Intermittent Fasting Calculator: Plan Your Eating Window and Meal Times

Intermittent fasting means eating only during set hours and going without food for the rest of the day. The daily version is called time-restricted eating, where you eat inside the same window every day, such as 8 hours, and fast for the other 16. This calculator works out the clock times for you. Choose 12:12, 14:10, 16:8, 18:6 or 20:4, set the time of your first meal or your last, and it shows when your window opens and closes, how long you fast, when to eat your meals and when to train.

This is a schedule tool. It does not count food or promise weight loss, because in the trials the window worked mainly by helping people eat less. Our review of 16:8 intermittent fasting results covers what those trials found at one week, one month, three months and a year.

How to use the intermittent fasting calculator

The calculator needs a schedule and one clock time, and it does the arithmetic below.

Window closes = first meal + eating hoursFasting hours = 24 − eating hoursWorkout = first meal + 2 hours

1. Pick a schedule. The first number is the hours you fast and the second the hours you eat, so 16:8 is a 16-hour fast and an 8-hour window. The two always add up to 24.
2. Set one time. Choose First meal to set when your window opens, or Last meal to set when you stop eating, and the calculator counts back. Finishing by 8:00 pm on 16:8 opens the window at 12:00 pm.
3. Read your day. Your first meal opens the window, and your last meal starts an hour before it closes so you finish in time. A window of 8 hours or more gets a third meal halfway between. A 6-hour or 4-hour window gets two meals.
4. Add calories or protein if you like. Enter a daily total and the calculator splits it evenly across your meals. The TDEE calculator estimates your calories, and the protein calculator sets a protein target.

Times move in 30-minute steps. The bar under the result shows your day on a 24-hour clock, with the eating window in orange and the workout in light blue, and it handles windows that run past midnight for night shifts.

Which intermittent fasting schedule should you start with?

If you have never fasted, start with 12:12 or 14:10 and shorten the window once it feels routine. Most people already eat across more of the day than they think. When Gill and Panda (2015) had 156 adults record every time they ate or drank with a phone app for 3 weeks, the median eating span was 14 hours 45 minutes a day, and fewer than 1 in 10 people kept their eating within 12 hours. For most people, 12:12 means a normal day of meals with late-night snacks cut out.

14:10 has a small trial behind it. In the same study, 8 overweight adults who usually ate across more than 14 hours a day kept to a window of about 10 to 11 hours for 16 weeks. They were not told to change what or how much they ate, yet their calorie intake fell by about a fifth and they lost 3.27 kg (7.2 lb) on average. A year after the start they had kept the weight off, and they said they slept better and had more energy.

Most of the time-restricted eating trials on this page used 16:8. The TREAT trial (Lowe et al. 2020) and a 12-month trial by Lin and colleagues (2023) both used noon to 8 pm, which mostly means skipping breakfast. In TREAT, people reported keeping to the window on 83.5 percent of days over 12 weeks. The weight-loss section below covers both trials.

Shorter windows leave less room for meals. In a trial by Cienfuegos and colleagues (2020), adults with obesity who ate only from 1 to 7 pm (18:6) or from 3 to 7 pm (20:4) ate about 550 calories a day less without counting, and lost about 3 percent of their body weight in 8 weeks compared with a control group. The 4-hour window did no better than the 6-hour one. On either schedule you get two meals, so each one has to carry about half of your day's food and protein.

Intermittent fasting and workouts

You can train on any of these schedules. The choice is whether to train inside your window, after a meal, or before it opens, on an empty stomach. For strength training, the trials found that both work.

Training inside your eating window

The calculator puts your workout about 2 hours after your first meal, with your next meal after it. Sports nutrition guidelines time the meal before a long event 1 to 4 hours ahead of the start (Burke 2015), and 2 hours sits inside that range. Our guide on how long after eating to work out explains why a full meal needs longer to settle than a snack.

Training inside the window has kept muscle in the trials. In a study by Moro and colleagues (2016), resistance-trained men ate at 1, 4 and 8 pm and lifted between 4 and 6 pm, three times a week for 8 weeks. Their fat mass fell from 10.9 to 9.3 kg (24.0 to 20.5 lb) while their fat-free mass held steady, and their strength did not differ from a group eating at 8 am, 1 pm and 8 pm. Women did as well. In a trial by Tinsley and colleagues (2019), resistance-trained women who ate only from noon to 8 pm gained 2 to 3 percent fat-free mass over 8 weeks of supervised training, the same as women who ate across the day, with protein at 1.6 g per kg of body weight a day in every group.

Training fasted

Training before your window opens works too. In a 2025 trial by Vieira and colleagues, young adults lifted twice a week for 12 weeks, either after an overnight fast or 1 to 2 hours after a carbohydrate-rich meal. Both groups gained the same muscle thickness, strength and power. If you prefer to train fasted, do it in the hour before your window opens and make your first meal the one after the session. The calculator shows that time under the workout.

Long cardio sessions are the exception. In a review of 46 studies, Aird and colleagues (2018) found that eating beforehand improved performance in prolonged aerobic exercise, but not in shorter sessions. So put long runs and rides inside your window, after a meal. Training fasted does not burn more body fat over weeks either, as our review of fasted cardio research explains.

Protein inside a short window

Your total protein for the day matters more than its timing. A meta-analysis by Schoenfeld, Aragon and Krieger (2013) found that once total protein intake was accounted for, eating protein close to a workout made no significant difference to gains in muscle or strength. So inside your window, aim to reach your daily total. The protein calculator sets one, and this calculator divides it across your meals.

Shorter windows mean bigger servings. On 18:6 or 20:4 you eat two meals, so 120 g of protein a day becomes 60 g a meal. A large serving is not wasted. In a 2023 study by Trommelen and colleagues, 100 g of protein after exercise produced a larger muscle-building response than 25 g, and it lasted more than 12 hours.

Does intermittent fasting work for weight loss?

It works about as well as eating less in any other way, mostly because it helps you eat less. Four lines of evidence point the same way.

The window on its own. In the TREAT trial, Lowe and colleagues randomized 116 adults with overweight or obesity to eat only between noon and 8 pm or to eat three structured meals a day, with no calorie targets. After 12 weeks the window group had lost 0.94 kg (2.1 lb) and the meal group 0.68 kg (1.5 lb), and the difference of 0.26 kg (0.6 lb) was not statistically significant. The two groups ate about the same number of calories, and the researchers concluded that time-restricted eating on its own was no more effective for weight loss than eating throughout the day.

The window on top of a calorie target. Liu and colleagues (2022) gave 139 adults with obesity the same daily calorie target, 1,500 to 1,800 for men and 1,200 to 1,500 for women, and asked half of them to also eat only between 8 am and 4 pm. After 12 months the window group had lost 8.0 kg (17.6 lb) and the calorie-only group 6.3 kg (13.9 lb), a difference that was not statistically significant.

The window without counting. In Lin's 12-month trial, 90 adults with obesity ate between noon and 8 pm without counting calories, cut their calories by 25 percent every day, or kept eating over 10 or more hours a day as a control group. The window group ate about 425 calories a day less and the calorie-cutting group about 405. Compared with the control group, the window group lost 4.61 kg (10.2 lb) and the calorie-cutting group 5.42 kg (11.9 lb), with no significant difference between the two.

All the trials together. A 2025 network meta-analysis in The BMJ by Semnani-Azad and colleagues pooled 99 randomized trials with 6,582 adults. Every fasting approach and ordinary calorie cutting reduced weight compared with eating freely, and the authors concluded that intermittent fasting has similar benefits to cutting calories every day.

So pick a schedule because it makes eating less easier for you, not because the hours themselves burn fat. If you want a calorie number to aim for inside your window, the TDEE calculator estimates one, and our page on calorie deficits explains how the gap between what you eat and what you burn drives weight loss.

5:2, alternate-day fasting and longer fasts

Not every fasting plan uses a daily window. On the 5:2 diet you eat normally five days a week and very little on the other two. In a 6-month trial, Harvie and colleagues (2011) compared two days a week at about 2,710 kJ (roughly 650 kcal) with a smaller cut every day, the same 25 percent over the week, in 107 overweight or obese women before menopause. They lost 6.4 kg (14.1 lb) and 5.6 kg (12.3 lb), and the difference was not significant.

Alternate-day fasting swaps fast days and normal days. Trepanowski and colleagues (2017) randomized 100 adults with obesity to eat 25 percent of their energy needs on fast days and 125 percent on the days between, or 75 percent every day, for a year. Weight loss was similar, 6.0 and 5.3 percent compared with a control group, but more people dropped out of the fasting group, 38 percent against 29 percent. In the 2025 BMJ meta-analysis, alternate-day fasting was the only approach that beat daily calorie cutting, by 1.29 kg (2.8 lb), and only in trials shorter than 24 weeks.

One meal a day (OMAD) works out to 23:1, beyond what this calculator plans. In a crossover trial by Stote and colleagues (2007), healthy adults ate the same calories as one meal or as three meals a day for 8 weeks each. On one meal a day they lost a little fat mass, but they were hungrier, and their blood pressure and total and LDL cholesterol rose.

Fasts of 24, 36 or 72 hours, and water-only fasts, are a different thing, and this calculator does not plan them. Do them only under medical supervision. Even in a supervised clinic, a review of 768 water-only fasts of 2 days or longer by Finnell and colleagues (2018) found a severe side effect in 212 of them and a life-threatening one in another, although most side effects were mild.

How accurate is this intermittent fasting calculator?

The clock arithmetic is exact, so a 16:8 window that opens at 12:00 pm always closes at 8:00 pm. The meal and workout times are starting points rather than rules. What the calculator cannot tell you is how much less you will eat inside the window, or how much weight you will lose, and the trials show how widely both vary.

How much less people eat

In Lin's trial, the window cut intake by an average of 425 calories a day, with a standard deviation of 531 calories. A spread larger than the average means people varied a lot. Some cut far more than 425 calories, and some cut little or nothing. In TREAT, the window group ate about the same as the regular-meal group.

How much weight people lose

Weight loss spreads widely too. A review of 27 fasting trials by Welton and colleagues (2020) found losses from 0.8 to 13.0 percent of starting weight, and only 2 of the trials lasted a year. In TREAT, the 95 percent confidence interval for the window group's 12-week loss ran from 0.20 to 1.68 kg (0.4 to 3.7 lb).

How well people stick with it

In TREAT, people reported keeping to the noon-to-8 pm window on 83.5 percent of days. Welton's review found dropout rates of 25 percent or more in several fasting studies, though fasting and calorie cutting had similar dropout when trials compared them directly. People occasionally reported dizziness, headache, weakness, sleep problems or constipation, and none of the 27 trials reported a serious adverse event.

Treat the times as a first plan, then adjust them to your hunger, your sleep and your work hours.

Worked examples: seven fasting schedules through the calculator

Each row runs the calculator once. The workout column gives the time inside the window first, with the fasted option in brackets. The last column splits 2,000 kcal and 120 g of protein a day evenly across the meals.

Two patterns stand out. Shorter windows mean bigger meals. At 2,000 kcal and 120 g of protein a day, each meal on 18:6 or 20:4 carries 1,000 kcal and 60 g of protein, against about 670 kcal and 40 g on three meals. And the window can sit anywhere in the day. A night-shift window from 8:00 pm to 4:00 am follows the same arithmetic as noon to 8 pm.

When to ignore this calculator

This calculator plans daily eating windows for healthy adults. Talk to a clinician before you fast at all if any of these apply to you.

• Diabetes, or medicine that lowers blood sugar. Going hours without food on insulin or similar medicine can drop your blood sugar too low. In a trial of adults with type 2 diabetes on these medicines, fasting two days a week roughly doubled the rate of low blood sugar episodes, even after their doses were reduced (Corley et al. 2018). Plan any fasting with your diabetes care team.
• Pregnancy or breastfeeding. Do not start fasting. Ask your obstetric care team about when and how much to eat.
• An eating disorder, now or in the past. Strict rules about when you may eat can be harmful. Work with your treatment team.
• Under 18. The trials on this page tested adults. Teenagers should keep regular meals unless their doctor says otherwise.
• Underweight, or losing weight without trying. Eating less is not the goal, and weight loss you cannot explain needs a doctor.
• Gout or kidney disease. Check with your doctor before you start.
• Medicine that must be taken with food. Keep the timing your doctor or pharmacist gave you and build the window around it.

If you feel faint, shaky or unwell during a fast, stop and eat. A longer window you can keep is better than a shorter one that makes you feel ill.

Related reading

References

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3. Lin S, Cienfuegos S, Ezpeleta M, et al. "Time-restricted eating without calorie counting for weight loss in a racially diverse population: a randomized controlled trial." Ann Intern Med. 2023;176(7):885-895. doi:10.7326/M23-0052, PMC11192144
4. Cienfuegos S, Gabel K, Kalam F, et al. "Effects of 4- and 6-h time-restricted feeding on weight and cardiometabolic health: a randomized controlled trial in adults with obesity." Cell Metab. 2020;32(3):366-378.e3. doi:10.1016/j.cmet.2020.06.018, PMC9407646
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6. Moro T, Tinsley G, Bianco A, et al. "Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males." J Transl Med. 2016;14(1):290. doi:10.1186/s12967-016-1044-0, PMC5064803
7. Tinsley GM, Moore ML, Graybeal AJ, et al. "Time-restricted feeding plus resistance training in active females: a randomized trial." Am J Clin Nutr. 2019;110(3):628-640. doi:10.1093/ajcn/nqz126, PMC6735806
8. Vieira AF, Blanco-Rambo E, Bandeira-Guimarães M, et al. "Impact of overnight fasted state versus fed state on adaptations to resistance training: a randomized clinical trial." Int J Sport Nutr Exerc Metab. 2025;35(4):291-302. doi:10.1123/ijsnem.2024-0215, PubMed 40335157
9. Aird TP, Davies RW, Carson BP. "Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: a systematic review and meta-analysis." Scand J Med Sci Sports. 2018;28(5):1476-1493. doi:10.1111/sms.13054, PubMed 29315892
10. Burke LM. "Re-examining high-fat diets for sports performance: did we call the 'nail in the coffin' too soon?" Sports Med. 2015;45(Suppl 1):S33-S49. doi:10.1007/s40279-015-0393-9, PMC4672014 Table 1 gives the pre-event meal: 1 to 4 g/kg of carbohydrate 1 to 4 hours before events usually longer than 45 minutes.
11. Schoenfeld BJ, Aragon AA, Krieger JW. "The effect of protein timing on muscle strength and hypertrophy: a meta-analysis." J Int Soc Sports Nutr. 2013;10(1):53. doi:10.1186/1550-2783-10-53, PMC3879660
12. Trommelen J, van Lieshout GAA, Nyakayiru J, et al. "The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans." Cell Rep Med. 2023;4(12):101324. doi:10.1016/j.xcrm.2023.101324, PMC10772463
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How this calculator differs from typical intermittent fasting calculators

Most online intermittent fasting calculators add the eating hours to a start time and stop there. Few explain what the trials found, that an eating window without eating less gave little or no extra weight loss and that results followed how much less people ate.

This calculator plans the whole day. It sets the window from your first meal or counts back from your last, places two or three meals, suggests a workout time with a fasted option, and splits your calories and protein evenly across the meals. It handles windows that cross midnight, names the trial behind each schedule, and reports in the accuracy section how widely intake and weight loss varied. It plans daily schedules only and leaves 24 to 72 hour fasts and water fasts to medical supervision.