Intermittent Fasting Calculator
Plan your fasting and eating windows.
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Intermittent fasting is about when you eat, not just what. Choose your fasting method and the time you'd like to start eating, and this calculator maps out your eating and fasting windows so you know exactly when to have your first and last meal.
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Intermittent Fasting — The Quick Answer
The popular 16:8 method means 16 hours of fasting and an 8-hour eating window. If you start eating at noon, your window runs to 8 PM, and you fast from 8 PM until noon the next day.
Shortcut
Eating window = 24 − fasting hours | Last meal = first meal + eating window
Common fasting methods
| Method | Fasting | Eating window | Level |
|---|---|---|---|
| 14:10 | 14 hours | 10 hours | Beginner |
| 16:8 | 16 hours | 8 hours | Most popular |
| 18:6 | 18 hours | 6 hours | Intermediate |
| 20:4 | 20 hours | 4 hours | Advanced (Warrior) |
During the fasting window, water, black coffee, and plain tea are generally fine and won't break the fast for most purposes. The eating window is when all your meals and calories should fit.
How It Works
The Method
Windows around the clock
Formula
Eating window = 24 − fasting hours Eat: start → start + eating window Fast: end of eating → start (next day)
Variables
Fast length
How long you go without eating — 16 hours in the popular 16:8 method.
Feeding period
The hours in which you eat all your meals. 24 minus the fasting hours.
First meal
When your eating window opens. Shifting it changes the whole schedule.
Note: The schedule is a planning tool — the health effects of intermittent fasting depend on overall diet quality and total calories, not just timing.
Worked Example: A 16:8 Schedule Starting at Noon
Mapping the eating and fasting windows
Choose a method
16:8 = 16 hours fasting, 8 hours eating
Set when your eating window opens
Noon
Add the eating-window length
Noon + 8 h → window closes at 8pm
Fast until it reopens
8pm to noon the next day
Reference Guide
| Method | Fasting | Eating window | Level |
|---|---|---|---|
| 14:10 | 14 h | 10 h | Beginner |
| 16:8 | 16 h | 8 h | Most popular |
| 18:6 | 18 h | 6 h | Intermediate |
| 20:4 | 20 h | 4 h | Advanced (Warrior) |
Does It Work, and Who Should Avoid It?
An honest look at intermittent fasting
Restricting your eating window tends to reduce total intake, which is where most of the weight-loss benefit comes from. When calories are matched, fasting generally produces similar results to ordinary calorie restriction — its real advantage is being a simple rule some people find easier to follow.
Best for: People who prefer a time window to counting calories
During the fasting window, calorie-free drinks won't meaningfully break the fast. Anything with calories — milk, sugar, juice, a snack — ends it. The point is simply to keep the eating window closed.
Best for: Knowing what you can have while fasting
Avoid or only try under medical supervision if you are pregnant or breastfeeding, have diabetes or take blood-sugar medication, have a history of disordered eating, are underweight, or are a child or teenager. If you have any health condition, check with a doctor first. This is general information, not medical advice.
Best for: Checking it's appropriate for you before starting
What Is Intermittent Fasting?
Intermittent fasting is unusual among diets because it mostly tells you when to eat, not what. The shorthand describes the split between fasting and eating hours: 16:8 means 16 hours fasting with an 8-hour eating window, 18:6 and 20:4 are stricter versions. This calculator maps your chosen method and start time onto exact windows. The evidence suggests fasting works largely by reducing total calories rather than through unique fat-burning — calorie-for-calorie it performs much like ordinary dieting, with its advantage being a simple, sustainable rule for some people. It isn't suitable for everyone, so the 'who should avoid it' guidance matters as much as the schedule.
Key Features
💡 Pro Tips
- →Pick a window that fits your life — many people skip breakfast and eat noon to 8pm, but an earlier window suits others better.
- →Let the overnight sleep do much of the fasting; it's the easiest stretch of hours to go without food.
- →Don't use the eating window as licence to overeat — fasting only helps if your overall calories support your goal.
- →Ease in with 14:10 before trying longer fasts, and stop if it leaves you light-headed or preoccupied with food.
Common Mistakes
Expecting fasting to beat normal dieting calorie-for-calorie
Research matching calories finds similar results. Fasting's benefit is adherence for some people, not a unique metabolic edge — so if calories creep up in the window, progress stalls.
Breaking the fast without realising
A splash of milk in coffee, juice, or a small snack adds calories and effectively ends the fast. Stick to water, black coffee, or plain tea during the fasting window.
Trying it when it isn't appropriate
For people who are pregnant, diabetic, underweight, or have a history of disordered eating, fasting can be harmful. Medical guidance should come first.
Research & Citations
Sources and standards referenced for the factual claims on this page
- [1]
de Cabo, R. & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 381(26).
Overview of intermittent fasting and its physiological effects.
View source - [2]
Trepanowski, J. F., et al. (2017). Effect of Alternate-Day Fasting on Weight Loss, Weight Maintenance, and Cardioprotection. JAMA Internal Medicine, 177(7).
Trial comparing fasting with daily calorie restriction.
View source
This calculator is a reference tool, not a substitute for professional advice. Verify important health, financial, legal, or safety decisions with a suitably qualified professional.
Last updated: October 8, 2026
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Frequently Asked Questions
What is 16:8 intermittent fasting?
It means fasting for 16 hours and eating within an 8-hour window each day. For example, eating only between noon and 8 PM, then fasting overnight until noon the next day. It's the most popular method because it's easy to fit around sleep.
When should I start my eating window?
Whenever suits your routine — many people skip breakfast and eat from around noon to 8 PM, while others prefer an earlier window. The best time is the one you can stick to consistently without disrupting sleep or social meals.
What can I drink while fasting?
Water, black coffee, and plain tea are generally fine and keep you hydrated without meaningfully breaking the fast. Anything with calories — milk, sugar, juice — effectively ends the fasting period.
Is intermittent fasting safe?
For many healthy adults it's safe, but it isn't for everyone. People who are pregnant or breastfeeding, have diabetes, take certain medications, or have a history of disordered eating should talk to a doctor first. This tool only helps with scheduling, not medical suitability.