Intermittent Fasting Calculator
How it works
Your eating window opens at your last meal time plus the fasting window's hours, wrapping past midnight when the fast runs into the next day — an 8pm last meal with a 16-hour fast opens the eating window at noon the following day. The next fast then starts at that time plus the eating window's hours, the same way.
The eating window field fills itself in as 24 minus the fasting window whenever you change the fasting hours, since that is the split most people mean — but typing your own number into it keeps whatever you typed, so an intentionally uneven split (a 16-hour fast with a 6-hour eating window, say) is never overwritten.
The water intake calculator and other health tools on this site are separate calculations if you are also tracking those alongside a fasting schedule.
Frequently asked questions
What does "16:8" mean?
The first number is hours spent fasting, the second is hours spent in the eating window — 16 hours fasting and 8 hours eating, repeating daily. It is one popular split, not the only one this tool works for: any two numbers can go in, so 18:6, 14:10 or 20:4 schedules all work the same way.
What if my fasting and eating hours don't add up to 24?
Nothing breaks — it just means the schedule doesn't repeat at exactly the same clock times every day. A 16-hour fast plus a 9-hour eating window is 25 hours total, so each day's times drift about an hour later than the day before, the same way a 25-hour routine would on any clock. That's a fact worth knowing, not an error this tool stops you from entering.
Is this telling me whether or how I should fast?
No. This is clock arithmetic on a schedule you already chose — it converts a last-meal time and a fasting window into the times your eating window opens and your next fast begins. Whether intermittent fasting is a good idea for you, and which split to use, is a separate question this tool has no opinion on.