Intermittent Fasting Window Calculator
Calculate your eating and fasting windows for any IF protocol: 16:8, 18:6, 20:4, 5:2, and more.
Find the best schedule for your lifestyle.
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Your Fasting Schedule
Intermittent fasting (IF) is not about what you eat — it is about when you eat. You cycle between periods of eating (your “eating window”) and fasting (your “fasting window”).
The most popular protocols:
| Protocol | Fasting | Eating |
|---|---|---|
| 16:8 | 16 hours | 8 hours |
| 18:6 | 18 hours | 6 hours |
| 20:4 | 20 hours | 4 hours |
| OMAD (23:1) | 23 hours | 1 hour |
| 5:2 | 2 days at 500 cal | Normal eating 5 days |
How the calculator works: You enter the time you want to stop eating (e.g., 8:00 PM). The calculator adds your chosen fasting duration to find when your fasting ends and your eating window begins the next day.
For example, with 16:8 and a last meal at 8:00 PM:
- Fasting window: 8:00 PM → 12:00 PM (noon next day)
- Eating window: 12:00 PM → 8:00 PM
What happens during fasting:
- Hours 0–4: Blood sugar and insulin drop
- Hours 4–8: Body shifts to fat burning (lipolysis begins)
- Hours 12–16: Autophagy increases (cellular cleanup)
- Hours 16+: Deep ketosis and accelerated fat metabolism
Tips for success:
- Black coffee, plain tea, and water are generally allowed during the fast.
- Break your fast with a balanced meal — avoid high-sugar foods first.
- Start with 12:12 if 16:8 feels too aggressive.
- Consistency matters more than the exact window you choose.
- Most people find skipping breakfast easier than skipping dinner.
- Electrolytes (sodium, potassium, magnesium) can reduce fasting headaches.
- Sleep counts toward your fast — align your window with your sleep schedule.
Who should be careful:
- Pregnant or breastfeeding women
- People with type 1 diabetes
- Those with a history of eating disorders
- Always consult a doctor before starting any fasting protocol.