Health Calculator
Intermittent Fasting Calculator
Work out exactly when to start and stop eating on your chosen fasting protocol. Pick 16/8, 18/6, 20/4, OMAD or set your own hours, choose when your eating window opens, and get the full daily schedule in 12-hour or 24-hour time.
Intermittent Fasting Calculator
Eating and fasting windows for your chosen protocol
Beginner · The most widely used protocol — 16 hours fasting, 8 hours eating. Often the easiest to sustain, since most of the fast covers sleep.
When you plan to eat your first meal of the day.
Eating window: 12:00 PM – 8:00 PM · 8 hours eating, 16 hours fasting
Results
Choose a protocol and start time, then click Calculate
Understanding Fasting Windows
Time-restricted eating divides each day into two blocks: a window in which you eat, and the rest of the day in which you do not. The notation makes this explicit — 16/8 means sixteen hours fasting and eight hours eating, and the two always add to 24.
That constraint is what makes the arithmetic simple and the cycle self-closing. Once you fix the fasting hours and the start time, everything else follows: the eating window is whatever is left of the day, the fast begins the moment eating stops, and the fast ends precisely when the next day's window opens. There is no drift.
The practical appeal is that most of the fast happens while you sleep. On 16/8 starting at noon, the fast runs 8:00 PM to noon — perhaps eight hours of it asleep, leaving only the morning to get through.
Fasting Window Formulas
1. Eating Window Duration
2. Window Boundaries
3. Why the Cycle Closes
Adding both windows returns you to the starting time exactly:
All times are computed in minutes past midnight and wrapped with a modulo, so a window crossing midnight needs no special handling — 8:00 PM plus eight hours simply becomes 4:00 AM the next day.
Fasting Protocols Compared
Each protocol shown with an eating window starting at noon:
| Protocol | Fast | Eat | Window from noon | Level |
|---|---|---|---|---|
| 16/8 | 16 h | 8 h | 12:00 PM – 8:00 PM | Beginner |
| 18/6 | 18 h | 6 h | 12:00 PM – 6:00 PM | Intermediate |
| 20/4 | 20 h | 4 h | 12:00 PM – 4:00 PM | Advanced |
| OMAD (23/1) | 23 h | 1 h | 12:00 PM – 1:00 PM | Advanced |
| 5:2 | — | — | Weekly pattern, no daily window | Intermediate |
| Custom | you set | 24 − fast | Any daily length | Beginner |
Why 5:2 Works Differently
5:2 appears alongside 16/8 in most lists of fasting methods, which obscures a real difference: it is a weekly pattern, not a daily clock schedule. There is no fasting-hours figure to subtract from 24, so the window formula simply does not apply.
What it actually prescribes is five days of normal eating and two non-consecutive days restricted to roughly 500 calories for women and 600 for men. Monday and Thursday is a common pairing, but any two days that are not back-to-back will do.
Worth noting: the restricted days are not true fasts. You still eat, just around a quarter of typical intake. Michael Mosley, who popularised the approach, later revised the target upward to about 800 calories, finding it more sustainable and similarly effective.
Because 5:2 defines no daily window, selecting it here returns the weekly pattern and calorie targets rather than clock times. The two approaches can be combined — a time window on restricted days — but neither requires the other.
Choosing Your Start Time
The arithmetic is identical whenever you start — only the clock times move. On 16/8:
| Eating Starts | Eating Ends | Fast Ends | Note |
|---|---|---|---|
| 6:00 AM | 2:00 PM | 6:00 AM | Early window |
| 8:00 AM | 4:00 PM | 8:00 AM | Breakfast kept, no dinner |
| 10:00 AM | 6:00 PM | 10:00 AM | Early dinner |
| 12:00 PM | 8:00 PM | 12:00 PM | Most common |
| 2:00 PM | 10:00 PM | 2:00 PM | Late dinner |
| 4:00 PM | 12:00 AM | 4:00 PM | Ends at midnight |
| 8:00 PM | 4:00 AM | 8:00 PM | Crosses midnight |
Note the fast-end column: it always returns to the start time, because the windows total 24 hours. From 4:00 PM onward with an eight-hour window the eating period crosses midnight, which the calculator flags — useful for shift workers, though eating very late can affect sleep for some people.
Benefits of Using the Intermittent Fasting Calculator
Example Calculations
Three protocols worked through step by step:
Example Scenario 1 — 16/8 Method
Protocol 16/8, eating starts at 12:00 PM.
Eating Window = 24 − 16 = 8 hours
Eating End = 12:00 PM + 8 hours = 8:00 PM
Fasting Start = Eating End = 8:00 PM
Fasting End = 8:00 PM + 16 hours = 12:00 PM (next day)
Eating window: 12:00 PM – 8:00 PM
Fasting window: 8:00 PM – 12:00 PM next day
The fast ends exactly when the next eating window opens
Example Scenario 2 — 18/6 Method
Protocol 18/6, eating starts at 1:00 PM.
Eating Window = 24 − 18 = 6 hours
Eating End = 1:00 PM + 6 hours = 7:00 PM
Fasting Start = 7:00 PM
Fasting End = 7:00 PM + 18 hours = 1:00 PM (next day)
Eating window: 1:00 PM – 7:00 PM
Fasting window: 7:00 PM – 1:00 PM next day
Example Scenario 3 — OMAD (23/1)
Protocol OMAD, eating starts at 6:00 PM.
Eating Window = 24 − 23 = 1 hour
Eating End = 6:00 PM + 1 hour = 7:00 PM
Fasting Start = 7:00 PM
Fasting End = 7:00 PM + 23 hours = 6:00 PM (next day)
Eating window: 6:00 PM – 7:00 PM
Fasting window: 7:00 PM – 6:00 PM next day
A one-hour window leaves very little room for a full day of nutrients
A Schedule, Not a Diet
This calculator works out clock times; it says nothing about what or how much to eat within the window, and a fasting schedule does not by itself create a calorie deficit. Longer fasts are not automatically better — the protocol you can actually sustain beats a stricter one you abandon, and 16/8 remains the sensible starting point for most people. Intermittent fasting is not suitable for everyone: it is generally not advised during pregnancy or breastfeeding, for anyone with a history of disordered eating, or for children and adolescents, and it requires medical supervision alongside diabetes medication and certain other prescriptions where meal timing affects dosing. Speak with a healthcare provider before starting.
Frequently Asked Questions
- How do you calculate an intermittent fasting window?
- Subtract the fasting hours from 24 to get the eating window, then add that to your chosen start time. On 16/8 starting at noon, the eating window is 24 − 16 = 8 hours, so eating runs from 12:00 PM to 8:00 PM and the fast runs from 8:00 PM until noon the next day.
- What does 16/8 mean?
- Sixteen hours fasting and eight hours eating within each 24-hour day. It is the most widely used protocol, largely because most of the fasting period falls during sleep. Skipping breakfast and eating from midday to 8:00 PM is the common arrangement.
- Which fasting protocol should I start with?
- 16/8 is the usual starting point, and many people ease into it from a 12-hour overnight fast before extending. Moving to 18/6 or 20/4 later is a matter of preference rather than a required progression — longer windows are not automatically better.
- Why does the fasting window end exactly when eating begins?
- Because the two windows always total 24 hours by construction. If you fast 16 and eat 8, the cycle closes precisely, so the fast always ends on the following day at the same clock time your eating window opened. This holds for every protocol and every start time.
- Can my eating window cross midnight?
- Yes, and the calculator handles it. Starting an 8-hour window at 8:00 PM means eating until 4:00 AM. That is perfectly workable for shift workers or night owls, though eating late can affect sleep quality for some people. The calculator flags when your window crosses midnight.
- Is 5:2 a time-window protocol?
- No, and this is a common source of confusion. 5:2 means five days of normal eating and two non-consecutive days restricted to roughly 500 calories for women and 600 for men. There is no daily fasting-hours figure, so it has no eating or fasting window. This calculator shows it as a weekly pattern instead.
- What is OMAD?
- One Meal A Day, generally written as 23/1 — a single hour of eating and twenty-three hours of fasting. It is among the most demanding protocols, and the short window makes it genuinely difficult to meet a full day of protein and micronutrient needs in one sitting.
- What can I drink while fasting?
- Water, black coffee and plain tea are generally considered acceptable, since they contribute no meaningful calories. Anything with sugar, milk or cream is usually treated as breaking the fast. Opinions differ on artificial sweeteners and zero-calorie drinks.
- Does the eating start time matter?
- The arithmetic is identical whenever you start, so it is largely a matter of what fits your life. Some evidence suggests earlier eating windows suit circadian rhythm better, but adherence matters more than timing — a schedule you can keep beats an optimal one you cannot.
- Is intermittent fasting suitable for everyone?
- No. It is generally not advised during pregnancy or breastfeeding, for anyone with a history of disordered eating, or for children and adolescents. It also needs medical supervision alongside diabetes medication and certain other prescriptions, since meal timing affects dosing. Speak with a healthcare provider before starting.