FitDiaryIntermittent fasting calculator
Choose a fasting plan and the time of your first meal to see your eating window, when your fast starts and how many calories to eat at each meal.
Your fasting plan
Your eating window opens at this time.
Your eating window
Eat between
12:00 and 20:00
Then fast for 16 hours, from 20:00 until 12:00 the next day.
Your meals
- Meal 1: 12:00
- Meal 2: 19:00
Add your age, height and weight below to see calories per meal.
For healthy adults. Don’t fast if you’re pregnant or breastfeeding or have had an eating disorder. If you take insulin or other diabetes medicines, talk to your doctor first. Who shouldn’t fast
What is intermittent fasting?
Intermittent fasting means eating only within set hours, or on set days, and fasting the rest of the time. Common plans are 16:8 (16 hours of fasting and an 8-hour eating window), 18:6, 20:4, one meal a day (OMAD) and 5:2, where two days a week are very low in calories.
The calculator above turns your plan into clock times and spaces your meals across the window. Add your details to see calories per meal.
Intermittent fasting schedules
Here are the time-restricted plans, using a first meal at 12:00 as an example.
| Plan | Fasting | Eating window | Example |
|---|---|---|---|
| 16:8 | 16 hours | 8 hours | 12:00 to 20:00 |
| 18:6 | 18 hours | 6 hours | 12:00 to 18:00 |
| 20:4 | 20 hours | 4 hours | 12:00 to 16:00 |
| OMAD (one meal a day) | 23 hours | 1 hour | 12:00 to 13:00 |
5:2 works by days instead of hours: you eat normally on five days and keep two days very low in calories, about 600 kcal in the plan described by Know Diabetes. A 2022 review in Nature Reviews Endocrinology lists 5:2, alternate-day fasting and time-restricted eating as the most studied forms.
The calculator spreads meals from the moment your window opens to an hour before it closes, so the last meal isn’t rushed.
What should my fasting hours be?
Choose a window that fits your day, because the best plan is one you can keep up. Most people sleep through a large part of the fast, so the hours that matter are the ones before your first meal and after your last.
Two windows have been tested in well-known trials. In the TREAT trial people ate between 12:00 and 20:00, which in practice means skipping breakfast. In a 12-month trial published in 2022 they ate between 8:00 and 16:00, which means an early dinner.
If you train in the morning or get hungry early, an earlier window may suit you better. If you often eat with family or friends in the evening, a later one usually works better.
Does intermittent fasting help you lose weight?
On its own, the eating window made little difference in two well-known trials. In the 2022 trial in the New England Journal of Medicine, 139 adults with obesity all ate a reduced-calorie diet for 12 months, and about half of them also ate only between 8:00 and 16:00. Those in the time-restricted group lost 8.0 kg on average and the others 6.3 kg, a difference the researchers found was not significant.
In the 12-week TREAT trial (116 adults), eating between 12:00 and 20:00 with no other changes did not lead to more weight loss than eating three meals through the day.
If a fasting window helps you eat less, it can still be a useful way to structure your day. To set a daily calorie target, use the TDEE calculator or the calorie deficit calculator.
Worked example
Take a 30-year-old woman who is 165 cm (5 ft 5 in) tall and weighs 80 kg (176 lb). She does light exercise 1–3 days a week and wants to lose weight on 16:8, with her first meal at 12:00:
- Her eating window is 12:00 to 20:00, and she fasts from 20:00 until 12:00 the next day.
- Her maintenance level is 2,090 kcal a day, so her target to lose about 0.5 kg a week is 1,590 kcal.
- With 2 meals, she eats at 12:00 and 19:00, about 795 kcal each.
- With 3 meals, she eats at 12:00, 15:30 and 19:00, about 530 kcal each.
On 5:2 instead, she would eat about 2,090 kcal on five days and about 600 kcal on two, an average of about 1,664 kcal a day. See how these numbers are worked out on the TDEE calculator.
Who shouldn’t try intermittent fasting
Fasting isn’t safe for everyone. Don’t try it if you:
- are pregnant or breastfeeding
- have had an eating disorder, such as anorexia or bulimia
If you take insulin or diabetes tablets that can cause hypos (low blood sugar), such as sulphonylureas, talk to your doctor or diabetes team before you start.
Source: Know Diabetes, which advises that people with a history of eating disorders and pregnant or breastfeeding women should not attempt intermittent fasting, and warns that fasting on insulin or some diabetes medicines could significantly increase the risk of hypos.
This calculator is for adults. If you have a medical condition or take regular medication, check with your doctor before changing when or how much you eat.
See what’s left for today before your window closes
Type what you ate in one sentence and FitDiary shows how much of today’s target is left, so you can plan the last meal of your window.
See how the FitDiary calorie counter works.
Frequently asked questions
What should my hours be for intermittent fasting?
Pick a window that fits your day. For 16:8, common choices are 12:00 to 20:00 (skip breakfast) or 8:00 to 16:00 (early dinner). Both have been tested in trials, and in neither did the window lead to significantly more weight loss than the comparison group.
How many meals should I eat on 16:8?
Two or three meals fit comfortably in an 8-hour window. The calculator splits your daily target evenly: with 1,590 kcal a day, that’s about 795 kcal for each of two meals or 530 kcal for each of three.
Can I lose 20 pounds in a month with intermittent fasting?
That would mean about 5 lb a week, far more than the 1 to 2 lb a week the CDC suggests aiming for. Fasting doesn’t change that, because weight loss depends on your total calories, whatever hours you eat them in.
Can I drink coffee while fasting?
Most fasting plans allow drinks with no calories, such as water, black coffee and plain tea. Milk, sugar and juice add calories, so they belong in your eating window.
Is 16:8 or 18:6 better?
The trials on this page didn’t compare them. Some people find a shorter window makes it easier to eat less, but only if they don’t make up for it at their meals. Start with the plan you can keep up, then adjust.
Is intermittent fasting safe if I have diabetes?
If you have diabetes, talk to your diabetes team before you start. Fasting while taking insulin or some diabetes tablets can cause hypos, and your medicines may need to change. Don’t start fasting to treat diabetes on your own.
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