Intermittent Fasting Calculator

What is intermittent fasting?

Intermittent fasting (IF) is an eating pattern that alternates between periods of eating and fasting. Unlike traditional diets that focus on what to eat, IF emphasises when to eat. The most common methods include 16:8, 5:2 and OMAD. During fasting periods, the body gradually shifts from burning mainly glucose to using more stored fat for energy, which may support fat loss, improve metabolic health and simplify meal planning.

What does this free intermittent fasting calculator do?

This fasting calculator helps you design an intermittent fasting plan that fits your life — not the other way around. Based on your usual eating times and chosen method (like 16:8 or 18:6), it shows when to start eating, when to stop, and how long you are fasting and eating each day. Whether you're looking for a fasting weight loss calculator or simply want to optimize your fasting time, this tool turns IF from a vague idea into a concrete daily schedule.

Which intermittent fasting methods can I use?

You can use gentle approaches like 12:12 or 14:10, more popular patterns like 16:8 or 18:6, or advanced methods such as 20:4, OMAD or 5:2. This fasting time calculator supports all these methods. Looking for an OMAD calculator? Just select 20:4 or adjust to your single-meal schedule. The table below gives you a numeric overview of common schedules:

How long has intermittent fasting been around?

Fasting has existed for centuries in religious, cultural and medical traditions. The modern “intermittent fasting” framing became popular in the early 2000s, as research and media coverage highlighted its potential effects on weight, blood sugar, brain health and longevity.

What should I have during the fasting period?

During your fasting window stick to zero‑calorie drinks such as water, sparkling water, black coffee, unsweetened tea and calorie‑free electrolytes. This is sometimes called water fasting — you consume only water and zero-calorie beverages. These keep you hydrated and comfortable without meaningfully interrupting the fast. Drinks containing sugar, milk, cream, juice or calories generally break a fast. Use a water fasting calculator approach: focus on hydration, not calories.

Does intermittent fasting have any benefits?

Systematic reviews and meta‑analyses in adults with overweight or obesity suggest that IF can reduce waist circumference, fat mass, LDL cholesterol, triglycerides, total cholesterol, fasting insulin and systolic blood pressure, while sometimes increasing HDL cholesterol and preserving lean mass. It may also support better appetite control and cognitive health. Results vary, and overall calorie intake, protein and lifestyle still matter more than any single protocol.

How many hours is considered intermittent fasting?

Any continuous fasting period of around 12 hours or more is usually considered intermittent fasting. In practice, most people use 12–24 hour fasts, with 14:10 and 16:8 being common daily patterns.

How long is the eating window while fasting?

Your eating window is simply 24 hours minus your fasting hours. For example, a 16:8 schedule has an 8‑hour eating window (such as 12:00 PM – 8:00 PM), while an 18:6 schedule leaves 6 hours for meals. Shorter windows usually mean a stronger fasting “signal” but aren’t necessary for everyone.

Do overnight hours count toward my fast?

Yes — sleep absolutely counts as part of your fasting period. If your last meal is at 8:00 PM and you eat again at 12:00 PM the next day, that’s a full 16‑hour fast (16:8), even though much of it happens while you’re asleep.

How do I calculate my fasting hours?

To work out your fasting hours, count the time between your last calorie‑containing food or drink one day and your first calorie‑containing food or drink the next day. Example: if you finish dinner at 7:30 PM and break your fast at 11:30 AM, that’s 16 hours of fasting. The calculator above automates this based on your selected schedule and first‑meal time.

Do I have to fast every day for it to work?

Not necessarily. Many people like the routine of a daily 14:10 or 16:8 schedule, but others do better with a few fasting days per week (for example 5:2 or alternate‑day fasting). What matters most is choosing a pattern you can repeat consistently over weeks and months, not perfection every single day.

Can I skip a day of fasting?

Yes — skipping a day will not “ruin” your progress. Life happens: travel, social events and busy days are normal. Simply return to your usual schedule at the next opportunity. Over time, your average pattern is what drives results, not a single day.

How fast will I see results with intermittent fasting?

Some people notice less bloating and better appetite control within a few days. Visible changes in weight and measurements usually take 2–4 weeks, depending on your calorie intake, protein, activity level and how closely you follow your chosen schedule.

How much weight can I lose in a month with IF?

Safe, sustainable weight loss is typically around 0.5–1.0 kg (1–2 lb) per week, or roughly 2–4 kg (4–8 lb) per month. Some people may lose weight faster at the beginning due to water shifts. This intermittent fasting weight loss calculator helps you pair your fasting window with realistic calorie and protein targets so the numbers line up with this range. Whether you use a fasting weight loss calculator or a simple fasting schedule, consistency matters most.

Can intermittent fasting be a bad idea for some people?

Intermittent fasting is generally safe for healthy adults, but it's not right for everyone. People who are pregnant, breastfeeding, underweight, very lean, under 18, taking certain medications or with a history of eating disorders should speak with a healthcare professional before trying IF. If fasting leaves you feeling unwell, overly anxious or obsessed with food, a more flexible eating pattern may suit you better.

What is the best intermittent fasting schedule?

The best intermittent fasting schedule depends on your lifestyle, goals, and what's tolerable for you. 16:8 is the most popular because it's sustainable for most people — you simply skip breakfast and eat between noon and 8 PM. More advanced options like 18:6, 20:4, and OMAD (one meal a day) work well for those with experience. Start with 14:10 or 16:8 and adjust based on how you feel.

How does intermittent fasting affect calorie intake?

Intermittent fasting naturally reduces calorie intake by limiting your eating window. When you have fewer hours to eat, you typically consume fewer meals and snacks. However, weight loss still depends on what and how much you eat during eating periods — IF is not a license to overeat. The intermittent fasting calculator above helps you set appropriate calorie targets.

Can intermittent fasting cause anxiety?

For some people, fasting may temporarily increase anxiety, especially if blood sugar drops or stress levels are high. This is usually temporary and improves as your body adapts. To minimize anxiety: stay hydrated, get enough electrolytes, ensure adequate sleep, and don't jump straight into aggressive fasting schedules. If anxiety persists, consider a gentler approach like 14:10.

How many months can you do intermittent fasting?

You can practice intermittent fasting indefinitely, as long as you maintain proper nutrition and balance. Many people adopt IF as a permanent lifestyle rather than a temporary diet. The key is to ensure you're getting enough calories, protein, vitamins, and minerals during your eating windows. Listen to your body — if you feel consistently fatigued or unwell, reassess your approach.

Bottom line

Intermittent fasting works best when:

• Calories are appropriate
• Protein intake is sufficient
• The schedule fits your lifestyle

This free fasting calculator gives you structure — the results come from consistency. Use it as your daily fasting time calculator and track your progress.