Intermittent Fasting Calculator

How the calculator works

Pick a fasting protocol and enter the time of your last meal. The calculator adds the fasting hours of that protocol to your last meal time to find when your fast ends, then adds the eating window to find when it closes. The progress bar tracks how far you are through the current fast in real time.

Common fasting protocols

• 12:12 fast 12 hours, eat within a 12-hour window. A gentle entry point.
• 16:8 fast 16 hours, eat within an 8-hour window. The most popular beginner protocol.
• 18:6 fast 18 hours, eat within a 6-hour window. A step up from 16:8.
• 20:4 fast 20 hours, eat within a 4-hour window. Closer to OMAD without committing.
• OMAD one meal a day. Fast 23 hours, eat within a 1-hour window.
• 24h / 36h / 48h extended water-only fasts of one to two days. For experienced fasters — read the safety note.

How the weight-loss projection works

We estimate your maintenance calories (TDEE) with the Mifflin-St Jeor equation and your activity level. For daily protocols, the projection assumes you eat below maintenance because of the shorter eating window; for extended fasts, it counts the full fasting hours as a deficit at your maintenance burn rate. Weight change is converted at roughly 7,700 kcal per kilogram of body fat. Real-world results vary — metabolism adapts, and the projection is an estimate, not a guarantee.

FAQ

Can I use this fasting calculator for weight loss?

Yes — that's what the weight-loss projection above is for. Enter your stats and it estimates how much you could lose in 4, 8, and 12 weeks with your protocol. Fasting helps mainly by making it easier to eat less; the deficit still does the work.

Which schedule should a beginner start with — 16:8?

16:8 is the most popular starting point: skip breakfast, eat between roughly noon and 8 pm. If that feels hard, start at 12:12 or 14:10 and extend gradually. Consistency beats aggressiveness.

Does fasting work differently by age or for women?

The mechanics are the same, but energy needs differ by age, sex, and size — the calculator accounts for that via the Mifflin-St Jeor equation. Some women prefer gentler protocols (12:12-16:8). If you are pregnant, breastfeeding, under 18, or have a medical condition, talk to a doctor before fasting.