Intermittent Fasting Window Calculator
Eating and fasting window from your schedule.
Enter your values
Choose one of: 16:8, 18:6, 20:4, 14:10.
Enter the first meal time (hh:mm, 24h) you are working with.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- Eating window
- 12:00 - 20:00
- Eating window length
- 8 hours
- Fasting window length
- 16 hours
Quick answer
Your eating window starts at your first meal and lasts for the number of hours defined by your chosen fasting protocol.
eating window = 24 - fasting hours
At a glance
| What it does | Eating and fasting window from your schedule. |
|---|---|
| Category | Health |
| Inputs needed | Protocol, First meal time (HH:MM, 24h) |
| Main output | Eating window |
| Formula | eating window = 24 - fasting hours |
| Cost | Free — no sign-up, no download |
How to use the Intermittent Fasting Window Calculator
- 1Choose your fasting protocol and enter the time of your first meal.
- 2The eating window is calculated as 24 hours minus the fasting duration.
- 3Intermittent fasting isn't suitable for everyone — check with a doctor if pregnant, breastfeeding, or managing diabetes or an eating disorder.
Inputs explained
- Protocol
- Choose one of: 16:8, 18:6, 20:4, 14:10.
- First meal time (HH:MM, 24h)
- Enter the first meal time (hh:mm, 24h) you are working with.
Worked example
Using the values the calculator loads with:
Inputs
- Protocol16:8
- First meal time (HH:MM, 24h)12:00
Results
- Eating window12:00 - 20:00
- Eating window length8 hours
- Fasting window length16 hours
Frequently asked questions
Which protocol is best?
There's no single best option — 16:8 is the most commonly sustained by beginners.
Can I drink water while fasting?
Yes, water, black coffee and plain tea are typically allowed and don't break a fast for most protocols.
What do I need to enter into the Intermittent Fasting Window Calculator?
Just 2 values: protocol, first meal time (hh:mm, 24h). Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Intermittent Fasting Window Calculator work out the answer?
Your eating window starts at your first meal and lasts for the number of hours defined by your chosen fasting protocol. It applies the formula eating window = 24 - fasting hours and shows the working so you can check each step by hand.
Is the Intermittent Fasting Window Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.