Intermittent Fasting Calculator
Pick a protocol — 16:8, 18:6, 20:4, OMAD, or 5:2 — tell us when you wake up and where you want your eating window, and you’ll get the exact hours you eat, the hours you fast, and when to put your meals.
Your Schedule
Your Fasting Schedule
Your fast runs until 12:00 PM the following day.
- Meal 1 of 212:00 PM
- Meal 2 of 27:00 PM
Meal times are a starting point, not a rule. Move them to fit your training and your appetite — the window is what matters.
Which IF protocol is right for you?
All five protocols above do the same thing: they use time as a constraint on how much you eat. None of them has a metabolic edge over the others at matched calorie intake. So the question isn’t which one burns the most fat — it’s which one you can run on a Tuesday in February without thinking about it.
- 16:8 — start here.Eight hours is wide enough to eat lunch with colleagues and dinner with your family, which is why it’s the protocol most people are still doing a year later. Skipping breakfast does most of the work.
- 18:6 — when 16:8 stops biting.Six hours trims one snack’s worth of opportunity off the day. It suits people who find the last two hours of an 8-hour window are where the extra calories creep in.
- 20:4 — tight, and it shows.Four hours is enough for two real meals if you plan them. The main cost is protein: getting a full day’s worth into that window takes deliberate effort rather than good intentions.
- OMAD — the simplest to describe, the hardest to fuel.One meal removes every decision except one. But a full day of calories in one sitting defeats a lot of people, and a single eating window can normalize very large portions — a poor fit for anyone with a history of binge eating.
- 5:2 — for people who hate daily restriction. Nothing is off-limits five days a week. In exchange, two days are genuinely hard. It moves the difficulty rather than reducing it, and some people much prefer it that way.
One honest note before you pick. The evidence comparing intermittent fasting to a plain daily deficit, at the same total intake, keeps landing in the same place: similar fat loss. Fasting earns its keep only if the closed window makes eating less feel automatic. If it makes you ravenous and you eat back the difference the moment the window opens, a flat daily deficit will serve you better. Track two weeks and let the scale settle the argument.
Calorie targets per protocol
This calculator sets your schedule. It deliberately doesn’t set your calories, because a window doesn’t have a calorie target — you do, and it comes from your bodyweight, height, age, and activity. Compressing eight hours into four changes when the food arrives, not how much of it you need.
| Protocol | Shape of the day | How the target is set | Run your numbers |
|---|---|---|---|
| 16:8 | 8-hour daily window, usually two meals | A standard daily deficit against your TDEE | 16:8 macro calculator |
| 18:6 | 6-hour daily window, two meals | Same daily deficit — the window is tighter, the target isn't | 16:8 macro calculator |
| 20:4 | 4-hour daily window, one or two meals | Same daily deficit, harder to hit protein inside it | OMAD calculator |
| OMAD | One meal, roughly a 1-hour window | The full daily target in a single sitting | OMAD calculator |
| 5:2 | No daily window; 2 low-calorie days per week | 500–600 cal on fast days, maintenance on the other five | 5:2 calculator |
The pattern in that third column is the point: for every daily-window protocol, the calorie target is identical. Only 5:2 changes the arithmetic, because it works on a weekly average rather than a daily one. If you want the maintenance number underneath all of them first, the TDEE calculator is the place to start.
Protein deserves one line of its own. The shorter the window, the harder it is to hit a protein target, and protein is what protects muscle while you’re in a deficit. If a 4-hour window or OMAD leaves you well short, that’s a reason to widen the window — not a reason to accept less protein.
What breaks a fast?
Calories break a fast. That’s the whole rule, and almost every argument on the internet is a footnote to it. What matters practically is whether something adds meaningful energy or provokes a meaningful insulin response.
- Doesn’t break a fast: water, black coffee, plain tea, unsweetened sparkling water. Negligible calories, no meaningful insulin response.
- Breaks a fast:anything with real calories — cream or milk in coffee, sugar, juice, protein shakes, bone broth, and BCAA or EAA supplements. Amino acids carry calories and are precisely the signal a fast is meant to withhold.
- The gray zone:zero-calorie sweeteners, diet soda, and sugar-free gum. The research on whether sweet taste alone triggers a meaningful insulin response is mixed and mostly points to “not much.” For fat-loss purposes they cost you nothing; whether they help or hurt your appetite is individual enough that it’s worth testing on yourself. We looked at the evidence in is diet soda bad for weight loss?
Keep the stakes in proportion. If you’re fasting to make a calorie deficit easier to hold — which is what most people are doing — then a splash of milk hasn’t ruined anything. It has added fifteen calories. The people for whom fast purity genuinely matters are chasing outcomes other than fat loss, and that’s a different page.
Frequently asked questions
What is the best intermittent fasting schedule?
The one you can hold for months without white-knuckling it. Window length is not what drives fat loss — calorie intake is. 16:8 is the usual starting point because an 8-hour window survives contact with a normal social life. Tighter protocols (20:4, OMAD) don't burn more fat at the same intake; they just concentrate the same calories into less time.
When should my eating window start?
Anchor it to when you're actually hungry and when you train, not to a number on a clock. Most people run a late window — skip breakfast, first meal around midday, window closed a few hours before bed — because morning hunger is usually the easiest to ride out. If you train hard in the morning, an early window is the more sensible fit.
Does intermittent fasting burn more fat than a normal calorie deficit?
No. When controlled trials match total calorie intake between intermittent fasting and continuous daily restriction, fat loss comes out roughly the same. Fasting is a behavioral tool: it helps if compressing your window means you eat less overall, and it does nothing if you eat the same calories in fewer hours.
What breaks a fast?
Calories break a fast. Water, black coffee, plain tea, and unsweetened sparkling water don't — the calorie load is negligible and there's no meaningful insulin response. Cream, milk, sugar, protein shakes, BCAAs, and bone broth all do. Zero-calorie sweeteners sit in a gray zone, but for fat-loss purposes they cost you nothing.
Can I train while fasted?
Plenty of people do, and for easy or moderate sessions it's usually a non-issue. Long or high-intensity training is where fasted performance tends to suffer, and hitting a protein target gets harder when the window closes early. If you lift hard, placing at least one meal after training is the simpler arrangement.
How long does it take to adjust to intermittent fasting?
Hunger during the closed window typically settles over the first couple of weeks as meal timing habits shift. If it doesn't — if you're lightheaded, sleeping badly, or bingeing the moment the window opens — that's a signal the protocol is too aggressive for you, not a hurdle to push through.
Intermittent fasting is a scheduling tool, not a treatment, and nothing here is medical advice. If you’re pregnant or breastfeeding, take medication that has to be timed with food, manage diabetes, or have a history of disordered eating, the decision to skip meals belongs with a clinician rather than a calculator.
Related tools
- 16:8 Fasting Macro Calculator — calories and macros split across a two-meal window.
- OMAD Calorie Calculator — the whole day in one sitting.
- 5:2 Diet Calorie Calculator — fast days, normal days, weekly average.
- TDEE Calculator— the maintenance number every protocol above is measured against.
- PE Diet with intermittent fasting — how protein interacts with a compressed eating window.
- Alternate-day fasting results — what happens when the fasting days get more frequent.