Start with your real schedule
A fasting routine only works if it fits your mornings, commute, workday, training, and family meals. Pick the window you can hit most days of the week without rearranging dinner.
Fasting windows & calories
Intermittent Fasting Calculator: Schedule, Eating Window, and Calories
Plan your fasting window, eating window, and daily calories for 16:8, 18:6, 20:4, OMAD, or 5:2.
By Jeff Beem
Updated
01
UnitsProfile
Biometrics
Height
Activity (TDEE)
Feeds Mifflin–St Jeor TDEE.
Goal
Protocol
Timing
Eating window start
02
Targets
Target about 1,941 kilocalories per day. Goal: fat loss, roughly 20 percent below estimated maintenance. Protocol: 16:8 (Standard). Estimated basal metabolic rate 1,765 kilocalories; total daily energy expenditure about 2,426 kilocalories.
Macros
Protein169.8g (35%)
Carbs145.6g (30%)
Fats75.5g (35%)
Per meal1,941 kcal
Fasting timeline (model)
Illustrates modeled metabolic phases over 24 hours after your eating window. For 5:2, scheduling is day-based rather than hourly. Hover reveals phase labels for sighted users only.
Schedule
Fast starts8:01 PM
Ketosis band (est.)10:01 AM
Late fast (est.)10:01 AM - 12:01 PM
First meal12:00 PM
~1h after first meal1:00 PM
Last meal8:00 PM
Plan notes
For fat loss, the fasting window is only part of the plan. The bigger win usually comes from having a structure that makes calories easier to control without feeling like every evening turns into catch-up eating.
What breaks a fast?
- Usually fine: black coffee, unsweetened tea, water, sparkling water.
- Often breaks fast: cream, milk, sugar, some sweeteners, bone broth, MCT oil.
How People Actually Use It
Most people want a fasting routine they can stick with through a normal week, with work, training, and family meals at their usual times. The arguments about whether coffee with milk really counts as fasting will sort themselves out once the schedule actually fits your life.
Practical starting points
Use 16:8 before harder protocols
Most people do better starting with 16:8 instead of jumping straight to OMAD or 20:4. The shorter fast leaves room for normal meals, recovery from training, and dinners that don’t need to start at 4 p.m.
Plan your first meal
Fasting goes more smoothly when you already know what your first meal will be. Decide it the night before, with real protein and enough total food, so you’re not picking through the kitchen the moment the window opens.
Treat longer fasts cautiously
Longer fasting windows sound appealing on paper, but they aren’t automatically better. If sleep, mood, training quality, or hunger get worse over a few weeks, the schedule is probably too aggressive for where you are right now.
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.
What This Calculator Does
This intermittent fasting calculator maps a fasting-and-eating schedule onto your day and estimates a calorie target based on your body stats and goal. Pick a protocol (16:8, 18:6, 20:4, OMAD, or 5:2) and the time your eating window opens. Enter age, sex, height, weight, and activity level, then choose fat loss, maintenance, or muscle gain. The results show your fasting window, eating window, and a daily calorie target you can plan meals around.
- Who it helps:Anyone starting or refining an intermittent fasting routine who wants a realistic eating window and a calorie target tied to their goal.
- What it outputs:Fasting and eating window times, a daily calorie target adjusted for your goal, and a visual clock showing each period.
- Limitations:Calorie estimates use Mifflin-St Jeor and a standard activity multiplier. They don’t model individual metabolic variation, medical conditions, or fasting-specific hormonal effects.
How the Math Works
The calorie target starts with Basal Metabolic Rate via the Mifflin-St Jeor equation:
where w is weight in kg, h is height in cm, and a is age in years. TDEE multiplies BMR by an activity factor (1.2 sedentary through 1.9 athlete). The goal adjustment applies a deficit or surplus: fat loss subtracts roughly 500 kcal, maintenance holds TDEE, and muscle gain adds roughly 250–500 kcal.
- Worked Example:30-year-old male, 80 kg, 178 cm, moderately active (1.55): BMR = 1,780, TDEE = 2,759, fat-loss target ≈ 2,259 kcal/day.
- Protocol Effect:The fasting protocol sets the eating window length but doesn’t change the calorie math. Shorter windows mean larger, more calorie-dense meals.
- 5:2 Variation:On two low-calorie days per week, intake drops to ≈500–600 kcal. The remaining five days use full TDEE or a mild deficit.
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
Calories, Meals, and Fasting Goals
How to use the calorie target
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.
- 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 to keep intake steady while you test whether a time-restricted schedule helps you stay consistent week to week.
- 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.
A daily calorie target turns the eating window from a clock into a budget. The schedule tells you when to eat; the number tells you how much.
What Breaks a Fast?
For most people, the practical rule is straightforward: if it has calories, count it as food. Save the fasting-window list for water, black coffee, and unsweetened tea.
- 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.
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. Food choice (keto, whole foods, counting macros) sits on top of that schedule and isn’t part of intermittent fasting itself. Most people pick a named routine such as 16:8, 18:6, 20:4, OMAD, or 5:2 and build the eating window around work, training, and appetite.
Which intermittent fasting schedule should a beginner start with?
For most beginners, 16:8 is the easiest place to start. The fasting window is clear, and you can usually still eat lunch and dinner at normal times if you skip breakfast or push it later. Once that feels easy, some people try 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 standard fasting-window drinks. Once you add cream, milk, sugar, juice, or any calorie-containing supplement, you’re not really fasting anymore.
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 gets more likely when calories stay too low for too long, protein intake is poor, or resistance training drops off. 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. Walking, easy cardio, and most lifting sessions are fine fasted; harder sessions usually feel better after the first meal. The practical rule: if training quality crashes, move the workout closer to your eating window.
Is intermittent fasting safe for everyone?
No. It’s not a good fit for everyone, especially people who are pregnant, breastfeeding, underweight, recovering from an eating disorder, taking medications that need to be taken with food, or managing certain medical conditions. If any of those apply, check with a qualified clinician before trying it.
What should I eat during my eating window?
Aim for meals that keep you full: protein, fiber-rich foods, fruit or vegetables, and enough total calories to support your goal. You don’t need a special fasting menu, but you do need meals that stop you from ending the evening hungry and overeating later.
Fitness Reference Note
Informational Use: These calculations (BMI, Calories, etc.) are based on standard statistical formulas and are intended for general reference and goal-setting purposes only.
Consult Experts: This tool does not provide medical advice, diagnosis, or treatment. Results may not be accurate for athletes, pregnant individuals, or those with underlying health conditions.
Health Safety: Always consult with a healthcare professional or qualified trainer before beginning any new diet or intensive exercise program.
Privacy First: All calculations are performed locally in your browser. No health data is stored or transmitted to any server.
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