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Intermittent Fasting Calculator
Use this free intermittent fasting calculator to plan your daily fasting schedule. Pick a method such as 16:8 or 18:6, enter the time of your last meal, and the calculator shows your eating window and fasting window for today and tomorrow, plus your total fasting hours for the week.
Leave the first-meal time blank to let the method set your eating window. For 5:2, the two low-calorie days are usually non-consecutive, for example Monday and Thursday.
How it works
Intermittent fasting is a pattern of eating, not a diet. Instead of telling you what to eat, it tells you when to eat: you eat all of your meals inside a short window each day and fast for the rest of the day. The method is written as two numbers, fasting hours first and eating hours second. In the 16:8 method, you fast for 16 hours and eat during an 8-hour window. In 18:6 the eating window shrinks to 6 hours, and in the 20:4 method, sometimes called the warrior diet, you eat during a single 4-hour window. OMAD, short for one meal a day, is the 23:1 method: one meal, then 23 hours of fasting.
This calculator works backwards from your last meal, because that is the anchor most people actually control. If your last meal ends at 8:00 PM and you follow 16:8, your eating window is 8 hours long, so it started at 12:00 PM. Your fast then runs from 8:00 PM until 12:00 PM the next day, which is 16 hours. The same arithmetic works for every daily method: eating window start = last meal time − eating hours. If you prefer to set your first meal instead, enter it in the optional field and the calculator will work the other way around.
The 5:2 method works differently, so it needs a different schedule. Instead of fasting every day, you eat normally for five days a week and eat very little — about 500 to 600 calories — on two non-consecutive days, such as Monday and Thursday. On a fast day you still eat, just far less, so there is no single eating window to draw on a clock. The calculator shows the 5:2 plan as two low-calorie days per week, roughly 48 fasting hours in total if you count each fast day as about a 24-hour stretch of very low intake, and it leaves the daily window blank because the method does not use one.
During the fasting window, water, black coffee, and plain tea are generally fine, because they add almost no calories. Anything with calories — milk in coffee, juice, a snack, a sweetened drink — breaks the fast in the practical sense, because it starts digestion. During the eating window there is no special rule in the method itself, but the results people report come mostly from eating a normal, balanced amount of food rather than using the window as a licence to eat twice as much.
Most people start with 14:10 or 16:8 because the fast is mostly overnight. If you finish dinner at 8:00 PM and sleep from 11:00 PM to 7:00 AM, eight of your sixteen fasting hours pass while you are asleep, and you only need to wait until noon to eat. That is why the 16:8 schedule in the worked example below feels manageable: lunch at noon, dinner at 8:00 PM, and nothing but water, coffee, or tea in between. Longer fasts such as 20:4 and OMAD are harder and suit people who have already spent a few weeks on 16:8. Ease in over one to two weeks rather than jumping straight to the longest fast.
Weekly fasting hours are the daily fast multiplied by the number of days you follow the method. On 16:8 every day that is 16 × 7 = 112 hours per week, though about 56 of those hours are sleep. Follow it five days a week and the total is 16 × 5 = 80 hours. The calculator lets you set the days per week so the weekly total matches your real plan.
Worked example
A person follows the 16:8 method and finishes their last meal at 8:00 PM.
Step 1: The eating window is 8 hours long, so it started 8 hours before the last meal — 8:00 PM minus 8 hours = 12:00 PM.
Step 2: The eating window is therefore 12:00 PM to 8:00 PM.
Step 3: The fasting window runs from the last meal until the eating window opens again — 8:00 PM to 12:00 PM the next day, which is 16 hours.
Step 4: Over a full week at 7 days, the total fasting time is 16 × 7 = 112 hours.
So the answer is: eat between 12:00 PM and 8:00 PM, fast for 16 hours overnight, and repeat. If the same person switched to 18:6 with the same 8:00 PM last meal, the eating window would become 2:00 PM to 8:00 PM and the fast would stretch to 18 hours, or 126 hours per week.
FAQ
What is the best intermittent fasting method for beginners?
Most beginners do best with 14:10 or 16:8. Both keep the fast short enough that most of it passes overnight, and both leave a wide enough eating window for two or three normal meals. Start with 14:10 for a week or two, move to 16:8 when it feels easy, and only try 18:6, 20:4, or OMAD after a few weeks on 16:8.
Can I drink coffee while fasting?
Black coffee, plain tea, and water are fine during the fast because they add almost no calories. Milk, cream, sugar, honey, and sweetened drinks do break the fast in the practical sense, because they add calories and trigger digestion. If you need something in your coffee, keep it to a very small splash and count it as part of your eating window.
Does intermittent fasting help with weight loss?
It can, mainly because a shorter eating window makes it easier to eat fewer calories without counting every bite. Two meals in eight hours are usually less food than three meals plus snacks spread over sixteen hours. The method itself does not burn extra fat — the calorie total still matters — but many people find the structure easier to stick to than a strict diet.
What should I eat during my eating window?
Eat normally and aim for a balanced plate: protein, vegetables, whole grains, and healthy fats, with plenty of water. The eating window is not a free pass to eat twice as much, and a fast followed by a very large, low-quality meal usually leaves people feeling worse, not better. If you have a weight goal, keep an eye on your total calories across the week.
Is intermittent fasting safe for everyone?
No. It is not suitable for children, teenagers, people who are pregnant or breastfeeding, people with a history of eating disorders, or people with certain medical conditions such as type 1 diabetes. If you take medication that must be taken with food, or you have any health condition, talk to a doctor before starting. Stop and seek advice if you feel dizzy, faint, or unwell while fasting.