Frequently asked questions

What is the intermittent fasting calculator?

Intermittent fasting is a diet pattern where you alternate between days of fasting and days of eating normally. By reducing calorie intake on your fasting days, you may decrease your overall calorie intake through the week. Some people find this approach helps them lose or maintain weight more easily than a traditional calorie deficit.

Our intermittent fasting calculator is based on your age, gender, height, weight and your level of physical activity. It outputs a 5:2 diet plan, with 5 normal days and 2 days of fasting, as well as a 4:3 plan, with 4 normal days and 3 days of fasting.

How can intermittent fasting help me lose weight?

The theory behind intermittent fasting is based on what happens to your body during periods of eating and fasting. After a period of eating, your insulin levels are high. This forces your body to store glucose as glycogen in your liver, or as fat around your organs or beneath your skin.

In contrast, after fasting for a period of time, your insulin drops to low levels. Instead of storing energy, your body now burns energy. Your liver converts the stored glycogen back into glucose and, more importantly for weight loss, your body starts to burn the fat it previously stored.

Because fasting leads to the burning of fat, having regular fasting periods can enhance your body’s ability to burn fat over a long period of time. That’s how intermittent fasting can help you lose excess fat.

Types of intermittent fasting

The two main types of intermittent fasting are whole day fasting and time-restricted eating:

• Whole day fasting – Whole day fasting sees you eat normally for most days of the week, while fasting for 1-3 days. On a normal day, you eat at your typical calorie intake, while on a fasting day you eat only 500-600 calories. The most popular whole day fasting diets are the 5:2 diet and the 4:3 diet. These diets see you fast for 2 and 3 days per week, respectively.
• Time restricted eating – Time restricted eating works differently because you fast for a long period of time every day. The most popular diet is the 16:8 diet where you fast for 16 hours each day. The remaining 8 hours are your “eating window” where you consume your full calorie intake for the day.

Is intermittent fasting effective?

Losing weight can be a challenging struggle – both physically and emotionally. Although calorie deficits are a proven way to lose weight, many people find it difficult to battle cravings and maintain motivation.

If you’ve tried to lose weight before, you’ll know how difficult it can be. You may lose weight for a few weeks or months, but after 6-8 weeks, you might hit a plateau or find it impossible to keep to your daily calorie goal.

The truth is that what works for one person may not work for another. Weight loss is about finding a diet you can sustain over an extended period of time. For some people, that’s a simple calorie deficit. Others prefer zig zag dieting. And for many, intermittent fasting is the diet of choice.

While intermittent fasting has been shown to result in weight loss, there’s not enough evidence to compare it against other methods of weight loss. There’s also little research into the risks and health consequences of intermittent fasting, and many healthcare professionals are cautious about recommending it.

Benefits of intermittent fasting

Weight loss may be the primary motivation for most people following intermittent fasting (IF) diets. But IF may offer several other benefits.

• Weight loss - Intermittent fasting can be an effective way to lose weight for some people. By limiting the days or hours you consume food, IF can make it easier to maintain a calorie deficit over a sustained period of time. Because fasting encourages your body to burn fat for energy, it may help you lose fat while maintaining muscle.
• Burn fat more effectively - After a period of fasting, your body releases ketones. Ketones are chemicals produced by your liver when it breaks down fat for energy. A state of 'ketosis' (or 'keto' for short) means your body is actively burning fat, rather than using glucose from your dietary carbs. Periods of fasting encourage the same 'keto state' seen in low-carb or keto diets.
• Metabolic health - Intermittent fasting may improve insulin sensitivity, which is your body's ability to use insulin to manage blood sugar levels. However, in recent clinical trials, the impact on insulin sensitivity was similar to other weight loss methods like calorie deficit. More research is needed to understand the impact of intermittent fasting on metabolic health.

Types of intermittent fasting

5:2 intermittent fasting

In the 5:2 diet, you eat a normal amount of calories 5 days per week, while fasting on the remaining 2 days each week. On the two fasting days, you eat up to 25% of your typical calories, with a minimum of 500-600 calories each day.

5:2 is a straightforward and flexible diet, allowing you to choose any two non-consecutive days to fast.

4:3 intermittent fasting

4:3 is a similar diet, except you eat normally for 4 days per week, while fasting on the remaining 3 days. Once again, on your fasting days you should eat up to 25% of your typical calories. Never eat less than 500-600 calories each day.

Because 4:3 contains more fasting days, your calorie intake on your non-fasting days will be much higher than in the 5:2 diet. You may prefer this style of diet if you want to eat more on the weekends, keeping Mondays, Wednesdays and Fridays for fasting, for example.

If you want to eat more calories on particular weekdays, compare 4:3 against the zig zag diet which uses a calorie restriction but does not use fasting.

What should I eat and drink on fasting days?

On your fasting days, you only need to eat 25% of your typical calorie intake, or between 500 and 600 calories (whichever is higher). On a typical day you may eat one or two small meals, plus snacks. You should also drink plenty of fluids.

Here are some fasting day meal ideas that are low in calories:

• Simple soups - Simple vegetable soups can be nutritious, low in calories and add some fiber to your diet. Make soups in portions of around 250 calories each, allowing you to have two meals on each fasting day.
• Seafood meals - Seafoods like prawns and salmon are high in protein, but lower in calories compared to meats like beef and pork. Try simple stir-fries or serve with a side of vegetables, aiming for a meal of 200-250 calories.
• Chicken breast - Chicken breast is a lean meat that is high in protein and lower in calories than other meats. A seasoned grilled chicken breast, with a side of vegetables can come in under 250 calories.
• Omelette - Eggs contain around 150 calories each are a great option while fasting because they are high in protein. Simple meals like scrambled eggs, omelettes or even eggs on toast can come in under 300 calories.

As well as meals, try to eat occasional snacks. Fruits like apples, pears and berries can keep you going throughout the day.

Finally, because food normally provides 20% of your water intake, it's even more important to drink fluids on your fasting days. Here are some low calorie drinks and foods that will keep you hydrated:

• Black coffee and tea - Try coffee and tea without milk or sugar. These are low calorie drinks and some people find the caffeine suppresses their appetite (your mileage may vary!).
• Water - Food provides around 20% of your water intake. But on your fasting days, you'll need to make up this up by consuming plenty of fluids.
• Bone and vegetable broths - Bone and vegetable broths are rich in nutrients and electrolytes. The electrolytes can help you stay hydrated during your fasting days.

How to use the intermittent fasting calculator

The intermittent fasting calculator works by taking a number of inputs about you. This includes your height and current weight, as well as your gender, age and activity level.

Input the information about your weight, height, gender, age and activity level into the calculator. This will be used to calculate your TDEE. By default, the calculator assumes you are maintaining your current weight. But you can enter a weight loss goal or weight gain goal of up to 2 lbs per week.

Using your TDEE along with your weight goal, the intermittent fasting calculator will output a weekly total calorie estimate. This includes a calorie deficit or a calorie surplus. See our calorie calculator guide for more information on how this calculation works.

The calculator will then output two IF schedules - the 5:2 schedule and 4:3 schedule. It will suggest which days you should fast on, but remember you can choose any 2 or 3 non-consecutive days to fast.

Which activity level should I choose?

Our intermittent fasting calculator estimates your TDEE based on your current weight and activity level. The activity levels are called Harris-Benedict Standard Activity Factor (SAF) scores, and they range from 1.2 (+20%) to 1.9 (+90%), depending on your level of physical activity.

To estimate your TDEE, you will need to determine which SAF score fits your lifestyle best:

• Sedentary (SAF = 1.2) – Your daily life involves little to no exercise. You may work a desk-based job and spend your spare time indoors with little physical activity.
• Light Activity (SAF = 1.375) – You take part in light exercise or sports 1-3 days per week. If you work in a job where you spend time on your feet for most of the day, this may be an appropriate factor.
• Moderate Activity (SAF = 1.55) – You take part in moderate exercise or sports 3-5 days per week. This can include activities like jogging, cycling or swimming for at least 30 minutes each day.
• Very Active (SAF = 1.725) – You take part in moderate-to-vigorous exercise or sports 6-7 days per week. This can include running or playing competitive sports.
• Extra Active (SAF = 1.9) – You take part in vigorous training two times a day or have job requiring hard physical labor.

How to calculate your intermittent fasting schedule

Our intermittent fasting calculator first estimates your total energy expenditure for the entire week. If you want to lose weight, this estimate will also include a calorie deficit.

To create a fasting schedule, the calculator tries to divide your weekly TDEE into 7 days, using the following rules:

• Minimum calorie intake - For most women, eating less than 1,200 calories per day is unsafe; while for most men, eating less than 1,500 calories is unsafe. However, intermittent fasting sees you eat as few as 500-600 calories on your fasting days. For that reason, the calculator will adjust your calorie intake up to ensure your diet averages the safe minimum of 1,200 and 1,500 calories per day.
• 25% rule - The calculator will aim to set your fasting calories to 25% of your average daily calorie intake. The remaining calories are spread equally across your non-fasting days. Because it has more fasting days, the normal days on the 4:3 diet will be higher in calories than on the 5:2 diet.
• Minimum fasting calories - If you are trying to lose weight, the 25% rule may set your calories too low. For this reason, the calculator will never let your fasting calories drop below 500 calories if you are a woman and 600 calories if you are a man.

With these rules, the calculator creates a 7-day schedule for both the 5:2 and 4:3 diet plans. Each day is labelled as a "Normal" or "Fasting" day.

Who should not use the intermittent fasting calculator?

Do not use our intermittent fasting calculator:

• If you are pregnant – Our calculator will not provide an accurate reflection of your health status as your body undergoes significant changes during pregnancy. Consult a doctor or medical professional if you are concerned about your weight.
• If you have diabetes - Intermittent fasting can cause swings in your blood glucose levels and you may find it difficult to manage your diabetes. Always consult a medical professional before making changes to your diet if you have diabetes.
• If you have high or low blood pressure, a heart condition or any other medical condition - Although intermittent fasting may help some people with high blood pressure, you should get medical advice and supervision when changing your diet. If you have hypotension, a heart condition or any other medical condition, do not follow an intermittent fasting diet. Talk to your doctor or a medical professional.
• If you have an eating disorder – Always consult a medical professional about your weight.
• If you are under the age of 18 – BMR and TDEE estimates are not a suitable measure for children and teens under the age of 18.

References

Welton, S., Minty, R., O’Driscoll, T., Willms, H., Poirier, D., Madden, S., & Kelly, L. (2020). Intermittent fasting and weight loss: Systematic review. Canadian Family Physician, 66(2), 117-125.

Horne, B. D., Muhlestein, J. B., & Anderson, J. L. (2015). Health effects of intermittent fasting: hormesis or harm? A systematic review. The American journal of clinical nutrition, 102(2), 464-470.

Stockman, M. C., Thomas, D., Burke, J., & Apovian, C. M. (2018). Intermittent fasting: is the wait worth the weight?. Current obesity reports, 7, 172-185.