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

  • Sedentary :
    Little or no exercise. This level is for someone who does not or cannot incorporate exercise into their daily life (eg drives rather than walks, takes the lift rather than the stairs, has a desk job or restricted mobility).
  • Lightly active :
    Light exercise or sports 1-3 days per week. This level would include people who incorporate walking and activity into their day to day activities but do not have an exercise regime at such or exercise or play sports fewer than three times a week.
  • Moderately active :
    Moderate exercise or sports 3-5 days per week. This level is for people who exercise or play very active sports at least 30 minutes non-stop at a time at least three times a week, every week. This is the level for people who keep up a good fitness regime that fits into their daily life.
  • Very active :
    Hard exercise or sports 6-7 days per week. This level would include serious non-professional athletes actively training for, eg, a triathlon that requires near daily hard exercise for at least an hour at a time.
  • Extremely active :
    Very hard exercise or sports more than once every day and a physical job. This level is for people doing exercise multiple times per day, at least an hour at a time and with the type of physical job that requires top fitness. This level is not common – most non-professional athletes in serious training will be in the “Very active” level at most.

Our online calculator

Fill out the form and we'll tell you your BMI (reminder: take this calculation with a grain of salt, see the long description for details), your BMR and your TDEE. You can see full explanations for all of these on the left hand side of the page.

  • male female
  • years old
  • In: feet and inches centimetres
    feet inches
  • In: pounds stone kilos
    stone pounds
    • Sedentary :
      Little or no Exercise/ desk job
    • Lightly active :
      Light exercise/ sports 1 – 3 days/ week
    • Moderately active :
      Moderate Exercise, sports 3 – 5 days/ week
    • Very active :
      Heavy Exercise/ sports 6 – 7 days/ week
    • Extremely active :
      Very heavy exercise/ physical job/ training 2 x/ day

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