Intermittent Fasting Calculator
Enter your last meal time and pick a protocol. See exactly when your fast ends, when to eat, and what's happening in your body along the way.
Your fasting plan
Tip: this is usually the time you finish your last meal.
Fast starts
8:00 PM
Fast ends — you can eat
12:00 PM (next day)
Eating window closes
8:00 PM (next day)
What happens during your fast
Blood sugar settles
~4h · 12:00 AM (next day)
Digestion wraps up and insulin starts to fall.
Fat burning begins
~12h · 8:00 AM (next day)
Glycogen runs low and your body turns to stored fat.
Ketosis ramps up
~16h · 12:00 PM (next day)
Ketone production increases as fat becomes the main fuel.
Timings are approximate and vary by person, activity, and last meal. This is educational, not medical advice.
Track this fast automatically
The free Fasting Tracker app runs this timer live and shows your stages in real time.
How the fasting calculator works
Intermittent fasting splits your day into a fasting window and an eating window. This calculator takes the time of your last meal and adds your protocol's fasting hours to find the moment your fast ends — the point when your eating window opens. For a 16:8 fast that finishes eating at 8:00 PM, your fast ends at 12:00 PM the next day.
New to fasting? Start with the 16:8 method, browse fasting schedules, or read our beginner's guide.
This tool is for education only and isn't medical advice. If you have a health condition, are pregnant or breastfeeding, or have a history of disordered eating, talk to your doctor before fasting.