Intermittent Fasting Calculator
Calculate your eating window, fasting window, and daily fasting schedule for 16:8, 18:6, 20:4, and other IF protocols.
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How to use this calculator
- 1
Select your intermittent fasting protocol (16:8 is the most popular starting point).
- 2
Enter the time you want to start eating each day.
- 3
Choose your goal to get goal-specific tips.
- 4
Review your eating window and fasting window.
- 5
Drink plenty of water, black coffee, or plain tea during your fasting hours.
Frequently asked questions
How are the eating and fasting windows calculated?
The protocol sets the number of eating hours: 10 for 14:10, 8 for 16:8, 6 for 18:6, 4 for 20:4 and 1 for one meal a day. The eating window runs from your start time for that many hours, and the fasting window is the rest of the 24 hours, ending at your start time the next day. Times are shown on a 12-hour clock.
What does the 5:2 option do?
It ignores the start time and prints a fixed text plan: five normal-eating days, and two non-consecutive days (suggested as Monday and Thursday) limited to 500 to 600 calories in two small meals. There is no clock-based window for 5:2.
What do the goal, sex and coffee options change?
Only the explanatory text below the schedule. The goal selects one note about that goal, the sex option adds an extra paragraph if you pick female, and the coffee option switches one sentence about what can be drunk during fasting hours. None of them alter the window times.
How are the meal times chosen?
The first meal is at your start time. The second is placed at half the eating window, and the last meal is one hour before the window closes. At 16:8 from noon that means 12:00 PM, 4:00 PM and 7:00 PM. These are suggestions for spacing, not nutrition advice.
What start times does it accept?
Write HH:MM in 24-hour form, such as 13:30. Hours are reduced modulo 24, minutes outside 0 to 59 are treated as 0, and text that cannot be read as a time falls back to 12:00. For example 8:75 is shown as 8:00 AM.
Does the calculator estimate weight loss or fat burning?
No. It makes no calorie, ketosis or weight-change predictions. It is a scheduler for the hours of the day, and any benefit from fasting depends on what and how much you eat in the eating window, which the tool does not measure.
Who should check with a doctor first?
The tool’s own warning lists people who are pregnant or breastfeeding, have diabetes or a history of eating disorders, or take medication that must be taken with food. Anyone with another medical condition should also ask. This is general information, not medical advice.
Intermittent fasting calculator — eating window and fasting window by protocol
What gets calculated
Intermittent fasting plans divide the day into an eating period and a fasting period, and the calculator does exactly that. You choose a protocol and the time you want to start eating. It adds the eating hours to the start time to find when the window closes, treats the remaining hours as fasting, and suggests three meal times inside the window. Everything is clock arithmetic on a 24-hour day; the goal, sex and coffee options change only the notes printed afterwards.
Schedules for a 12:00 start
The table lists the windows the calculator returned when the eating start was set to noon. The one-meal option has a one-hour window and no separate meal suggestions.
| Protocol | Eating window | Fasting | Meals suggested |
|---|---|---|---|
| 14:10 | 12:00 PM – 10:00 PM | 14 h | 12:00 PM, 5:00 PM, 9:00 PM |
| 16:8 | 12:00 PM – 8:00 PM | 16 h | 12:00 PM, 4:00 PM, 7:00 PM |
| 18:6 | 12:00 PM – 6:00 PM | 18 h | 12:00 PM, 3:00 PM, 5:00 PM |
| 20:4 | 12:00 PM – 4:00 PM | 20 h | 12:00 PM, 2:00 PM, 3:00 PM |
| OMAD (23:1) | 12:00 PM – 1:00 PM | 23 h | one meal |
Worked examples with other start times
Starting at 13:30 on the 18:6 plan gives an eating window of 1:30 PM to 7:30 PM, a fasting window from 7:30 PM until 1:30 PM the next day, a second meal at 4:30 PM and a last meal at 6:30 PM. Starting at 11:00 on 20:4 gives 11:00 AM to 3:00 PM with meals at 11:00 AM, 1:00 PM and 2:00 PM. Choosing one meal a day at 18:00 gives a window of 6:00 PM to 7:00 PM. Minutes are carried through, so a 13:30 start produces a 7:30 PM close and not 7:00 PM.
Choosing a protocol
The calculator lists the protocols in no particular order of difficulty and does not recommend one. A longer fast leaves a shorter window for eating, which makes it harder to fit meals and can be harder to sustain, while a 14:10 pattern asks for a smaller change from a typical day. Its own tips suggest starting with 14:10 or 16:8, and then treating consistency as more important than the length of any single fast. Whether the pattern suits you depends on work, family routine, training schedule and health, none of which the tool knows.
What it does not tell you
The page schedules hours, not nutrition. It does not count calories, set protein targets, or predict weight change, and the statements it prints about drinks that do or do not break a fast are general guidance, not a ruling from the calculator. People who are pregnant, breastfeeding or diabetic, who take medicine that needs food, or who have had disordered eating should speak to a doctor first, and anyone who feels unwell while fasting should stop and seek advice.
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Results are estimates for informational purposes only and do not constitute professional financial, medical, legal, or technical advice. Read full disclaimer →