Intermittent Fasting Window Calculator
Calculate exact fasting and eating window times for 16:8, 18:6, 20:4 or OMAD protocols.
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Intermittent fasting is not a diet — it's a timing pattern. The core idea is to compress your eating into a defined window each day so your body spends more time in a fasted state, which affects insulin sensitivity, cellular repair (autophagy), and for many people, calorie intake. This calculator converts your preferred protocol and wake time into exact eating window start and stop times, so there's no guesswork.
Example
Wake time: 7:00am Fasting window: 7:00pm → 11:00am (16 hours) Eating window: 11:00am → 7:00pm (8 hours) First meal: 11:00am
Window Calculation
Eating window start = Wake time + Fasting hours remaining after wake Eating window end = Eating window start + Eating hours Fasting resumes: at Eating window end (For 16:8 starting from sleep: fasting begins at dinner, eating starts 16h later)
How Different Protocols Compare
16:8 is the most popular starting point — accessible, sustainable, and well-studied. 18:6 extends the fast by skipping breakfast entirely and works well for people who aren't hungry in the morning. 20:4 (the Warrior Diet) allows one larger meal and some light snacks, creating a strong autophagy signal. OMAD (one meal a day) is the most extreme and hardest to sustain but produces the longest fasted window.
Frequently Asked Questions
Can I drink coffee or tea while fasting?
Black coffee and plain tea contain near-zero calories and do not meaningfully break a fast. Milk, cream, or sweeteners do.
Will I lose muscle from fasting?
At 16–18 hours, muscle catabolism is minimal for most people, especially with adequate protein in the eating window. Extreme protocols (48h+) carry more risk.
Is it safe to exercise while fasted?
Yes for moderate intensity. Some people perform better fasted; others feel weak. Try both and see what suits you. Keep protein intake high in the eating window.
Why isn't 16:8 working for me?
Fasting doesn't override calorie surplus. If the eating window contains excess calories, weight won't drop. Track total intake for a week to diagnose.