What is the Intermittent Fasting Window Calculator?
Intermittent fasting (IF) alternates set periods of eating and fasting each day. This calculator turns your chosen schedule — 16:8, 18:6, 20:4 or 14:10 — into concrete clock times. Enter when your eating window opens and instantly see when fasting begins and when your next meal window starts.
How to use it
Pick the hour (0–23) and minutes when you plan to eat your first meal, then select your fasting ratio. The first number in a ratio is the fasting hours, the second is the eating hours. For 16:8 you eat within an 8-hour window and fast for 16 hours.
The formula explained
The day is treated as a 24-hour clock. Your eating window length equals 24 minus the fasting hours. Fasting begins when the eating window closes:
$$\text{fasting start} = (\text{eating start} + \text{eat hours}) \bmod 24$$
Your next eating window opens after the fast completes:
$$\text{next eating start} = (\text{fasting start} + \text{fast hours}) \bmod 24$$
The modulo handles windows that roll past midnight.
Worked example
Say you start eating at 12:00 on a 16:8 plan. Eating hours = \(24 - 16 = 8\), so the window closes at \(12 + 8 = 20:00\) (8 PM). Fasting then runs 16 hours, reopening at $$20 + 16 = 36 \to 36 - 24 = 12:00.$$ So you eat noon to 8 PM and fast 8 PM to noon — a clean repeating cycle.
FAQ
Is intermittent fasting safe for everyone? No. People who are pregnant, underweight, have diabetes, or a history of eating disorders should consult a doctor first.
Can I drink during the fast? Water, black coffee and plain tea are generally allowed; calories break a fast.
Which schedule is best? 16:8 is the most popular and easiest to sustain; longer fasts like 20:4 are more advanced.
