Intermittent fasting calculator.
Pick a protocol like 16:8, 18:6, 20:4 or OMAD and one meal time, and this tool gives you your exact eating and fasting windows as clock times - plus a live readout of whether you are eating or fasting right now.
Build your fasting schedule
your schedule
Eating window
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Fasting window
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Where you are right now
a different kind of fast
5:2 is calorie-based, not clock-based. You eat normally on 5 days and cut to about 500 to 600 calories on 2 non-consecutive days, so there is no eating window to set - the lever is the calorie total on your two low days. Size those days with the calorie deficit calculator, and check your everyday intake with the maintenance calorie calculator.
Windows are shown in 24-hour clock time and the live readout uses your device clock. This is general information, not medical advice.
the short answer
An intermittent fasting calculator turns a protocol and one meal time into your exact eating window and fasting window as clock times. Choose 16:8, 18:6, 20:4 or OMAD, tell it when your first or last meal is, and it works out the rest. A 16:8 schedule means 16 hours fasting and an 8 hour eating window, so a last meal at 20:00 gives you an eating window of 12:00 to 20:00 and a fast from 20:00 to 12:00 the next day.
What is intermittent fasting?
Intermittent fasting is not a diet in the sense of what you eat. It is a schedule for when you eat. The more precise name is time-restricted eating: you group all of your food into a set window each day and take only water, black coffee, or plain tea outside it. Nothing on the plate is off limits by default, and no food is added. All that changes is the clock.
The windows are written as a ratio of fasting hours to eating hours. A 16:8 schedule means 16 hours fasting and an 8 hour eating window. Push the fast longer and the eating window shrinks: 18:6, then 20:4, and at the far end one meal a day, or OMAD, compresses all your food into roughly a single hour. 5:2 works differently again, which the calculator flags rather than trying to draw a clock for.
People reach for intermittent fasting because it is simple. There are no points to log and no foods to ban, just a start time and a stop time. For some people, closing the kitchen after dinner and skipping breakfast is a far easier rule to follow than counting every calorie - and that simplicity, not any special property of fasting itself, is where the results tend to come from.
How does each fasting protocol work?
16:8 is the default entry point. Sixteen hours of fasting and an eight hour eating window usually means skipping breakfast and eating from around midday to 20:00. Most people who try intermittent fasting pick this one first because it barely disrupts a normal day and is easy to hold long term.
18:6 tightens the window to six hours. It suits people who are genuinely not hungry in the morning and would rather have two solid meals than three rushed ones. With one fewer eating hour, getting enough protein at each meal takes a little more planning.
20:4, sometimes called the Warrior style, gives a four hour window, usually one large meal and one smaller one close together. It appeals to people who like structure and a clear finish line, but the short window makes it genuinely hard to eat enough protein and enough real food, so it rewards planning.
OMAD, one meal a day, is the strictest clock-based version - roughly a 23 hour fast with a single sitting. Some people love the freedom of one big meal, but most cannot fit adequate protein or food quality into one plate, so it is better treated as an occasional tool than a daily default.
5:2 is the odd one out. It is not clock-based at all: you eat normally on five days and cut to about 500 to 600 calories on two non-consecutive days. Because the lever is a calorie total rather than an eating window, this calculator gives 5:2 an explanatory note and points you to the calorie deficit calculator for the number that actually matters on those two days.
Does intermittent fasting actually work?
For weight loss, intermittent fasting works mainly for one reason: a shorter eating window makes it easier to eat less without counting. Cut off snacking after dinner and remove breakfast, and a lot of people quietly drop a few hundred calories a day. That is a calorie deficit, and a calorie deficit is what drives fat loss. The window is the tool that makes the deficit easier to reach for some people, not a separate mechanism.
This is the part worth being clear about. Skipping breakfast is not metabolic magic, and the eating window itself does not melt fat while you sit still. When researchers match calories between a fasting group and a normal three-meals group, the difference in fat loss tends to shrink or disappear. So the useful question is not how many hours you fast, but whether the window helps you hold a sensible deficit you could not hold otherwise. If it does, it is a great fit. If you just eat the same calories in a tighter window, the scale will not move.
That is why the fasting window and the calorie side belong together. Use this tool to set the schedule, then use the calorie deficit calculator to find the intake that actually creates the deficit, and the maintenance calorie calculator to see where you are starting from. If you have a target weight and a date in mind, the goal weight date calculator turns that deficit into a realistic timeline.
How do you make a fasting window fit your day?
Anchor the window to real life. If dinner with family is non-negotiable at 19:00, set that as your last meal and let the calculator work backward - that is exactly what the last-meal anchor above is for. If you are a morning person who cannot skip breakfast, anchor by your first meal instead and let the window run forward. The best window is the one that fits the meals you were never going to move anyway.
During the fast, water, black coffee, and plain tea are your friends and keep hunger manageable. What breaks the point of the fast is calories, so watch the add-ons: milk, cream, sugar, and syrups turn a coffee into a small meal. Training in a fasted window suits plenty of people for easy sessions, but if a hard workout leaves you flat, schedule it near the start of your eating window so you can eat around it.
Ease in rather than jumping straight to a four hour window. Start at 12:8 or 14:10, push the fast a little wider once it feels normal, and stop where it stays comfortable. And protect protein: a tighter window leaves fewer meals to reach your daily target, so make each one count. The protein per meal calculator and the calories per meal calculator split your daily totals across however many meals your window allows.
How do coaches manage clients who prefer IF?
When a client already likes intermittent fasting, a good coach works with it rather than against it. The schedule is a preference, and forcing a client onto three meals they will not eat is a fast way to lose adherence. So the coach fits the plan to the window the client actually keeps: the same calorie and protein targets, delivered across two or three meals inside their eating hours instead of spread through the day.
The real coaching work is the same as always - set a sensible calorie and protein target, then check the weekly data and adjust. Protein timing gets a little more attention with a short window, because fewer meals means each one has to carry more, and the protein per meal calculator makes that split concrete. The coach also watches for the trap of a client using the window as permission to overeat, and for the client who is genuinely under-eating because the window is too tight to hit their numbers.
None of this needs a different plan for every client. It needs a plan built around each client's real schedule, kept current as their preferences and results change - which is exactly the sort of thing a coaching platform is for.
Coach around the schedule your client actually keeps.
A calculator sets one person's window. Coachway lets a coach build meal plans and check-ins around whatever schedule a client keeps, whether that is 16:8, one big evening meal, or three normal meals - the calorie and protein targets stay the same, delivered in a branded client app and tracked automatically. Explore the meal planner or browse the full set of free coaching tools.
No charge today - cancel anytime under Billing. Card required.Intermittent fasting calculator FAQ.
Is 16:8 better than 18:6?
Neither window is better on its own. 16:8 is easier to hold day after day, and adherence is what actually drives results, so most people do best starting there. 18:6 gives a slightly longer fast but leaves one fewer hour to eat, which makes hitting your protein and total calories harder. Pick the widest window you can keep consistently, not the strictest one you can survive for a week.
Can I train fasted?
Many people train fine in the fasted part of their window, especially for easy cardio or a morning session done out of habit. For hard strength work or long, intense sessions, some people feel stronger having eaten first, and protein around training helps recovery. There is no single right answer. Try it, notice how your sessions feel, and schedule the harder workouts near your eating window if fasted training leaves you flat.
Does coffee break a fast?
Plain black coffee, plain tea, and water have essentially no calories and are fine during the fasting window for the purpose most people are after, which is eating less overall. What changes things is what you add. Milk, cream, sugar, sweetened syrups, or a splash of oat milk add calories, and enough of them turns a fast into a small meal. Keep it black, or keep the additions tiny, and the fast holds.
Is intermittent fasting safe for everyone?
No. For most healthy adults a moderate eating window is safe, but several groups should not fast without professional guidance first: people who are pregnant or breastfeeding, anyone with a history of disordered eating, people with diabetes or others on medication that affects blood sugar, and minors. If you take regular medication or have a medical condition, talk to your doctor before changing your eating pattern. This tool is for education, not medical advice.
How many hours should I fast to lose weight?
There is no magic number of hours. Fasting helps weight loss only when it leads you to eat fewer calories across the day, so the useful window is the one that quietly cuts your intake without leaving you so hungry that you overeat later. For most people a 14 to 16 hour fast, meaning an 8 to 10 hour eating window, is a sustainable place to start. Longer fasts are not automatically better and often just get harder to keep.
Do I still need to count calories on intermittent fasting?
Not always, and that is part of the appeal. A shorter eating window naturally trims snacking and late-night eating for a lot of people, which creates a deficit without formal tracking. But the window is a container, not a limit. It is fully possible to overeat in eight hours, so if the scale is not moving, the calories inside the window are the thing to check. The companion calorie deficit calculator gives you the target to aim for.
This calculator and page provide general information for education, not medical, dietary, or clinical advice. Intermittent fasting is not right for everyone. People who are pregnant or breastfeeding, anyone with a history of disordered eating, people with diabetes or on medication that affects blood sugar, and minors should not fast without professional guidance. Consult a qualified professional before making significant changes to how or when you eat, especially if you have a medical condition.
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