What is 16:8 intermittent fasting?

In 16:8 intermittent fasting, you fast for 16 hours and eat within an 8-hour window. For example, if you stop eating at 8pm, your eating window opens at noon the next day. This is one of the most popular and sustainable IF protocols.

When does autophagy start during fasting?

Autophagy (cellular self-cleaning) begins to increase noticeably after about 12–16 hours of fasting. It peaks during prolonged fasts (24–72 hours). For most 16:8 practitioners, some autophagy occurs during the fasting window, especially overnight.

What is the best fasting protocol for weight loss?

The best protocol is the one you can sustain. 16:8 is easiest for beginners and works well for most people. More aggressive protocols (20:4, OMAD) burn more calories but are harder to maintain. The 5:2 method (2 low-calorie days per week) is effective and flexible.

What can I consume during a fast?

During a fast, you can typically drink water, black coffee, plain tea, and sparkling water without breaking ketosis or autophagy. Adding cream, sugar, or calories will break the fast. Electrolytes (sodium, potassium, magnesium) without calories are generally acceptable.

Fasting Calculator

Calculate your intermittent fasting schedule for 16:8, 18:6, 20:4, OMAD, 5:2, and custom protocols. See eating window times, autophagy timeline, fat burning zone, and fasting support tips.

Fasting Protocol

Fast Start Time ?

:00 (24h clock)

Fasting Hours

—

Eating Window Opens —

Eating Window Closes —

Next Meal Time —

Extended More scenarios, charts & detailed breakdown ▾

Protocol

Fast Start Hour

:00

Eating Window Start

—

Eating Window End —

Fast Duration —

Eating Window —

Professional Full parameters & maximum detail ▾

Fasting Hours

Fast Start Hour

:00

Daily TDEE ?

cal

Meals in Eating Window

Hydration Goal

oz/day

Schedule

Eating Window Opens —

Eating Window Closes —

Calories per Meal —

Physiological Benefits

Autophagy Status —

Fat Burning Zone —

Growth Hormone Spike —

Fasting Support

Hydration Reminder —

Electrolyte Schedule —

Break-Fast Meal Suggestion —

How to Use This Calculator

Select your fasting protocol (16:8, 18:6, 20:4, or OMAD) and enter the hour your fast starts (24h clock — e.g., 20 = 8pm).
The calculator shows your eating window start and end times.
Use the Weekly (5:2) tab to plan alternate-day or 5:2 fasting with calorie tracking.
Use the Custom Schedule tab to set any custom fast/eat ratio.
Use the Professional tab for autophagy timeline, fat burning status, hydration reminders, and electrolyte schedule.

Formula

Eating window = 24h − fasting hours
Eating window start = fast start + fasting hours (mod 24)
Autophagy onset: ~12h fasting | Fat burning zone: ~16h | GH spike: ~24h

Example

Example: 16:8 protocol, fast starts at 8pm (20:00). Fast hours: 16. Eating window opens at 12:00 PM (noon) and closes at 8:00 PM.

Frequently Asked Questions

In 16:8 intermittent fasting, you fast for 16 hours and eat within an 8-hour window. For example, if you stop eating at 8pm, your eating window opens at noon the next day. This is one of the most popular and sustainable IF protocols.
Autophagy (cellular self-cleaning) begins to increase noticeably after about 12–16 hours of fasting. It peaks during prolonged fasts (24–72 hours). For most 16:8 practitioners, some autophagy occurs during the fasting window, especially overnight.
The best protocol is the one you can sustain. 16:8 is easiest for beginners and works well for most people. More aggressive protocols (20:4, OMAD) burn more calories but are harder to maintain. The 5:2 method (2 low-calorie days per week) is effective and flexible.
During a fast, you can typically drink water, black coffee, plain tea, and sparkling water without breaking ketosis or autophagy. Adding cream, sugar, or calories will break the fast. Electrolytes (sodium, potassium, magnesium) without calories are generally acceptable.

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Sources & References (5) ▾