Intermittent fasting is not about what you eat — it is about when. You fast for a set number of hours, then take your normal meals inside a shorter daily window. This intermittent fasting calculator does the clock math for you: pick a plan, set the time of your first meal, and it shows your exact eating window, your fasting window, and the moment your fast ends.
I want to START eating (first meal) at:
What Is Intermittent Fasting?
Intermittent fasting is an eating schedule, not a diet. It tells you nothing about which foods to eat — it only splits your day into a fasting window, when you take in no calories, and an eating window, when your meals happen. Most people already fast every night while they sleep; intermittent fasting simply stretches that overnight fast a few hours longer, usually by skipping or delaying breakfast.
Why do people bother? Mainly for weight control. A shorter eating window usually means fewer meals and fewer snacks, which makes a calorie deficit easier to hold without counting every bite. Fasting does not burn fat by magic — the deficit does the work, and the window is the tool that helps you keep it. To find the calorie target your window should deliver, start with our maintenance calories calculator, then eat a few hundred calories below it inside your window.
The best window is the one that fits your actual life. If your family eats dinner together at 7 PM, a window that ends at 6 PM will fail by Friday; shift it later instead. The plan you can repeat on your worst week beats the perfect plan you abandon in nine days.
Popular Fasting Plans
Every plan below is written as fasting hours : eating hours. The daily-window plans are the ones the calculator above handles.
| Plan | Fast | Eat | Best for |
|---|---|---|---|
| 12:12 | 12 hours | 12 hours | Total beginners — barely feels like fasting |
| 14:10 | 14 hours | 10 hours | A gentle first step up |
| 16:8 | 16 hours | 8 hours | The most popular plan — sustainable for most people |
| 18:6 | 18 hours | 6 hours | Experienced fasters who want a tighter window |
| 20:4 (Warrior) | 20 hours | 4 hours | Advanced — usually one small meal and one big meal |
| OMAD | About 23 hours | About 1 hour | One meal a day — the hardest to keep nutritionally complete |
One popular variant is not a daily window at all: on 5:2 you eat normally on five days and take two very-low-calorie days (around 500-600 kcal) each week. Because it works by the week rather than by the clock, the calculator above does not cover it — but the same honesty applies: it works when it creates a weekly calorie deficit.
What Breaks a Fast?
The simple rule: calories break a fast. Anything with energy in it — food, milk, sugar, juice, alcohol — ends the fasting state, because your body starts processing it.
What does not break it, for practical purposes: water, black coffee and plain unsweetened tea. They carry zero (or near-zero) calories and do not meaningfully interrupt a fast taken for weight control. This is also why most fasting guides allow them freely — hydration makes fasting far easier.
The grey zone is the splash of milk in coffee, at roughly 10-20 kcal. Strictly, yes, those calories technically break the fast. Practically, for weight goals, 15 kcal of milk will not undo anything — the honest answer is that purists count any calories as the end of the fast, while most people fasting for weight loss treat coffee with a splash as close enough. Decide which kind of faster you are, and be consistent.
Who Should NOT Fast
Fasting is a tool for healthy adults, and it is not for everyone. Do not start intermittent fasting without medical guidance if any of these apply to you:
- You are pregnant or breastfeeding — your calorie and nutrient needs are higher, and this is not the time to restrict eating windows.
- You are under 18 — growing bodies need regular fuel.
- You have a history of eating disorders — scheduled restriction can feed the very patterns you are recovering from. Please skip this tool entirely.
- You take diabetes or blood-pressure medication — fasting changes blood sugar and blood pressure, and doses often need adjusting; talk to your doctor first.
And for everyone: if fasting makes you dizzy, shaky, unwell or obsessed with food, stop and eat. A smaller window is always there to try another week; your health is not negotiable.
First-Week Tips
Start smaller than your ambition. Run 12:12 or 14:10 for three or four days before you touch 16:8 — your hunger hormones need a few days to learn the new schedule, and an easy first week is what turns a plan into a habit.
Anchor the eating window to your social life, not to willpower. If evenings are when you eat with people, make lunch your first meal and keep dinner normal. Put real protein at both ends of the window — a solid first meal and a solid last meal — using your daily target from our protein calculator; protein is what carries you through the last hungry hours.
Expect hunger waves around days 2-4. They arrive at your old meal times, peak for about twenty minutes, and pass — water, black coffee and a short walk get most people through them. By the end of the first week, the wave usually shows up at the new mealtime instead.
Finally, keep the flexibility valve: a birthday dinner or a family breakfast is not a failure. Eat, enjoy it, and restart the window the next day. Plans survive real life by bending, not by being perfect.
Frequently Asked Questions
What is the best intermittent fasting schedule for beginners?
Start with 12:12 or 14:10 for about a week — both are gentle enough to learn on. Once the schedule feels normal, step up to 16:8, the plan most people settle on long-term. There is no prize for starting at the hardest level.
Does coffee break a fast?
Black coffee, no — it has effectively no calories, and neither do water or plain tea. Add milk or sugar and the honest answer changes: a splash of milk is only about 10-20 kcal, but strictly, calories end a fast. For weight-loss purposes most people allow the splash; purists drink it black.
How much weight can I lose with 16:8?
Fasting itself burns nothing extra — the window only helps if it makes you eat less overall. If your 16:8 window creates a deficit of about 500 kcal a day, expect roughly a pound a week, the same sane rate as any other method; our guide to the quickest way to lose weight safely explains the math.
Can I exercise while fasting?
Yes. Plenty of people train in the fasted state and feel fine, especially for walking, easy cardio and moderate lifting. A common pattern is to train near the end of the fast and eat the first meal right after. If hard sessions leave you light-headed, move the workout inside the eating window instead — performance matters more than the perfect schedule.