How does the Fast Diet work?

(A TDEE / BMR / BMI calculator)

Go straight to our online calculator

If we were to distill the Fast Diet into a single sound-bite, it would all come down to 5:2. That’s five days of normal eating, with little thought to calorie control and a slice of pie for pudding if that’s what you want. Then, on the other two days, you reduce your calorie intake to 500 calories for women and 600 calories for men.

Since you are only fasting for two days of your choice each week, and eating normally on the other five days, there is always something new and tasty on the near horizon. In short, it’s easy to comply with a regime that only asks you to restrict your calorie intake occasionally. It recalibrates the diet equation, and stacks the odds in your favour.

Bear in mind that the programme is designed as a well-signposted path towards a longer, healthier life; weight loss is simply a happy adjunct to all of that.

How many calories on a non-Fast Day?

You may have wondered how we came up with the recommendation that women have 500 calories and men have 600 calories on a Fast Day. We used the rule of thumb that women need 2000 calories and men need 2400 calories per day and on a Fast Day you should eat a quarter of a normal day’s recommended calories. Some of you have also wondered exactly how many calories you should be eating on days when you’re not fasting.

We thought we’d bring everything together in one place for you so you can do the calculations here. You can use the calculator on the right to calculate your BMI, BMR (basal metabolic rate) and TDEE (total daily energy expenditure). We’ll go into these in more depth below.

Calculation of BMI (body mass index)

BMI is a calculation of body fat based on height and weight. It has several limitations: it’s not accurate for pregnant women, people under 5 feet tall, and people with very muscular builds. It also does not account for age and the standard recommendations do not apply children or teens.

Calculations of BMR (basal metabolic rate)

Basal metabolic rate – the amount of calories you expend sitting for 24 hrs doing nothing. We are using the Mifflin-St. Jeor equations (above) to estimate your BMR which is believed to be more accurate than the more commonly used Harris-Benedict equation.

Calculation of the TDEE (total daily energy expenditure)

TDEE is the number of calories burnt in a day scaling BMR to level of activity. This is the number of calories you need daily to maintain your current weight and is about the amount you should eat on the days you’re not fasting. It depends on how active you are. Regular exercise burns up calories and is good for you. Do it!

People have a tendency to overestimate the exercise they do, so if in doubt, choose the lower activity level.

Activity levels