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— Intermittent Fasting Calculator

Find Your Eating Window in Seconds

Select your fasting protocol, enter your last meal time, and get your exact fasting and eating window — with science-backed recommendations for your goal.

A product that hits that number

Legion Whey+

A shorter eating window makes the daily protein target harder, not easier — the same grams in fewer meals. Whey+ puts 22 g into one shake, which is the simplest way to close that gap.

Legion is the only supplement brand this site earns a commission from, and it is said on every page that links to them.

Check the label → How we ranked it →
Free · printable

A shorter window makes the protein target harder

The printable chart: 30+ foods ranked by protein per 100 calories — which is what decides whether the total fits in your eating window.

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1 — Choose Your Fasting Protocol
2 — Enter Your Details
— Your Fasting Schedule
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Fasting Starts (last meal)
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Eating Window Opens
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Eating Window Closes
16h
Fasting Window
8h
Eating Window
16:8
Protocol
— Recommendation for Your Goal
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?? Medical Disclaimer: This tool is for educational purposes only. If you have diabetes, are pregnant or breastfeeding, take glucose-lowering medication (insulin, metformin, GLP-1 agonists), have a history of eating disorders, or have any chronic medical condition — consult a qualified healthcare professional before beginning any fasting protocol.

— Protocol Guide
Which Fasting Protocol Is Right for You?
Choose based on your lifestyle, training schedule, and goal — not what's trending.
12:12
The Foundation
The natural overnight fast most people already do. A good starting point before moving to longer protocols. Minimal disruption to social eating.
Beginners Easy to maintain Social-friendly
14:10
The Step Up
A gentle extension of the overnight fast. Effective for appetite control without significant lifestyle disruption. Ideal for people with early-morning training.
Habit building Appetite control Early risers
16:8
The Gold Standard
The most researched protocol. Skipping breakfast and eating from midday to 8pm. Clinically shown to reduce calorie intake and improve insulin sensitivity.
Fat Loss Metabolic Health Most studied
18:6
The Serious Protocol
An 18-hour fast with a 6-hour eating window. Requires fitting all meals between e.g. 12:00–18:00. Effective for fat loss but can conflict with evening social eating.
Aggressive fat loss Experienced fasters
20:4
The Warrior Diet
A 20-hour fast with a 4-hour eating window. Originally called the Warrior Diet. Requires consuming all daily calories in a very tight window — challenging but effective.
Advanced Maximum fat loss High discipline
OMAD
One Meal a Day
A 23-hour fast with a 1-hour eating window. Extreme protocol requiring careful planning to hit protein and micronutrient targets. Not recommended for most people.
Extreme Expert only Maximum simplicity
— Fasting Support
Products That Support Your Fasting Protocol
Zero-calorie products that don't break your fast and help maintain energy, hydration, and focus during the fasting window. Affiliate links — we earn a commission at no cost to you.
Electrolytes
Zero-Calorie Electrolyte Mixes
Electrolytes are depleted faster during fasting. A zero-calorie electrolyte mix prevents fasting headaches, fatigue, and muscle cramps without breaking your fast.
Appetite Control
Quality Black Coffee & Green Tea
Caffeine suppresses ghrelin (the hunger hormone) and enhances fat oxidation during fasted states. Black coffee and plain green tea do not break a fast.
Breaking the Fast
High-Protein First Meal Options
Breaking your fast with protein and fibre — not carbohydrates — prevents the blood sugar spike and subsequent hunger crash that can derail the rest of your eating window.
— Research Basis
The Science Behind This Calculator
This tool is built on the following peer-reviewed research.
Study 01
Intermittent Fasting and Human Metabolic Health — de Cabo & Mattson, 2019
New England Journal of Medicine — IF improves insulin sensitivity, blood lipids, and body composition across multiple protocols. PubMed ?
Study 02
Effect of 16:8 Time-Restricted Eating on Body Weight — Lowe et al., 2020
JAMA Internal Medicine — 12-week RCT showing 16:8 reduced calorie intake and body weight without calorie counting. PubMed ?
Study 03
Time-Restricted Eating and Muscle Mass — Tinsley & La Bounty, 2015
Nutrition Reviews — Adequate protein intake within the eating window preserves muscle mass during IF protocols. PubMed ?