Intermittent Fasting Window Calculator
Work out your daily eating and fasting windows for popular intermittent fasting protocols, with a live countdown to your next window change. This is a scheduling tool only — it does not calculate calories or predict weight loss.
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Intermittent Fasting Window Calculator
Our Intermittent Fasting Window Calculator works out your daily eating and fasting windows for popular fasting protocols like 16:8, 18:6, 20:4, 14:10, OMAD and 5:2. Pick a protocol and a start time, and see exactly when your eating window opens and closes, with a live countdown to your next window change. This tool is for scheduling only — it does not calculate calories, meal plans, or expected weight change.
What Is Intermittent Fasting?
Intermittent fasting (IF) is an eating pattern that cycles between periods of eating and periods of not eating. Unlike a diet, it does not typically prescribe which foods to eat, only when to eat them. The most common approaches restrict eating to a daily window (such as 16:8) or reduce intake on certain days of the week (such as 5:2).
Common Intermittent Fasting Protocols
| Protocol | Fasting Period | Eating Window | Typical Use |
|---|---|---|---|
| 14:10 | 14 hours | 10 hours | Often used as a gentle starting point |
| 16:8 | 16 hours | 8 hours | The most widely used daily protocol |
| 18:6 | 18 hours | 6 hours | A step up once 16:8 feels comfortable |
| 20:4 (Warrior Diet) | 20 hours | 4 hours | A more advanced daily protocol |
| OMAD (23:1) | 23 hours | 1 hour | One meal a day; considered more advanced |
| 5:2 | 2 non-consecutive days/week | 5 normal days/week | Weekly pattern rather than a daily window |
How to Use This Calculator
- Choose your protocol from the dropdown — 14:10, 16:8, 18:6, 20:4, OMAD, or 5:2.
- For daily protocols, enter the time you plan to start your eating window (your first meal or drink containing calories).
- For 5:2, select the two days of the week you plan to reduce intake — they must not be next to each other.
- Click Calculate My Windows to see your personalised schedule, a 24-hour timeline, and a live countdown to your next transition.
What Can I Have During the Fasting Window?
Most intermittent fasting approaches allow water, black coffee, and plain tea during the fasting window, as these contain negligible or no calories. Anything with meaningful calories — milk, sugar, cream, juice, or food — is generally considered to break the fast. If you are combining fasting with medication that must be taken with food, check with a pharmacist or GP about timing.
Is Intermittent Fasting Right for Everyone?
No. Intermittent fasting is not recommended for pregnant or breastfeeding women, children and teenagers, people with type 1 diabetes or who use insulin, people on medication that requires food, and anyone with a current or past history of disordered eating. If you have any underlying health condition, speak to a GP before changing your eating pattern.
Frequently Asked Questions
What is intermittent fasting?
Intermittent fasting is an eating pattern that alternates between periods of eating and periods of fasting. It focuses on when you eat rather than what you eat, and common approaches include daily time-restricted windows (like 16:8) and weekly patterns (like 5:2).
What’s the difference between 16:8, 18:6 and 20:4?
The numbers refer to hours of fasting and hours of eating in a 24-hour day. 16:8 means 16 hours fasting and an 8-hour eating window; 18:6 and 20:4 follow the same pattern with progressively shorter eating windows. Many people start with a shorter fast, such as 14:10 or 16:8, before considering longer windows.
Is intermittent fasting safe for everyone?
No. It is not suitable for pregnant or breastfeeding women, under-18s, people with type 1 diabetes or who take medication requiring food, and anyone with a current or past eating disorder. Anyone with an existing health condition should speak to a GP before starting.
Can I drink water or coffee during the fasting window?
Most intermittent fasting approaches allow water, black coffee and plain tea during the fasting window since they contain negligible calories. Adding milk, sugar, cream or sweeteners is generally considered to break the fast, though views on this vary between approaches.
How long does it take to adapt to fasting?
Many people report hunger, low energy or irritability during the first one to two weeks as their body adjusts. This varies significantly between individuals. If symptoms are severe or persistent, it’s worth reconsidering the approach and speaking to a healthcare professional.
What is OMAD and is it safe?
OMAD stands for “One Meal a Day” and involves eating within a roughly one-hour window, fasting for the remaining 23 hours. It is considered a more advanced and restrictive approach than shorter daily windows, and is not generally recommended as a starting point. It should be approached with particular caution and, ideally, professional guidance.
How does 5:2 fasting work?
5:2 involves eating normally five days a week and significantly reducing intake on two non-consecutive days. This calculator helps you schedule which two days to use but does not set a calorie target for those days — for that, speak to a GP or registered dietitian.
Should I combine intermittent fasting with calorie counting?
Some people combine fasting with calorie awareness during their eating window, while others do not track calories at all. This calculator focuses purely on timing. If you want to understand general daily energy needs, our Calorie Calculator can help, though it’s separate from this scheduling tool.