Intermittent Fasting Schedule Calculator: 16:8, 18:6, OMAD

Eating and fasting windows: calculated in both directions

The tool builds your fasting day from either the end of your last meal or the time of your first meal, as you choose, then gives the exact clock time of each fasting stage (4, 8, 12, 16 and 20 h). On 18/6 with a first meal at 11 am, the eating window runs from 11 am to 5 pm and the fast from 5 pm to 11 am.

The tool starts either from the end of your last meal or from the time of your first meal, and converts each stage into a time of day. From the last meal, the 19:00, 20:00, 21:00 and 22:00 shortcuts fill in the time in one click. The weekly 5:2 and alternate-day protocols show a day-by-day schedule instead. A table gives the time of the next day's first meal for each daily protocol and a dinner finished at 19:00, 20:00, 21:00 or 22:00. The site's fasting timer, for its part, tracks a session live.

Calculation

• From the last meal: fast from the time entered to that time + fasting duration, then the eating window.
• From the first meal: eating window from the time entered to that time + window length, then the fast.
• Stages: start of fast + 4, 8, 12, 16 h, and + 20 h for protocols of at least 20 h.
• Table: time dinner ends + the protocol's fasting duration.
• 5:2: restricted days on the first day chosen and three days later; the other five days are normal.
• Alternate-day: one fast day in two from the first day chosen, over 14 days, because a 7-day week does not repeat identically.
• Daily protocols: 14/10, 16/8, 18/6, 20/4, 23/1 (OMAD) and 24/0 (24 h with no eating window). Weekly protocols: 5:2 and alternate-day.

Examples

18/6, first meal at 11 am: eating from 11 am to 5 pm, fasting from 5 pm to 11 am, stages at 9 pm, 1 am, 5 am and 9 am. 23/1, last meal finished at 7:30 pm: fast until 6:30 pm, meal from 6:30 pm to 7:30 pm, stages at 11:30 pm, 3:30 am, 7:30 am, 11:30 am and 3:30 pm. In the table, a dinner finished at 9 pm gives a first meal at 1 pm on 16/8 and at 3 pm on 18/6.

5:2 starting on Monday: restricted days on Monday and Thursday. Alternate-day starting on Monday: fast on Monday, Wednesday, Friday and Sunday in the first week, then Tuesday, Thursday and Saturday in the second. In the trial by Harvie et al. (2011), the 5:2 restricted days provided about 2,710 kJ (650 kcal); the review by Varady et al. (2022) describes fast days of 0 to 500 kcal for alternate-day fasting.

Contraindications

Do not start without medical advice if you are pregnant or breastfeeding, under 18, have a current or past eating disorder, or have diabetes treated with insulin or a glucose-lowering drug. The review by Vasim et al. (2022) advises against intermittent fasting during pregnancy and breastfeeding, in young children and in people with eating disorders, and warns of a risk of severe hypoglycaemia with diabetes drugs. In the trial by Corley et al. (2018), in people with type 2 diabetes on glucose-lowering medication, fasting multiplied the rate of hypoglycaemia by 2.05 despite reduced doses. If a medicine must be taken with a meal or at a set time, ask your doctor or pharmacist how to adjust the doses.

Limits

Stages are fixed benchmarks counted from the start of the fast, regardless of what you last ate or who you are. On 14/10 the 16 h stage is still shown although the fast stops at 14 h. The stage descriptions are benchmarks, not measurements. ANSES has not assessed intermittent fasting: its 2024 opinion covers overnight fasting, for which it found the data insufficient to conclude on a link with chronic disease, and it set no fasting duration; as a precaution, it only advises reducing the share of energy eaten in the evening and having dinner about two hours before bedtime. It reports an average overnight fast of 11 h 22 min among adults in France (aged 18 to 79, Inca 3 survey of 2014-2015). The intakes quoted above for restricted days are those of the studies, not a prescription. Talk to a health professional before starting.

Sources and dates:

• ANSES (French food safety agency), opinion on updating the national nutrition guidelines: timing of food intake (request 2019-SA-0001), 6 March 2024 (checked on 28/10/2026)
• Corley et al., “Intermittent fasting in Type 2 diabetes mellitus and the risk of hypoglycaemia: a randomized controlled trial”, Diabetic Medicine 35(5):588-594, 2018 (accessed 22/10/2026)
• Vasim et al., “Intermittent Fasting and Metabolic Health”, Nutrients 14(3):631, 2022 (accessed 22/10/2026)
• Varady et al., “Clinical application of intermittent fasting for weight loss: progress and future directions”, Nature Reviews Endocrinology 18(5):309-321, 2022 (accessed 22/10/2026)
• Harvie et al., “The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers”, International Journal of Obesity 35(5):714-727, 2011 (accessed 22/10/2026)

Frequently Asked Questions

When should I eat on 16/8 if I have dinner at 8 pm?

With a last bite at 8 pm, the tool puts the end of the fast at noon the next day and the eating window from noon to 8 pm. Finishing dinner at 9 pm shifts everything by one hour: first meal at 1 pm, window until 9 pm.

What is the difference between 14/10, 16/8, 18/6 and 20/4?

The first number is the fasting duration, the second the eating window, adding up to 24 h. With dinner finished at 8 pm, the first meal falls at 10 am on 14/10, noon on 16/8, 2 pm on 18/6 and 4 pm on 20/4.

How do I work out my fasting times from my first meal?

Choose the "first meal" mode and enter the time you want to break your fast: the tool adds the window length to find its end, then the fasting duration. On 16/8 with a first meal at noon, you eat from noon to 8 pm and fast from 8 pm to noon.

At what time does each fasting stage occur?

The tool adds 4, 8, 12, 16 and, for protocols of at least 20 h, 20 hours to the start of the fast. With dinner finished at 8 pm, the stages fall at midnight, 4 am, 8 am and noon. These are fixed benchmarks, not a measurement of your metabolism.

What is the difference between OMAD 23/1 and the 24-hour fast?

On 23/1 the tool plans a one-hour eating window each day: with a last meal finished at 7:30 pm, the next meal takes place from 6:30 pm to 7:30 pm. 24/0 has no eating window: the fast runs from one dinner to the next day's dinner, and the tool's description limits it to once or twice a week.

Is intermittent fasting officially recommended?

In France, ANSES has not assessed intermittent fasting: its opinion of 6 March 2024 covers overnight fasting, for which it finds the data insufficient to conclude on a link with the risk of chronic disease. As a precaution, it advises eating less in the evening and having dinner about two hours before bedtime. Ask a health professional before starting.

How do I plan a week on 5:2 or alternate-day fasting?

Choose 5:2 or Alternate-day, then the first fast day. On 5:2 starting on Monday, the tool puts the restricted days on Monday and Thursday; the other five days are normal. In the trial by Harvie et al. (2011), those days provided about 650 kcal. On alternate-day, the schedule covers 14 days: starting on Monday, you fast on Monday, Wednesday, Friday and Sunday in the first week, then Tuesday, Thursday and Saturday in the second.

Who should not do intermittent fasting?

Without medical advice, it is not advised during pregnancy and breastfeeding, under 18, with a current or past eating disorder, or with diabetes treated with insulin or a glucose-lowering drug. In the trial by Corley et al. (2018), fasting doubled the rate of hypoglycaemia in people with type 2 diabetes despite lower doses. If a medicine must be taken with a meal, ask your doctor or pharmacist how to adjust the doses.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.