Intermittent Fasting Calculator

Give it your wake time, your sleep time and a protocol, and it places the eating window for you. On 16:8 waking at 7am, eating early means noon to 8pm and eating late means 3pm to 11pm. Most of the 16 hours are spent asleep, and the calculator prints that figure rather than hiding it.

Your Schedule

Wake Time

Sleep Time

Fasting Protocol

Window Preference

Your Recommended Schedule

Eating Window

12:00 – 20:00

Stop Eating By

20:00

24-Hour Timeline

00:00 06:00 12:00 18:00 00:00

Sleep Fasting Eating

Fasting Phases

00:00 06:00 12:00 18:00 00:00

Fed State (0-4h) Catabolic (4-8h) Fat Burning (8-12h) Ketosis (12-16h) Deep Ketosis (16h+)

You'll reach Fat Burning at approximately 04:00.

Fasting Hours

Eating Hours

Fasting While Asleep

How this is calculated

This is clock arithmetic, not physiology. You give a wake time, a sleep time, a protocol and where in the day you would rather eat. The protocol is split on the colon, so 16:8 is 16 fasting hours and an 8-hour eating window; the options are 14:10, 16:8, 18:6, 20:4 and 23:1 for one meal a day. Your waking hours are the difference between wake and sleep, and your sleeping hours are whatever is left of the 24.

The window is then placed. "As early as possible" starts it at your wake time and runs it forward; "as late as possible" ends it at your sleep time and runs it backwards; "midday-centred" finds the middle of your waking hours and centres the window on it. One rule overrides all three: an eating window is never allowed to run past your sleep time, so a window that would have ended after bed is pulled back to end at it.

The zone strip below the clock lays the site's fasting-zone hours over your own fast — fed for 4 hours, then catabolic, fat burning and ketosis at 4 hours each, then deep ketosis for the rest. Because sleep is counted as fasting time, most of a 16-hour fast is spent asleep, and the calculator prints that figure separately rather than hiding it.

Where this comes from

1. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting — Anton et al., Obesity, 2018 (PubMed Central)

Nothing on this site is medically reviewed — it cites its sources so you can check them. Read our editorial policy. Method last checked 21 August 2026.

Why Your Fasting Window Matters

Intermittent fasting is not just about when you eat, but about aligning your eating window with your circadian rhythm and daily schedule. A well-timed fasting window maximises the hours you spend in fat-burning and ketosis states while fitting naturally around your sleep, work and social life.

Research shows that eating earlier in the day (time-restricted eating aligned with daylight hours) may provide additional metabolic benefits. However, the best fasting schedule is the one you can stick to consistently. This calculator helps you find that sweet spot.

The fasting phases shown above reflect the metabolic transitions your body goes through during a fast: from the fed state where your body processes your last meal, through the catabolic phase where glycogen stores are tapped, into fat burning and eventually ketosis where your body primarily uses fat for fuel.

Frequently Asked Questions

How do I choose the right eating window for intermittent fasting?

The best eating window fits your daily routine. Consider when you wake up, when you feel hungriest, your work schedule, and social meal times. Most people find it easiest to skip breakfast and eat from midday to early evening.

What if I work shifts or have an irregular schedule?

Shift workers can still practise intermittent fasting successfully. The calculator adjusts to your specific wake and sleep times. The key is keeping a consistent window relative to your sleep schedule, even if that shifts day to day.

Does sleeping count as fasting time?

Yes, sleep hours count toward your fasting window. This is one of the reasons intermittent fasting is so practical — most people already fast for 7-9 hours overnight. You only need to extend that window by a few hours to reach a 16:8 protocol.

What is the best intermittent fasting protocol for beginners?

Most experts recommend starting with 14:10 or 16:8, which gives you a 10- or 8-hour eating window. These protocols are sustainable, easy to follow, and still deliver metabolic benefits. You can always tighten the window later as your body adapts.

Go deeper

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How to Calculate Your Fasting Window: Windows, Phases, and Timing

Learn how to calculate your ideal fasting window, understand fasting phases, and set up a schedule that matches your goals and lifestyle. Step-by-step guide.

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Best Time Window for Intermittent Fasting: Morning vs Evening

What's the best time window for intermittent fasting? Compare morning and evening eating windows backed by science, and find out which one fits your life.

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16:8 Intermittent Fasting: The Most Popular Schedule Explained

Learn how 16:8 intermittent fasting works, what the science says, and how to build a sustainable 16-hour fasting routine. Complete guide with meal timing tips.

Set this schedule in the Fasted app and track every fast

Once you find your ideal window, the Fasted app lets you lock it in with one tap. You will get reminders, automatic tracking, and a streak calendar to keep you on course.

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