Intermittent Fasting Calculator
Choose a fasting method and your last meal time to see when your fast ends and your eating window.
General guidance — not medical advice. This tool is for general scheduling and education only. Intermittent fasting is not suitable for everyone — including people who are pregnant, have diabetes, or have a history of disordered eating. It is not medical advice; consult your doctor before starting a fasting routine.
Intermittent fasting splits the day into a fasting period and an eating window. A 16:8 plan means 16 hours fasting and an 8-hour window. If you finish eating at 8 pm on 16:8, your fast ends and eating can resume at 12 pm the next day, giving an 8-hour window until 8 pm.
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How to Use This Calculator
Choose your fasting method — the ratio of fasting hours to eating hours — and enter the time you finished your last meal. The calculator tells you when your fast ends, when your eating window opens and closes, and how long the window is.
The most popular method is 16:8, but the tool also covers 18:6, 20:4 and one-meal-a-day (23:1). Pick the one you are following to plan your day.
How the Calculation Works
The method sets the number of fasting hours. Adding those hours to the time you last ate gives the moment your fast ends and your eating window opens. The window length is simply 24 minus the fasting hours.
The eating window then runs from when the fast ends until the same number of window hours later, at which point the next fast begins. The tool handles times that cross midnight.
fast ends = last meal + fasting hours; window = 24 − fasting hours
Worked Example
On 16:8, if you finish eating at 8:00 pm, add 16 hours: your fast ends at 12:00 pm (noon) the next day.
Your eating window is 24 − 16 = 8 hours, running from noon to 8:00 pm, after which the next 16-hour fast begins.
Assumptions and Limitations
- Schedules the window from your last meal time and chosen method
- Times that cross midnight are handled automatically
- Fasting is not suitable for everyone — see the disclaimer
- A planning aid, not medical or nutritional advice
Who Uses a Fasting Calculator
People doing intermittent fasting use this to find when a fast ends and when the eating window opens for a chosen method like 16:8 or 18:6. Use it to plan meals around your schedule and stay consistent. Enter your start time and method to see your window at a glance.
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Open calculator →Frequently Asked Questions
What does 16:8 fasting mean?
It means 16 hours of fasting followed by an 8-hour eating window each day. You eat only within the window and consume no calories during the 16-hour fast.
When does my fast end on 16:8?
Sixteen hours after your last meal. If you finish eating at 8 pm, your fast ends at noon the next day, and your eating window runs from noon to 8 pm.
Which fasting method is best?
It depends on your goals and lifestyle. 16:8 is the most popular and sustainable for many people; longer fasts like 20:4 or OMAD are more restrictive. Choose what you can maintain safely.
Can I drink anything during the fast?
Water, black coffee and plain tea are generally considered fine during a fast because they have essentially no calories. Anything with calories breaks the fast.
Is intermittent fasting safe for everyone?
No. It is not recommended during pregnancy, for people with diabetes on certain medications, or for those with a history of disordered eating. Check with your doctor before starting.
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