Intermittent fasting duration calculator
Enter when you last ate and your fasting protocol. See exactly when your fast ends and how much time remains.
Common fasting protocols
- 16:8 - Fast for 16 hours, eat within an 8-hour window. The most popular protocol; compatible with most lifestyles when the eating window is set at noon–8 PM.
- 18:6 - A stricter variant; 18-hour fast leaves a 6-hour eating window. Popular with experienced IF practitioners.
- 20:4 (Warrior Diet) - 20-hour fast with a 4-hour evening eating window. Often involves one large meal.
- OMAD (One Meal A Day) - A 23–24 hour fast. It calls for attention to nutrient density in the single meal.
- 36 hours - An extended fast, usually done occasionally rather than daily. Longer fasts carry more risk, so talk to a doctor before attempting one.
- 5:2 - Not a fasting window: you eat normally on five days of the week and keep to about 500-600 calories on two non-consecutive days. Because this calculator counts hours, it does not model 5:2.
Is fasting right for you?
Fasting is not suitable for everyone. If you are pregnant or breastfeeding, under 18, have diabetes or another medical condition, have had an eating disorder, or take regular medication, talk to a doctor before you fast. Mayo Clinic has an overview of intermittent fasting. This calculator only works out times; it does not give medical advice.
What breaks a fast?
Caloric intake breaks a fast. Water, black coffee, and plain tea are generally considered fast-safe. Milk, cream, sugar, or any caloric addition ends the fast. Some practitioners also allow zero-calorie electrolytes for extended fasts.
Track your fast with Left
Add your fast end time as a countdown in Left. The widget on your Lock Screen will show you exactly how much fasting time remains - without needing to open an app or do the mental math.
Left also has a fasting timer with a Live Activity. Fasting is part of Left Premium. For a free countdown to your fast's end, see the countdown widget.
Start noticing what matters.
Download Left on your iPhone to see the time you have left, dates you are looking forward to, build the habits you want to keep, and become a better version of yourself.
Scan or search "Left" on the App Store.