Intermittent Fasting Calculator

How to Use This Calculator

• Enter your sex, age, height and current weight
• Select your activity level
• Choose your preferred IF method
• Click Calculate

Your result shows your personalised eating window with start and finish times, your fasting window duration, your daily calorie target and your recommended calorie intake on fasting days where applicable.

Intermittent Fasting Methods Explained

The calculator supports the four most popular IF methods. Here is how each one works:

• 16:8 Method
  The 16:8 method involves fasting for 16 hours and eating within an 8-hour window. You might eat all your meals and snacks between 10am and 6pm, then fast for the remaining 16 hours.This is the most popular starting point for beginners. It fits naturally around daily life, many people simply skip breakfast, eat from midday to 8pm, and fast overnight. It is flexible, sustainable and does not require counting calories on fasting days.
• 5:2 Diet
  In the 5:2 plan, you eat normally for five days a week and on the other two days you dramatically restrict your diet, cutting calories by at least 75%. For example, if you normally aim for 2,000 calories a day, you would eat no more than 500 calories on fast days.The two fasting days should be non-consecutive, Mondays and Thursdays work well for most people. This method offers more weekly flexibility and is easier to schedule around social events.
• Alternate Day Fasting (ADF)
  Alternate day fasting involves fasting every other day or reducing calorie intake to around 25% of your normal intake on fasting days.ADF is more demanding than 16:8 or 5:2 and is better suited to people who have already tried a simpler method first. Research shows it is effective, but it is harder to sustain long-term for most people.
• 14:10 Method
  A gentler entry point into time-restricted eating. You fast for 14 hours and eat within a 10-hour window. This suits people who find 16:8 too restrictive or who exercise in the morning and need breakfast. It still produces circadian rhythm benefits and reduces evening snacking.

Your Eating Window Schedule

The calculator generates a personalised schedule based on your preferred wake time. Here are sample schedules for the most popular methods:

16:8 examples:

5:2 examples:

Research suggests earlier eating windows, breaking your fast at breakfast rather than lunch, may have additional metabolic benefits.

How Many Calories Should You Eat on IF?

This is one of the most commonly misunderstood points about intermittent fasting.

IF is not an excuse to eat anything you like during your eating window. If you consume more calories than your body needs during the eating window, you will not lose weight, regardless of the fasting hours.

The main reason IF helps with weight loss is that it makes it easier to eat fewer calories overall. If you eat massive amounts during your eating periods, you may not lose any weight at all.

For weight loss, aim to eat at a modest calorie deficit during your eating window, typically 300 to 500 kcal below your TDEE. The calculator above works out your TDEE and suggests your daily target for your chosen IF method.

For weight maintenance, eat to your TDEE within your eating window.

Prioritise protein (aim for 25 to 30% of calories), complex carbohydrates, vegetables and healthy fats. High protein intake preserves muscle mass and reduces hunger during fasting periods, both of which matter a great deal for long-term IF success.

What Can You Drink During a Fast?

Staying hydrated during your fasting window makes a significant difference to how you feel, and some drinks break a fast while others do not.

These do not break your fast:

• Plain water: as much as you like
• Black coffee: no milk, sugar, or flavouring
• Plain herbal tea: no sweeteners
• Sparkling water: plain, not flavoured
• Black tea: no milk or sugar

These will break your fast:

• Milk or cream in coffee (even a small amount)
• Fruit juice
• Smoothies
• Protein shakes
• Any sweetened drinks, including diet drinks in strict fasting approaches
• Alcohol

Black coffee during fasting is particularly useful. It reduces hunger, supports focus and contains almost no calories. Most regular IF practitioners use black coffee as a standard tool for getting through the last few hours of a fasting window.

Health Benefits of Intermittent Fasting

Research shows that intermittent fasting can help you lose weight. Most studies also show this eating approach lowers blood pressure, blood sugar and blood fats such as LDL cholesterol. Research also indicates that intermittent fasting reduces inflammation, which is linked to many diseases.

• Weight loss. IF reduces overall calorie intake naturally for most people and helps the body shift to burning stored fat during fasting periods. A 2022 study found mean weight loss of 8kg over 12 months in the time-restricted eating group compared to 6.3kg in the daily calorie-restriction group.
• Improved insulin sensitivity. Fasting periods lower blood insulin levels, which improves the body's response to insulin and reduces insulin resistance, a key driver of type 2 diabetes and metabolic syndrome.
• Autophagy. During extended fasting, the body enters a state called autophagy, a cellular housekeeping process where damaged or dysfunctional cells are broken down and recycled. This process is linked to healthy ageing and reduced disease risk. Autophagy typically begins after around 16 to 18 hours of fasting.
• Reduced inflammation. Chronic low-grade inflammation is a driver of cardiovascular disease, cancer, and neurodegenerative conditions. IF has been shown to reduce inflammatory markers in multiple studies.
• Cardiovascular health markers. Research has found reductions in blood pressure and oxidative stress markers in people following intermittent fasting protocols.

Early vs Late Eating Windows : Does Timing Matter?

This is a section almost no IF calculator page addresses, and it genuinely matters.

Not all eating windows are created equal. A growing body of research suggests that early time-restricted eating, consuming your calories earlier in the day, produces better metabolic outcomes than the same calories eaten later.

Recent studies suggest that eating earlier in the day may be more beneficial, breaking your fast with breakfast and ending with an earlier dinner, rather than skipping breakfast and eating into the evening.

The mechanism is circadian rhythm alignment. The body's metabolic processes, insulin sensitivity, digestion efficiency, hormonal regulation, all function better during daylight hours. Eating late in the evening works against these natural cycles.

A practical approach: if you are using the 16:8 method, an eating window of 8am to 4pm or 9am to 5pm may produce better metabolic results than the popular noon to 8pm window, even though both are technically 16:8.

That said, adherence matters more than perfect timing. A noon to 8pm window you can stick to is more effective than an 8am to 4pm window you abandon within a fortnight.

Intermittent Fasting for Women : What You Should Know

IF works well for many women, but there are specific considerations worth understanding before you start.

Hormonal sensitivity: Women's bodies can be more sensitive to significant calorie restriction than men's. For some women, extended fasting can affect cortisol levels and in some cases, disrupt the menstrual cycle. Intermittent fasting may affect the menstrual cycle and cause headaches or mood swings in some women.

Starting with a gentler method, 14:10 or 5:2, rather than jumping straight to 16:8 or ADF is a sensible approach for women new to fasting.

Pregnancy and breastfeeding: Intermittent fasting is unsafe during pregnancy or breastfeeding. Do not fast during these periods. Calorie restriction of any kind during pregnancy or breastfeeding can affect foetal development and milk supply.

Menopause: IF can be particularly effective for managing the weight changes associated with menopause, specifically the tendency towards central fat accumulation that comes with declining oestrogen. However, protein intake during eating windows becomes even more important for preserving muscle mass during this life stage.

Women who exercise heavily: For women doing significant resistance training or endurance sport, aggressive fasting can impair recovery and performance. A 12:12 or 14:10 approach tends to work better than strict 16:8 for active women.

Common IF Mistakes and How to Avoid Them

• Overeating during the eating window: The most common reason IF does not work. Condensing your calories into a smaller window does not mean you can eat without limit. Stick to your TDEE or a modest deficit.
• Not eating enough protein: Protein keeps hunger in check and protects muscle during fasting periods. Aim for at least 1.6 to 2g of protein per kg of body weight across your eating window.
• Jumping straight to an extreme method: Starting with OMAD or ADF when you have never fasted before is a recipe for giving up in week one. Begin with 12:12, then 14:10, then 16:8. Give each stage two to three weeks before moving on.
• Expecting immediate results: Give it at least six weeks minimum to measure results. Your body needs time to adapt to the new eating pattern. The first two weeks often involve hunger, irritability and no visible scale change, this is normal.
• Drinking the wrong things. Milky coffees, flavoured sparkling water and fruit juice during the fasting window add calories that break the fast and reduce its effectiveness.
• Exercising poorly around fasting. Working out at the very end of a long fasting period can impair performance and recovery. Train either before you break your fast (light to moderate sessions only) or after eating.

How to Break Your Fast Without Undoing Your Progress

What you eat to break your fast matters, particularly after longer fasting periods.

After a fasting window, blood sugar is low and digestion has been resting. Breaking the fast with large volumes of refined carbohydrates or sugary foods causes a sharp blood sugar spike and rebound hunger.

Better options to break your fast:

• Eggs, protein-rich, easily digested, filling
• Greek yoghurt with berries, protein and fibre
• Oats with seeds and nuts, complex carbs and healthy fats
• A piece of whole fruit followed by a balanced meal
• Avocado on wholemeal toast with eggs

Start with a moderate-sized meal rather than a large one, your digestive system will be more comfortable after a slow start.

After the 5:2 or ADF fasting days, the same principle applies. Resist the urge to eat significantly more the following day to "make up" for the fast. Overeating after fasting days is the most common way people accidentally cancel out the calorie benefit of fasting.

Who Should Not Try Intermittent Fasting

IF is safe for most healthy adults, but it is not right for everyone.

Intermittent fasting is generally not recommended for people who have an eating disorder, are pregnant or breastfeeding, are at high risk of bone loss and falls, or take medications such as blood thinners, diuretics, blood pressure medications or medications that affect blood sugar.

Additionally, people with type 1 diabetes, those on insulin therapy, children and teenagers and people who are significantly underweight should not attempt IF without close medical supervision.

If you have any ongoing health condition, are on regular medication or have a history of disordered eating, speak to your GP before starting any intermittent fasting protocol.

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