Intermittent Fasting Planner
Plan a week of intermittent fasting from the time of your last meal: the eating window and the time each fast ends for 14:10, 16:8, 18:6, 20:4 and one meal a day, a 5:2 or alternate-day calendar, or how long you have fasted so far.
How to Use
- Choose Weekly plan to schedule your eating windows, or Time fasted so far to time a fast in progress.
- Pick a plan: 14:10, 16:8, 18:6, 20:4, one meal a day (23:1), a custom number of fasting hours, 5:2 or alternate-day.
- For a daily plan, enter the time you finish your last meal (8 pm, 20:00) and the first day of the week.
- For 5:2 or alternate-day, choose the low-calorie days and the calorie allowance for them.
- For the timer, enter when the fast started and the time now (Now fills it in), then read the time left and when the fast ends.
Worked Example
16:8 with dinner finished by 8:00 PM. The eating window is 8 hours, so it opens at 20:00 − 8 h = 12:00 noon. The fast runs 16 hours from 20:00, ending at 20:00 + 16 h = 12:00 noon the next day. Every day of the week is the same, so the week holds 16 × 7 = 112 hours of fasting.
How far along is a fast? It started at 8:00 PM yesterday and it is now 10:30 AM. From 20:00 to midnight is 4 hours, and midnight to 10:30 is 10 hours 30 minutes, so you have fasted 14 hours 30 minutes. On 16:8 that is 14.5 ÷ 16 = 90.6 % done, with 1 hour 30 minutes to go until noon.
The common mistake: counting from the start of the last meal. If dinner runs from 7:00 to 8:00 PM and you count the 16 hours from 7:00, you stop at 11:00 AM, which is only a 15-hour fast. Count from the last bite, 8:00 PM, and the fast ends at 12:00 noon.
Show Work
Formulas
Fasting Plans and the Research
Intermittent fasting is about when you eat rather than what. The National Institute on Aging (NIH) describes it as hours or days of no or minimal food without going short of essential nutrients, and names three commonly studied regimens: alternate-day fasting, 5:2 (fasting two days each week) and daily time-restricted eating, such as eating only within a six-hour window. Its 2019 review in the New England Journal of Medicine (de Cabo and Mattson) found wide-ranging benefits in animal and human studies, but noted that human trials have mostly been short, and mostly in overweight, middle-aged adults.
Mark Mattson, who has studied intermittent fasting for about 25 years and is now at Johns Hopkins, explains the idea as metabolic switching: after hours without food the body runs through its sugar stores and starts burning fat. Johns Hopkins Medicine describes the daily approach as eating within one six- to eight-hour period (16:8 is the common example) and the 5:2 approach as eating regularly five days a week with one 500–600 calorie meal on the other two.
About This Tool
This planner turns the time you finish your last meal into a daily eating window and fast end time for each day of the week, draws it on a 24-hour dial, lays out a 5:2 week or two weeks of alternate days with your calorie allowance, and times a fast in progress. It is a scheduling aid: it does not set calorie targets or say which plan suits you. The plan descriptions and cautions follow Johns Hopkins Medicine’s page “Intermittent Fasting: What is it, and how does it work?” and the National Institute on Aging.
Check with your doctor before starting. Johns Hopkins Medicine lists children and teens under 18, people who are pregnant or breastfeeding, people with type 1 diabetes who take insulin and anyone with a history of eating disorders as people who should steer clear; if you have diabetes of any type or take regular medicines, ask first. Everything runs in your browser; nothing you enter is sent anywhere.
For general information only — not medical advice. Talk to a doctor or other qualified professional about your own health.
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Frequently Asked Questions
What is 16:8 fasting?
Eating within an 8-hour window each day and fasting for the other 16. Johns Hopkins Medicine describes the daily approach as restricting eating to one six- to eight-hour period. If you finish your last meal at 8:00 PM, the window is noon to 8:00 PM and the fast ends at noon the next day: 112 hours of fasting a week.
How does the 5:2 diet work?
You eat regularly five days a week, and on the other two you limit yourself to one 500–600 calorie meal, according to Johns Hopkins Medicine, which gives Mondays and Thursdays as an example. Keep the two days apart; over a year that is 104 low-calorie days, about 8.7 a month.
Who should not try intermittent fasting?
Johns Hopkins Medicine says to check with your doctor before you start, and lists children and teens under 18, people who are pregnant or breastfeeding, people with type 1 diabetes who take insulin (because of the risk of low blood sugar while fasting) and anyone with a history of eating disorders as people who should steer clear. If you have diabetes of any type, ask your doctor first.
What can I drink while fasting?
Johns Hopkins Medicine says water and zero-calorie drinks such as black coffee and tea are fine during the fasting hours. Drinks with milk, sugar or juice have calories, so they belong in the eating window.
Is a longer fast better?
Not necessarily. Johns Hopkins Medicine says longer periods without food, such as 24-, 36-, 48- and 72-hour fasts, are not necessarily better for you and may be dangerous. It also notes it can take two to four weeks to get used to a new schedule, and to talk to your doctor about unusual anxiety, headaches or nausea.
How do I use the Intermittent Fasting Planner?
Simply type your numbers and read the result, which refreshes the instant you change something. There is nothing to submit and nothing to wait for.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
A 16:8 working day
Finishing dinner by 8:00 PM puts the eating window at noon to 8:00 PM; the fast ends at noon the next day, 16 hours later, 112 hours over a week.
An earlier finish
On 18:6 with a last meal at 7:00 PM, the window runs from 1:00 PM to 7:00 PM and each fast ends at 1:00 PM the next day.
One meal a day
A 23:1 plan with the meal finished by 6:00 PM means eating between 5:00 and 6:00 PM, 161 hours of fasting a week.
Planning 5:2
Mondays and Thursdays as 500-calorie days leave five days of normal eating; Johns Hopkins Medicine describes one 500–600 calorie meal on the two low days.
Checking a fast in progress
A 16:8 fast that started at 8:00 PM yesterday has run 14 hours 30 minutes at 10:30 AM: 90.6 % done, with 1 hour 30 minutes to go until noon.
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