Intermittent Fasting Window Calculator
Pick your wake-up time and fasting protocol, and see the exact clock times your eating window opens and closes.
*Estimates only, not medical advice. Consult a doctor before starting any fasting protocol, especially if you have a medical condition.
Tracking calories inside your eating window too?
BMI Calculator⏱️ Timing Your Fast
How the eating window is calculated and how to make it work for your routine.
How it Works
You give the calculator your wake time and chosen protocol. It assumes a 2-hour buffer after waking (a common pattern many people follow, e.g. black coffee first, food later), then opens your eating window for the number of hours your protocol allows.
The Formula
Eating Window Start = Wake Time + 2 hours. Eating Window End = Start + Protocol Eating Hours (8h for 16:8, 6h for 18:6, 4h for 20:4, ~1h for OMAD).
Pro Tip
Anchor your eating window around your biggest workout or your family dinner time — adherence matters far more than hitting a "perfect" window shown by any calculator.
Frequently Asked Questions
What's the best intermittent fasting protocol for beginners? +
16:8 is the most common starting point because it usually just means skipping breakfast and eating lunch through dinner. Most people adapt within a week or two, while 18:6, 20:4, and OMAD are more advanced protocols that further compress the eating window.
Does the exact time I start eating matter? +
What matters most is consistency, not the specific clock time. Picking a window that fits your work schedule, social life, and workout timing and sticking to it daily produces better adherence than chasing a theoretically optimal window.
Can I drink coffee or water during the fasting window? +
Yes. Water, black coffee, and plain tea contain effectively zero calories and do not break a fast for most practical purposes. Adding cream, sugar, or milk introduces calories and technically ends the fasted state.
How long does it take to adapt to intermittent fasting? +
Most people report reduced hunger and stabilized energy within 1 to 4 weeks as the body adjusts hunger hormone patterns. Starting with a shorter window like 12:12 and gradually extending toward 16:8 tends to be easier than jumping straight into 20:4 or OMAD.
Is OMAD (one meal a day) safe for everyone? +
OMAD is a more extreme protocol and is not appropriate for pregnant or breastfeeding people, those with a history of disordered eating, individuals with diabetes on insulin or sulfonylureas, or anyone with significant medical conditions. Check with a doctor before attempting OMAD or extended fasting.
Will intermittent fasting alone cause weight loss? +
Intermittent fasting is a timing strategy, not a calorie-counting method. Weight loss still depends on eating fewer calories than you burn during your eating window. Fasting can make that easier by naturally reducing eating occasions, but it does not override a calorie surplus.