Intermittent Fasting Calculator
Pick a fasting protocol and your first meal time to see your exact eating window, fasting hours and when to eat next.
How it works
eating hours = protocol (16:8→8, 18:6→6, 20:4→4, OMAD→1); window end = first meal + eating hours; fasting hours = 24 − eating hours
Intermittent fasting splits the day into an eating window and a fasting window. The protocol you choose sets how many hours you can eat: 16:8 gives an 8-hour window, 18:6 gives 6, 20:4 gives 4, and OMAD (one meal a day) compresses it to roughly 1. This tool takes your first meal time, adds the eating hours to find when the window closes, and shows the fast running from there until your first meal the next day.
Worked example
On 16:8 with a first meal at 12:00, the 8-hour window runs 12:00 to 20:00. The fast then lasts 16 hours, from 20:00 until your first meal at 12:00 the next day.
Frequently asked questions
When should I place my eating window?
Whatever you can keep consistent. Many people prefer a midday-to-evening window so they can eat with family, but an earlier window may suit morning appetite better. Consistency matters more than the exact clock time.
Can I drink anything during the fast?
Water, black coffee and plain tea are generally considered fine and do not meaningfully break a fast. Anything with calories — milk, sugar, juice — starts the digestive clock and ends the fasting state.
Does fasting alone cause weight loss?
Only if it puts you in a calorie deficit. Fasting can make that easier by shrinking the hours you eat, but it is still possible to overeat in a short window, so total intake is what counts.
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Educational estimate, not medical advice. Consult a qualified professional for health decisions.