, 16:8, 18:6, 20:4, OMAD (23:1), 5:2 and alternate-day fasting in one tool.
Set either end
Plan from your first meal or from when dinner finishes — the calculator works out the other end of the window.
24-hour clock view
The eating window is drawn as an arc on a clock face, so windows that cross midnight are easy to read.
Weekly table
Seven days with eating and fasting times, or fast and normal days for 5:2 and ADF.
Live fasting timer
Elapsed time, time to go and the clock time your fast ends, saved in your browser with a history of recent fasts.
12- or 24-hour times
Switch the display to am/pm whenever you like.
How the eating window is calculated
A daily plan written as fast:eat always adds up to 24 hours. If your first meal is at 12:00 on 16:8, the window closes 8 hours later at 20:00, and you fast for 16 hours until 12:00 the next day. If you set the last meal instead — say dinner ends at 19:30 on 18:6 — the window opens 6 hours earlier, at 13:30. Times are added on a 24-hour clock, so a window that starts at 22:00 on 20:4 correctly ends at 02:00.
The 5:2 approach is different: on two days a week intake is cut to about 2,000–3,000 kJ (roughly 480–720 kcal), as described by Dietitians Australia, and the other five days are normal eating. Alternate-day fasting switches between a fast day and a normal day; because a week has seven days, the weekdays shift from one week to the next.
| Plan | Fast | Eat | Example window |
|---|---|---|---|
| 12:12 | 12 h | 12 h | 08:00 – 20:00 |
| 14:10 | 14 h | 10 h | 09:00 – 19:00 |
| 16:8 | 16 h | 8 h | 12:00 – 20:00 |
| 18:6 | 18 h | 6 h | 13:00 – 19:00 |
| 20:4 | 20 h | 4 h | 15:00 – 19:00 |
| OMAD | 23 h | 1 h | 18:00 – 19:00 |
Who should not fast — and safe habits
Dietitians Australia (May 2026) states that intermittent fasting is not suitable for people who are pregnant or breastfeeding, have a history of eating disorders, or take medicines for diabetes, and that it is not appropriate for adolescents. Diabetes medicines can cause low blood sugar when meals are skipped. If you have any long-term condition, are underweight or take regular medicines, speak to a doctor or dietitian before you start.
- Start gently. 12:12 or 14:10 for a week or two before 16:8.
- Drink water during the fast; plain tea or black coffee are usual. The water intake calculator gives a daily target.
- Make the window count. Fasting does not change what a balanced meal looks like — plan meals with the diet chart maker and check energy needs with the calorie calculator.
- Stop if you feel faint, unwell or preoccupied with food.
Intermittent Fasting Calculator FAQ
How do I calculate my 16:8 eating window?
Add 8 hours to the time of your first meal: a first meal at 11:00 means eating until 19:00, then fasting 16 hours until 11:00 the next day. The intermittent fasting calculator does this for any start time, or works backwards from your last meal.
Is this intermittent fasting calculator free?
Yes. The calculator and the fasting timer are free, need no account, and save only to this browser.
Does the fasting timer keep running if I close the page?
Yes. The start time is stored in your browser, so when you reopen the page the timer shows the correct elapsed time. It does not send notifications.
What can I have during the fasting hours?
Most plans allow water, plain tea and black coffee. Anything with energy — milk, juice, snacks — breaks the fast. Follow any medical advice you have been given.
How many calories should I eat on 5:2 fast days?
Dietitians Australia describes the 5:2 diet as two days a week of 2,000–3,000 kilojoules, which is roughly 480–720 kilocalories, with normal eating on the other days.
Who should not try intermittent fasting?
People who are pregnant or breastfeeding, have a history of eating disorders, take diabetes medicines, or are under 18. Anyone with a medical condition should ask a doctor first. This tool gives times, not medical advice.