Free intermittent fasting calculator
When should you eat, and when does your fast end?
Pick a schedule and enter your times to see your eating window, a live timer and a 7-day plan.
An intermittent fasting calculator turns a fasting schedule, such as 16:8, into clock times: when to eat, when to stop and how much of the fast falls during sleep. It plans timing only. It cannot predict weight loss.
Intermittent fasting at a glance
- Definition: an eating plan that switches between fasting and eating on a regular schedule, focused on when you eat rather than what you eat.[1]
- Daily approach: all eating fits in one six- to eight-hour window each day, as in 16:8.[1]
- 5:2 approach: normal eating five days a week and one meal of 500 to 600 calories on the other two.[1]
- Metabolic switch: fat becomes the main fuel typically beyond 12 hours after the last meal, usually between 12 and 36 hours.[2]
- Weight: in two randomized trials, a set eating window did not lead to significantly more weight loss than the comparison diet.[3][4]
- Adjustment: it can take two to four weeks for the body to get used to a fasting routine.[1]
How to use the intermittent fasting calculator
- Pick a schedule. Choose 12:12, 14:10, 16:8, 18:6, 20:4, OMAD, 5:2 or a custom number of fasting hours.
- Enter your first or last meal. If you know when you want to stop eating, switch to “Last meal” and the calculator works backward.
- Enter your bedtime and wake-up time. The calculator uses your real sleep, not a guess, to show how much of the fast you sleep through.
- Read your plan. You see the eating window, a live timer, a fasting timeline, a way to space meals and a 7-day plan.
- Save it. “Add to my calendar” downloads a file for Google Calendar, Apple Calendar or Outlook. “Copy link to this plan” opens it on any device.
How is the intermittent fasting window calculated?
Subtract the fasting hours from 24 to get the eating window, then add the window to your first meal. On 16:8 with a first meal at noon, the window is 8 hours and closes at 8:00 PM. The fast then runs until noon the next day.
last bite = first meal + (24 − fasting hours)
The calculator then checks your bedtime against your last bite. It counts, minute by minute, how much of the fast overlaps your sleep. On 16:8 from noon to 8:00 PM, with sleep from 11:00 PM to 7:00 AM, 8 of the 16 fasting hours pass asleep.
| First meal | 14:10 window | 16:8 window | 18:6 window | 20:4 window |
|---|---|---|---|---|
| 7:00 AM | 7:00 AM to 5:00 PM | 7:00 AM to 3:00 PM | 7:00 AM to 1:00 PM | 7:00 AM to 11:00 AM |
| 8:00 AM | 8:00 AM to 6:00 PM | 8:00 AM to 4:00 PM | 8:00 AM to 2:00 PM | 8:00 AM to 12:00 PM |
| 10:00 AM | 10:00 AM to 8:00 PM | 10:00 AM to 6:00 PM | 10:00 AM to 4:00 PM | 10:00 AM to 2:00 PM |
| 12:00 PM | 12:00 PM to 10:00 PM | 12:00 PM to 8:00 PM | 12:00 PM to 6:00 PM | 12:00 PM to 4:00 PM |
| 2:00 PM | 2:00 PM to 12:00 AM | 2:00 PM to 10:00 PM | 2:00 PM to 8:00 PM | 2:00 PM to 6:00 PM |
Which intermittent fasting schedule should you choose?
Start with the shortest fast you can keep every day. For most people that is 12:12 or 14:10, moving to 16:8 later. Mark Mattson and Rafael de Cabo suggest starting slowly, for example with a 10-hour eating window.[6]
| Schedule | Fast | Eating window | Example (first meal at noon) | Typical fit |
|---|---|---|---|---|
| 12:12 | 12 hours | 12 hours | 12:00 PM to 12:00 AM | A first step. For many people it only means no late-night snacks. |
| 14:10 | 14 hours | 10 hours | 12:00 PM to 10:00 PM | A gentle step up from 12:12. |
| 16:8 | 16 hours | 8 hours | 12:00 PM to 8:00 PM | The most common daily schedule. Usually a later breakfast or no breakfast. |
| 18:6 | 18 hours | 6 hours | 12:00 PM to 6:00 PM | People already settled on 16:8. |
| 20:4 | 20 hours | 4 hours | 12:00 PM to 4:00 PM | Experienced fasters. Two meals in a short window. |
| OMAD (23:1) | 23 hours | 1 hour | 12:00 PM to 1:00 PM | One meal a day. Hard to meet protein and nutrient needs in one sitting. |
| 5:2 | Two low-calorie days a week | Normal eating on five days | For example, low days on Monday and Thursday | People who prefer a normal routine on most days. |
Johns Hopkins Medicine describes the daily approach as one six- to eight-hour eating window. It also warns that longer fasts of 24, 36, 48 and 72 hours are not necessarily better and may be dangerous.[1] This calculator stops at 23 hours for that reason.
The first weeks are the hardest. Mattson’s research shows it can take two to four weeks for the body to get used to the routine, and hunger or irritability is common at first.[1]
How does 5:2 fasting work?
On 5:2, you eat normally five days a week. On the other two days, intake drops to one meal of 500 to 600 calories, for example on Monday and Thursday.[1] The calculator lets you pick your two days and, if you like, keep a daily window on the other five.
What happens in the body during a fast?
The body shifts from sugar to fat as its main fuel. After a meal, it runs on glucose from food, then on glycogen stored in the liver. When that runs low, fat is released and the liver makes ketones. Researchers call this the metabolic switch.[2]
A 2018 review in Obesity places the switch typically beyond 12 hours after the last meal. It usually happens between 12 and 36 hours, depending on how much glycogen the liver held at the start and how active a person is during the fast.[2] That is why the calculator marks the 12-hour point on your timeline and shows how many hours of your fast go past it.
Many claims about fasting go further than the human evidence. The 2019 review by de Cabo and Mattson stresses that much remains unknown about the long-term effects of intermittent fasting in people.[5]
Does intermittent fasting help you lose weight?
Not more than eating fewer calories in other ways, according to two randomized trials. In both, a set eating window did not lead to significantly more weight loss than the comparison diet.[3][4]
| Trial | People | What the groups did | Result |
|---|---|---|---|
| TREAT, 2020 [3] | 116 adults with overweight or obesity, 12 weeks | Ate from noon to 8 PM, or ate three structured meals a day | About 0.94 kg lost with the window and 0.68 kg with three meals. No significant difference between groups. |
| Liu and colleagues, 2022 [4] | 139 adults with obesity, 12 months | Both groups cut calories. One group also ate only from 8 AM to 4 PM. | 8.0 kg lost with the window and 6.3 kg without it. The difference was not significant. |
Two details matter. In the 12-month trial, both groups also cut calories, and the authors said calorie restriction explained most of the benefit.[4] In the 12-week TREAT trial, the window group lost more lean mass from the arms and legs than the three-meal group.[3]
So a window works best as a tool to eat less, not as a shortcut. Johns Hopkins Medicine notes that people are unlikely to lose weight if they pack the window with high-calorie food or much larger meals.[1] For the food side, see the guide to a diet for weight loss and the calorie calculator.
Is an 8-hour eating window bad for your heart?
There is no proof that it is. A 2024 conference abstract linked eating within under 8 hours a day to a 91% higher risk of heart-related death. It was observational, and its author said it cannot show cause.[7][9]
| Question | What is known |
|---|---|
| What was studied? | More than 20,000 US adults from national nutrition surveys, 2003 to 2018, compared by how many hours a day they ate.[7] |
| What was found? | Eating within under 8 hours a day was linked to a 91% higher risk of death from heart disease than eating across 12 to 16 hours.[7] |
| How strong is it? | It was a conference abstract, not a full peer-reviewed paper. The lead author said the findings need replication and cannot show that the eating window caused the deaths.[9] |
| What did critics note? | People in the shortest-window group differed in other ways. For example, they were more likely to smoke and had a higher body mass index.[8] |
The study does justify one step. If you have heart disease or another long-term condition, talk to your doctor before starting a short eating window.
What can you drink while intermittent fasting?
Water and zero-calorie drinks, such as black coffee and tea, are fine during fasting hours, according to Johns Hopkins Medicine. Anything with calories, including milk, sugar and creamer, belongs in the eating window.[1]
| Drink | During the fast? | Why |
|---|---|---|
| Water, plain or sparkling | Yes | No calories. |
| Black coffee | Yes | Listed by Johns Hopkins Medicine as a zero-calorie drink for fasting hours.[1] |
| Plain tea | Yes | Listed by Johns Hopkins Medicine as a zero-calorie drink for fasting hours.[1] |
| Coffee with milk, cream or sugar | No | The milk, cream and sugar add calories. |
| Coffee with butter or MCT oil | No | Fats are calorie-dense. Keep them for the eating window. |
| Juice, smoothies, broth | No | They contain calories. |
Coffee late in a fast can still affect sleep. The caffeine calculator shows how much caffeine will be left at bedtime.
Who should not try intermittent fasting?
Johns Hopkins Medicine advises against intermittent fasting for four groups: children and teens under 18, people who are pregnant or breastfeeding, people with type 1 diabetes who take insulin, and people with a history of eating disorders.[1]
For type 1 diabetes, the concern is blood sugar dropping too low during the fast. For eating disorders, strict windows can trigger or worsen restrictive or obsessive eating.[1] Anyone taking medicine, especially for blood sugar or blood pressure, should talk to a doctor before changing meal timing.
Tips to make your eating window easier
- Put the fast over your sleep. The more fasting hours you sleep through, the fewer you spend awake and hungry.
- Keep the same window every day. A steady routine is easier to keep, and the calendar file helps.
- Close the window before bed. A gap of a few hours between the last bite and bedtime keeps late eating out of the plan.
- Build meals around protein and fiber. Johns Hopkins Medicine points to leafy greens, healthy fats, lean protein and whole grains.[1] The guide to a diet for gut health has fiber-rich ideas.
- Stay active. The VO2 max calculator shows where your aerobic fitness stands.
Ayurveda has long favored eating the main meal at midday and eating lightly in the evening. A noon-to-evening window fits that pattern. The free dosha quiz shows which dosha leads for you.
What this calculator cannot know
The calculator plans clock times. It cannot measure your blood sugar, ketones or hunger. It cannot know what you eat in the window, which matters at least as much as when. The 12-hour mark is a typical point from research, not a reading from your body.
Stop fasting and eat if you feel dizzy, faint or unwell, and talk to a doctor if symptoms continue.
More free tools for daily health decisions.
Intermittent fasting calculator FAQ
How do I calculate my intermittent fasting window?
Subtract your fasting hours from 24 to get the eating window, then add it to your first meal time. On 16:8 with a first meal at 12:00 PM, the window closes at 8:00 PM and the fast lasts until 12:00 PM the next day.
What is the best intermittent fasting schedule for beginners?
A shorter fast, such as 12:12 or 14:10, is a gentler start than 16:8. Mark Mattson and Rafael de Cabo suggest starting slowly, for example with a 10-hour eating window. Johns Hopkins Medicine notes that adjusting can take two to four weeks.
Does sleep count as fasting?
Yes. Every hour without calories counts, including sleep. That is why most schedules place the fast overnight. Enter your bedtime and wake-up time above to see how many fasting hours you sleep through.
When does the body start burning fat during a fast?
The body burns some fat at all times. A 2018 review in Obesity places the metabolic switch, when liver glycogen runs low and fat becomes the main fuel, typically beyond 12 hours after the last meal. It usually happens between 12 and 36 hours, depending on the last meal and activity.
What can I drink while intermittent fasting?
Johns Hopkins Medicine lists water and zero-calorie drinks, such as black coffee and tea, as fine during fasting hours. Milk, sugar, creamer, juice and MCT oil add calories, so they belong in the eating window.
How does 5:2 fasting work?
On 5:2, you eat normally five days a week. On the other two days, Johns Hopkins Medicine describes limiting intake to one meal of 500 to 600 calories, for example on Monday and Thursday. The calculator lets you pick the two days.
How much weight will I lose with intermittent fasting?
A fasting schedule alone cannot predict it. In a 12-week trial of 116 adults, eating from noon to 8 PM did not lead to more weight loss than three meals a day. In a 12-month trial of 139 adults, adding an 8-hour window to calorie restriction did not lead to significantly more weight loss.
Is 16:8 fasting bad for your heart?
There is no proof that it is. A 2024 abstract linked eating within under 8 hours a day to higher heart-related death, but it was observational and not a full published paper. Its author said it cannot show cause. Talk to a doctor if you have heart disease.
Is it better to eat early or late in the day?
Research has not settled this. Studies have used both early windows, such as 8 AM to 4 PM, and later ones, such as noon to 8 PM. A window that ends a few hours before bed and that you can keep every day is the practical choice.
Can I do intermittent fasting every day?
The daily approach, eating within a six- to eight-hour window each day, is one of the plans Johns Hopkins Medicine describes. Keeping the same window daily makes the routine easier. Longer fasts of 24 to 72 hours are not necessarily better and may be dangerous, Johns Hopkins notes.
Who should not do intermittent fasting?
Johns Hopkins Medicine advises against it for children and teens under 18, people who are pregnant or breastfeeding, people with type 1 diabetes who take insulin, and people with a history of eating disorders. Anyone on medication should talk to a doctor first.
Does this calculator store my data?
No. The calculator runs in your browser. Your plan is saved only in this browser so it is still there on your next visit. A copied link carries the plan inside the link itself, and the calendar file is created on your device.
Sources
- Johns Hopkins Medicine. Intermittent Fasting: What Is It, And How Does It Work? Featured expert: Mark Mattson, PhD. hopkinsmedicine.org/health/wellness-and-prevention/intermittent-fasting-what-is-it-and-how-does-it-work
- Anton SD, Moehl K, Donahoo WT, et al. (2018). Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity, 26(2), 254-268. pmc.ncbi.nlm.nih.gov/articles/PMC5783752
- Lowe DA, Wu N, Rohdin-Bibby L, et al. (2020). Effects of time-restricted eating on weight loss and other metabolic parameters in women and men with overweight and obesity: the TREAT randomized clinical trial. JAMA Internal Medicine, 180(11), 1491-1499. doi.org/10.1001/jamainternmed.2020.4153
- Liu D, Huang Y, Huang C, et al. (2022). Calorie restriction with or without time-restricted eating in weight loss. New England Journal of Medicine, 386(16), 1495-1504. pubmed.ncbi.nlm.nih.gov/35443107
- de Cabo R, Mattson MP (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381, 2541-2551. nejm.org/doi/full/10.1056/NEJMra1905136
- Johns Hopkins Medicine (2020). Intermittent Fasting: Is It for You? Dome, January/February 2020. hopkinsmedicine.org/news/articles/2020/01/intermittent-fasting-is-it-for-you
- American Heart Association (2024). 8-hour time-restricted eating linked to a 91% higher risk of cardiovascular death. EPI|Lifestyle Scientific Sessions 2024, Abstract P192. newsroom.heart.org/news/8-hour-time-restricted-eating-linked-to-a-91-higher-risk-of-cardiovascular-death
- TCTMD (2024). Intermittent fasting study sparks debate over increases in CV mortality. tctmd.com/news/intermittent-fasting-study-sparks-debate-over-increases-cv-mortality
- The Hospitalist (2024). Intermittent fasting linked to higher CVD death risk. Includes comments from lead author Victor Wenze Zhong, PhD. blogs.the-hospitalist.org/content/intermittent-fasting-linked-higher-cvd-death-risk
This calculator plans meal timing for education. It does not predict weight change and is not medical advice. It cannot diagnose, treat or prevent any condition. Do not try intermittent fasting if you are under 18, pregnant or breastfeeding, have type 1 diabetes and take insulin, or have a history of an eating disorder. Talk to a doctor first if you take any medicine or live with a health condition.
