Intermittent Fasting Calculator

Use the Intermittent Fasting Calculator

Build your daily eating and fasting windows (12:12, 14:10, 16:8, 18:6, and more) and see a clear schedule with a live countdown.

Start your IF schedule

How to Use Intermittent Fasting Calculator

1. Step 1: Pick a schedule

   Choose a preset (12:12, 14:10, 16:8, 18:6, 20:4, OMAD) or select Custom.

2. Step 2: Set your reference time

   Decide whether the time marks eating start or fasting start, then select the clock time.

3. Step 3: Adjust options

   Use the 12‑hour/24‑hour toggle and, if Custom, set your eating window length.

4. Step 4: Review today’s plan

   See your eating and fasting windows with a live countdown to the next phase.

5. Step 5: Save and follow

   Copy the schedule and keep times consistent day‑to‑day for best results.

Key Features

• Preset IF protocols (12:12, 14:10, 16:8, 18:6, 20:4, OMAD)
• Custom eating window builder (1–23 hours)
• Local time schedule with 12/24‑hour toggle
• Live countdown to next window
• Weekly preview and one‑tap copy
• Privacy‑first: no data stored

Understanding Results

How times are calculated

The calculator uses your selected schedule and reference time to build a 24‑hour plan. If you choose “Eating starts at 12:00,” an eating window of the specified length begins at 12:00 and ends when that time elapses; the fasting window fills the remaining hours until the next day’s start time. If you instead choose “Fasting starts at,” the tool offsets the eating block by the fasting duration.

Typical ranges & interpretation

Common time‑restricted eating patterns are 12:12 (gentle), 14:10 (moderate), and 16:8 (popular balance). Tighter windows like 18:6 and 20:4 are usually for experienced users who enjoy fewer, larger meals. There is no single “best” window—consistency and overall nutrition drive results. Use the live countdown to plan your next meal, and aim to end your window 2–3 hours before bedtime for better sleep.

Assumptions & limitations

All times use your device’s local clock. The tool does not store data. It does not prescribe foods or calories; pair it with appropriate nutrition targets if you have weight or performance goals. Intermittent fasting is not suitable for everyone—people who are pregnant or breastfeeding, under 18, underweight, or managing certain conditions (such as diabetes) should consult a clinician before fasting.

Complete Guide: Intermittent Fasting Calculator

Use our intermittent fasting calculator to plan eating/fasting windows for 16:8, 18:6, and more. Set start times and view a clear daily schedule with reminders.

Intermittent fasting (IF) is a time‑based way to organize meals, not a specific list of foods. The goal is to create consistent eating and fasting windows so your body has predictable periods of digestion and rest. Our intermittent fasting calculator keeps the tool above the fold and gives you a clear, mobile‑first schedule with a live countdown to your next window. It is simple enough for beginners and flexible enough for experienced users who prefer custom hours or OMAD.

What is intermittent fasting?

Intermittent fasting groups your day into two blocks: a fasting window and an eating window. Popular daily schedules include 12:12, 14:10, 16:8, 18:6, and 20:4. Some people also practice OMAD (one meal a day, roughly 23:1). The approach is valued for its simplicity—you pick a schedule and stick to it—making it easier to build routines around meals, workouts, and sleep. IF does not prescribe what to eat; it specifies when to eat. Your results still depend on overall calorie balance, protein intake, and sleep quality.

Research on fasting is ongoing. Early evidence suggests that time‑restricted eating can support weight management by reducing late‑night snacking and improving dietary consistency. Some individuals also report better appetite awareness and fewer decisions about grazing. However, IF is not a magic trick—calorie intake and activity remain central to weight change. If you live with a medical condition (especially diabetes), are pregnant or breastfeeding, or take medications, speak with your clinician before making changes.

How the intermittent fasting calculator works

The calculator asks for three things: your schedule (preset or custom), whether your reference time marks the start of eating or fasting, and the clock time for that reference. From those inputs it computes your eating and fasting windows for the current 24‑hour day and shows a live countdown to the next boundary. You can toggle 12‑hour or 24‑hour format, and copy the plan for quick notes or sharing with a coach.

• Presets: 12:12, 14:10, 16:8, 18:6, 20:4, OMAD.
• Custom: Choose any eating window from 1–23 hours; fasting adjusts automatically.
• Reference: Use “Eating starts at” (most common) or “Fasting starts at” if you begin after dinner.
• Local time: All calculations use your device’s local clock.

For weight‑related goals, pair your schedule with a calorie target. Use the TDEE calculator to estimate daily energy needs, then set a modest deficit or surplus with the calorie calculator. If you prefer ketogenic macros, the keto calculator can help distribute protein, carbs, and fat inside your eating window.

How to choose an intermittent fasting schedule

The best schedule is the one you can repeat most days without friction. A 16:8 plan is popular because it leaves an 8‑hour eating window—enough room for two meals and a snack—while removing late‑night grazing. If your mornings are busy and you naturally eat later, a 16:8 or 18:6 starting near noon often fits well. If you eat breakfast early with family, a 14:10 window centered on morning and midday may be better. New to IF? Start with 12:12 for a week, then tighten to 14:10 and see how you feel.

• 12:12: Gentle and flexible; a good starting point.
• 14:10: Moderate; curbs late snacks while staying social.
• 16:8: Common balance of structure and freedom.
• 18:6 or 20:4: Tighter windows for advanced users who like fewer, larger meals.
• OMAD (23:1): One meal a day; simplicity at the cost of flexibility. Many people use it only a few days per week.

For weekly patterns like 5:2 (two lower‑calorie days, five typical days), the daily schedule still helps. Pick a gentle window on busier days and a tighter one on lighter days. Keep hydration and protein steady across the week to support recovery.

Picking a start time that matches your life

Anchor your eating window to an event you already do every day. If lunch is your first meal, set “Eating starts at” to 12:00. If family dinner is non‑negotiable at 6:30 PM, place your 6–8‑hour window so it includes dinner and ends a few hours before bedtime. Consistency beats perfection: a ±30‑minute drift won’t ruin your plan, but a repeating rhythm simplifies hunger cues and grocery planning.

Sleep is the other anchor. Avoid pushing large meals right against bedtime; give your body 2–3 hours to digest to support sleep quality. For better nights, experiment with wind‑down routines and check your optimal bedtime with the sleep calculator or your natural cycles with the sleep cycle calculator.

What to eat during your window

IF sets a schedule; nutrition fills it. Aim for balanced meals built around protein, fiber‑rich carbohydrates, and healthy fats. Protein supports satiety and muscle maintenance, fiber aids digestion, and healthy fats make meals satisfying. Plan your largest meal when you’re most active or social, and keep a quick, protein‑forward backup meal on hand for busy days.

• Protein: lean meats, fish, eggs, Greek yogurt, tofu, legumes.
• Carbohydrates: fruits, vegetables, whole grains, beans.
• Fats: olive oil, avocado, nuts, seeds.

If you’re targeting weight change, set a daily calorie goal and distribute it across your window. Two meals plus a snack works well for many 16:8 users. You can estimate daily intake with the calorie intake calculator and check your progress with the weight loss calorie calculator.

Hydration, coffee, and supplements

Water is always in. Most IF approaches allow plain water, black coffee, and unsweetened tea during fasting windows. Many people find a glass of water on waking and a cup of coffee mid‑morning keeps hunger manageable. If you add milk, cream, sugar, or flavored syrups, you are consuming calories—use your judgment about whether that fits your goals and plan. For a quick estimate of fluids, see the hydration calculator or the water intake calculator.

Supplements are personal. Non‑caloric electrolyte tablets can be helpful if you train while fasting or live in a hot climate. If a supplement contains calories (e.g., oils), treat it as part of your eating window. When in doubt, read the label and consider asking a clinician or registered dietitian.

Training while fasting

Many people lift or do cardio late in a fast and then eat their first meal soon after. This pattern often feels good: you train, then refuel. If your sessions are long or intense, you may prefer a small pre‑workout snack and a larger meal post‑training. There is no universal rule—performance and adherence matter more than theory. Keep protein intake consistent across the week to support recovery.

Use our performance tools to plan smarter sessions. Calculate endurance paces with the running pace calculator or zone training with the heart rate zone calculator. If you’re tracking calories burned, the calories burned calculator can provide a rough estimate to combine with your intake.

Troubleshooting plateaus and hunger

• Hunger early in the fast: increase meal protein/fiber; add sparkling water or unsweetened tea.
• Late‑night snacking: move your window earlier and build a satisfying dinner plate.
• Weekend drift: choose a wider window on social days (e.g., 14:10) and return to normal on weekdays.
• Weight plateau: verify calorie intake and activity; small changes compound. Re‑estimate needs with the TDEE calculator.
• Low energy during workouts: try a small pre‑workout snack, or schedule training closer to your eating start time.

Calories, macros, and weight goals

IF can simplify adherence by reducing decision points, but calories still govern weight change. A modest deficit (e.g., 250–500 kcal/day) is typical for fat loss; a small surplus (e.g., 200–300 kcal/day) supports muscle gain when paired with strength training. Use the daily calorie calculator to pick a target and the macro calculator or keto calculator to allocate protein, carbs, and fat.

Results depend on the blend of intake, activity, sleep, and stress. Small, sustainable changes beat extremes. A consistent 14:10 with balanced meals can outperform a sporadic 18:6 with chaotic weekends.

Safety, limitations, and who should not fast

Intermittent fasting is not appropriate for everyone. Do not begin a fasted regimen without medical supervision if you are pregnant or breastfeeding, under 18, underweight, have a history of eating disorders, or live with conditions such as diabetes that require careful glucose management. Medications that must be taken with food may constrain your schedule. When in doubt, ask your clinician.

For general readers, focus on habits that are safe and broadly helpful: regular sleep, daily movement, and whole‑food meals. If you feel unwell while experimenting with fasting, stop and reassess. Tools like our fasting calculator can help you explore time spans without committing to a specific eating window.

Two example day plans

These examples show how different lifestyles can use the same 16:8 schedule with different anchor points. Adjust portions to your calorie target and keep protein high for satiety.

Office day (eating starts at 12:00)

• 07:00 – Water, black coffee; light walk.
• 12:00 – First meal (protein + veggies + whole grains).
• 16:00 – Snack (Greek yogurt and fruit; or nuts).
• 19:30 – Dinner (protein + vegetables + starch or beans).
• 20:00 – Eating window ends; herbal tea if desired.

Early‑riser training (eating starts at 08:00)

• 05:30 – Light cardio or strength; water or coffee.
• 08:00 – First meal (eggs, fruit, whole‑grain toast).
• 12:30 – Lunch (salad with chicken or tofu + grains).
• 15:30 – Snack (protein smoothie or cottage cheese).
• 16:00 – Eating window ends; evening walk; wind‑down.

References and further reading

For health decisions, prioritize guidance from your clinician and evidence‑based organizations. These resources explain intermittent fasting and time‑restricted eating in accessible language.

• National Institute on Aging. What do we know about intermittent fasting? CDC — Healthy Weight
• Harvard T.H. Chan School of Public Health. Intermittent fasting: What is it? Harvard Nutrition Source

Keep exploring: estimate your needs with the TDEE calculator, set a goal with the calorie calculator, plan hydration with the hydration calculator, and improve sleep timing with the sleep calculator.

Frequently Asked Questions