⏱️ Intermittent Fasting Calculator
By ToolNimba Health Team · Reviewed by ToolNimba Editorial Review, nutrition content review · Updated 2026-06-19
This calculator is for general information and planning only. It is not medical advice. Intermittent fasting is not suitable for everyone, including people who are pregnant or breastfeeding, those with diabetes or a history of disordered eating, children, and anyone on medication that must be taken with food. Talk to a qualified doctor or dietitian before starting any fasting routine.
This intermittent fasting calculator turns a fasting method and a start time into a clear daily schedule. Choose 16:8, 18:6, 20:4 or OMAD, tell it when you eat your last meal (when the fast begins), and it works out exactly when your fasting window ends, when your eating window opens and closes, and when the next fast starts. No counting on your fingers, just a plan you can follow.
What is the Intermittent Fasting Calculator?
Intermittent fasting is not a diet that tells you what to eat, it is an eating pattern that tells you when to eat. You split each day into a fasting window, when you take in only water, black coffee or plain tea, and an eating window, when all of your meals happen. The method is written as two numbers: the first is the fasting hours and the second is the eating hours, and they always add up to 24. So 16:8 means 16 hours fasting and an 8 hour eating window, while OMAD (one meal a day) is effectively 23:1.
The maths is simple. Take the time your last meal finishes, that is when the clock starts. Add the fasting hours to get the moment you can break the fast and your eating window opens. Add the eating hours to that, and you are back at the start of the next fast. Because the cycle is 24 hours, the eating window the next day lines up with the same clock times, which makes the routine easy to repeat. The calculator above does this arithmetic and rolls past midnight correctly, so a fast started at 8 PM under 16:8 ends at 12 PM (noon) the next day.
Most beginners start with 16:8 because it is the gentlest: you simply skip breakfast, stop eating after dinner, and let sleep cover a big chunk of the fast. Longer fasts (18:6, 20:4, OMAD) shrink the eating window further, which some people find makes a calorie deficit easier to hit, but they are also harder to sustain and leave less room to meet your nutrition needs. The best schedule is the one you can keep up consistently and that fits your work, sleep and social life.
When to use it
- Planning a 16:8 routine around your work day so the eating window covers lunch and dinner.
- Working out what time you can eat tomorrow when you finished dinner late tonight.
- Comparing how 16:8, 18:6, 20:4 and OMAD change the length and timing of your eating window.
- Setting a consistent last-meal cut-off time so your fasting window lines up the same way every day.
How to use the Intermittent Fasting Calculator
- Pick your fasting method: 16:8, 18:6, 20:4 or OMAD.
- Enter the time you start fasting, which is when your last meal ends.
- Read off when the fast ends and your eating window opens.
- Check the schedule table for the eating window close and the next fast start.
Formula & method
Worked examples
You follow 16:8 and finish your last meal at 8:00 PM.
- Fasting hours = 16, so eating hours = 24 - 16 = 8
- Fast ends = 8:00 PM + 16 h = 12:00 PM (noon) the next day
- Eating window = 12:00 PM to 12:00 PM + 8 h = 8:00 PM
- Next fast starts = 8:00 PM (the cycle repeats)
Result: Eat between 12:00 PM and 8:00 PM, fast from 8:00 PM to noon.
You try OMAD (23:1) with your one meal starting at 6:00 PM.
- OMAD means fasting hours = 23, so eating hours = 24 - 23 = 1
- If your single meal starts at 6:00 PM, the eating window is 6:00 PM to 7:00 PM
- The fast then runs from 7:00 PM all the way to 6:00 PM the next day
- That is 23 hours of fasting before the next meal
Result: One meal between 6:00 PM and 7:00 PM, then a 23 hour fast.
Common intermittent fasting methods and their windows
| Method | Fasting hours | Eating hours | Typical for |
|---|---|---|---|
| 16:8 | 16 | 8 | Beginners, daily routine |
| 18:6 | 18 | 6 | Intermediate, tighter window |
| 20:4 (Warrior) | 20 | 4 | Experienced fasters |
| OMAD | 23 | 1 | Advanced, one meal a day |
Example 16:8 eating windows by last-meal time
| Last meal ends | Fast ends (eat from) | Eating window closes |
|---|---|---|
| 7:00 PM | 11:00 AM | 7:00 PM |
| 8:00 PM | 12:00 PM (noon) | 8:00 PM |
| 9:00 PM | 1:00 PM | 9:00 PM |
| 10:00 PM | 2:00 PM | 10:00 PM |
Common mistakes to avoid
- Counting the start time as when you sit down, not when you finish. The fasting clock starts when your last bite or sip of anything caloric is done, not when the meal begins. Starting the count too early makes you break the fast sooner than planned.
- Breaking the fast with calorie drinks. Coffee with milk, juice, soda or a sports drink contains calories and ends the fast. During the fasting window stick to water, plain black coffee or unsweetened tea.
- Jumping straight to long fasts. Starting with 20:4 or OMAD on day one often leads to intense hunger, low energy and giving up. Most people do better easing in with 16:8 and extending the window gradually.
- Eating anything you like in the window. Fasting controls timing, not quality. If the eating window is filled with ultra-processed food and you overeat, the benefits people are after may not show up.
Glossary
- Fasting window
- The hours each day when you do not eat and take in only water, black coffee or plain tea.
- Eating window
- The hours when all of your meals and caloric drinks happen, the second number in the ratio.
- 16:8
- A method with 16 hours of fasting and an 8 hour eating window, the most popular starting point.
- OMAD
- One Meal A Day, an advanced pattern with roughly a 23 hour fast and a single short eating window.
- Time-restricted eating
- The clinical name for limiting daily food intake to a set window of hours, the basis of methods like 16:8.
Frequently asked questions
What is the 16:8 intermittent fasting schedule?
The 16:8 method splits the day into 16 hours of fasting and an 8 hour eating window. A common pattern is to finish your last meal at 8 PM, fast overnight and through the morning, then eat between noon and 8 PM the next day. This calculator builds the exact times from whatever start time you enter.
When does my eating window open and close?
Your eating window opens when the fast ends, which is your start time plus the fasting hours, and it closes after the eating hours have passed. For 16:8 starting at 8 PM, the window opens at noon and closes at 8 PM the next day. The calculator shows both times for you.
Can I drink anything during the fasting window?
Yes. Water, plain black coffee and unsweetened tea have effectively no calories and are generally considered fine during a fast. Anything with calories, such as milk, juice, soda or a snack, breaks the fast.
Which fasting method should a beginner choose?
Most people new to intermittent fasting start with 16:8 because much of the fast happens during sleep, so it is the easiest to sustain. You can move to 18:6, 20:4 or OMAD later if you want a tighter window, but there is no need to rush.
Is intermittent fasting safe for everyone?
No. It is not recommended for people who are pregnant or breastfeeding, children, those with diabetes or a history of disordered eating, or anyone whose medication must be taken with food. This tool is for planning only, so speak to a doctor or dietitian before starting.
Does the calculator handle fasts that cross midnight?
Yes. If you start your fast in the evening, the fast end and eating window will roll into the next day correctly, and the schedule notes when a time falls on the next day so the plan stays clear.
Sources
- Intermittent Fasting: What is it, and how does it work? , Johns Hopkins Medicine
- Diet Review: Intermittent Fasting for Weight Loss , Harvard T.H. Chan School of Public Health