Intermittent Fasting Calculator: Fasting Window, Eating Window, and Daily Calories
Use this intermittent fasting calculator to plan a 16:8, 18:6, 20:4, OMAD, or 5:2 routine, set your eating window, and estimate daily calories for fat loss, maintenance, or muscle gain.
Common Intermittent Fasting Schedules
Protocol Comparison Table
16:8
- Difficulty
- Beginner
- Primary Benefit
- Easy to fit into a normal day
18:6
- Difficulty
- Intermediate
- Primary Benefit
- Smaller eating window
20:4
- Difficulty
- Advanced
- Primary Benefit
- Short eating window
OMAD
- Difficulty
- Advanced
- Primary Benefit
- One main meal per day
5:2
- Difficulty
- Intermediate
- Primary Benefit
- Works by weekly calorie structure
Use the calculator to map the fasting window and eating window onto a time you can actually keep.
Calories, Meals, and Fasting Goals
How to use the calorie target
- Fat loss:The calculator lowers calories to create a moderate deficit. For most people, a routine like 16:8 or 18:6 is easier to sustain than very long fasts.
- Maintenance:Use the calorie target as a way to keep your intake steady while testing whether a time-restricted schedule helps you stay organised and consistent.
- Muscle gain:If muscle gain is your goal, the challenge is fitting enough food and protein into the eating window. Shorter fasting windows usually make that easier.
Your fasting plan should fit your goal, but it also has to fit your appetite, training, and daily routine. A plan you can keep beats a perfect plan you quit after four days.
This calculator gives you a realistic daily target so the eating window is not just smaller, but also more structured.
What Breaks a Fast?
- Usually fine during the fast:Water, sparkling water, black coffee, unsweetened tea, and plain electrolytes without calories.
- Usually breaks the fast:Cream, milk, sugar, juice, calorie-containing supplements, bone broth, or anything else that turns the fasting window into intake.
- When in doubt:Keep the fasting window simple. If you have to argue with yourself about whether it counts, it usually does.
For most people, the practical rule is straightforward: if it contains calories, treat it as food, not a fasting beverage.
If your goal is a clean intermittent fasting schedule, plain drinks make the window much easier to manage.
Intermittent Fasting Calculator FAQ
What is intermittent fasting?
Intermittent fasting is a schedule for when you eat rather than a food list of what you can or cannot eat. Most people pick a routine such as 16:8, 18:6, 20:4, OMAD, or 5:2 and use it to create a consistent eating window that fits work, training, and appetite.
Which intermittent fasting schedule should a beginner start with?
For most beginners, 16:8 is the easiest place to start because it gives you a clear fasting window without making meals hard to fit into a normal day. Once that feels easy, some people experiment with 18:6 or a shorter eating window, but the best schedule is the one you can repeat without feeling miserable.
Can I drink coffee or tea during the fasting window?
Usually yes. Plain water, black coffee, and unsweetened tea are the usual choices during a fasting window. Once you add cream, milk, sugar, juice, or calories from supplements, you are no longer really fasting.
What breaks a fast?
In practical terms, anything with meaningful calories breaks a fast. Black coffee, plain tea, sparkling water, and water are the usual exceptions people keep during fasting hours. If your goal is a clean fasting window, keep it simple and avoid anything that turns your fast into a snack.
Will intermittent fasting make me lose muscle?
Not automatically. Muscle loss is more likely when your calories are too low for too long, protein intake is poor, or you stop resistance training. If you want to keep muscle while fasting, eat enough protein during your eating window and keep lifting.
Can I exercise while fasting?
Yes, but your best training time depends on the workout and how you feel. Many people are fine doing walking, easy cardio, or lifting during a fast, while hard sessions may feel better after the first meal. The practical rule is simple: if training quality crashes, move your workout closer to your eating window.
Is intermittent fasting safe for everyone?
No. It is not a good fit for everyone, especially people who are pregnant, breastfeeding, underweight, recovering from an eating disorder, taking medications that require food, or managing certain medical conditions. If any of those apply to you, check with a qualified clinician before trying it.
What should I eat during my eating window?
Aim for meals that actually keep you full: protein, fiber-rich foods, fruit or vegetables, and enough total calories to support your goal. You do not need a special fasting menu, but you do need meals that stop you from spending the whole evening hungry and overeating later.