Calculating your calorie needs: a quick guide

When to use the calculators?


Knowing your TDEE/personal  energy &need* helps you understand what your body needs to maintain weight 每 and why you gain or lose.

The 5:2 fast day limits of 500 or 600 calories on a Fast Day are based on eating a quarter or 25% of what the &average* woman or man needs to eat to keep their weight stable - but how many of us are &average*?

Being taller, shorter, heavier or more/less active than average will affect how many calories you need.

Using these calculators will also help you understand how much you can eat on a non-fast/normal day without undoing your hard work on a fast day. I don*t recommend calorie-counting every day 每 but knowing your BMI and your TDEE (Total Daily Energy Expenditure) helps give you a guideline and track progress.

TDEE calculator

Your TDEE or Total Daily Energy Expenditure is how many calories you need each day to maintain your current weight.  By eating at around that level 5 days a week 每 and then fasting for 2 每 you*re creating a calorie deficit 每 or shortfall 每 which means your body has to burn fat for energy - so you lose weight.

To work out your Fast Day Limit: simply use the calculator below to work out your estimated &total daily energy expenditure*.

THEN divide by 4 to get a personalised fast day guideline. For most of us, this will be fairly close to 500 for women/600 for men. It*s simply a guideline 每 if you exceed it by up to 10%/50 calories,  you should still lose weight! 

TDEE calculator

Why is your TDEE useful?

  • It gives you a guideline for how many calories you can eat on non-fasting days without gaining weight: by also fasting twice a week, you will create a calorie deficit/shortfall, so you lose weight.

  • That doesn*t mean calorie-counting every day, unless you really want to. Most people use their TDEE as a guideline; we naturally eat less on some days, and more on others.

  • It*s also helpful if you*re fasting but not losing weight 每 use your TDEE to check you*re not consistently over-eating on your non-fasting days, which could undo the good work on fasting days.

  • For much more about enjoying your food and losing weight, read the 5:2 Books, which all explain how to balance your eating habits.

BMI calculator

BMI - Body Mass Index - is a basic calculation of whether you are a healthy weight or not (18-25 is the number you are aiming for 每 lower, and you are underweight, higher and you may be running a health risk by being overweight or obese). It*s a useful guideline but does have limitations, as it doesn*t recognise lean mass i.e. how muscular you are.

Calculating your BMI before starting 5:2 or The Dirty Diet每 and then on a regular basis after that 每 will show your progress.

Keep a note of your BMI so you can see what you*ve achieved.


  • If in doubt, underestimate your activity levels 每 we can all be a little, um, optimistic about how active we are

  • These are estimates, so you don*t need to be obsessive about the numbers: they are a tool to help you make good, healthy choices

Real Life Examples

Jean, 62

is 5ft/60inches/152cm tall 每 and weighs 157 pounds/11 stone 3 pounds/71.2kg 每 after a recent injury, she*s had a break from swimming, which she loves, and her BMI of 30.8 puts her just into the obese category. Her height, age and activity levels mean she*s burning less energy than average, so her TDEE using this calculator is 1430, lower than the average for women of 1,800-2,000.

Recommendation: she can aim for an average of around 1,400-1,500 or less so Jean doesn*t cancel out the calories she &saves* on her Fast Days as it*s only by creating a deficit that she*ll lose weight. If she can only add a couple of walks or gentle swims to her week, she will have more to play with - up to around 1,700 on non-fast days.

Mark, 31

is 6ft/72 inches/183cm tall and weighs 210lbs/15 stone/95.3kg 每 he works in a very physically active job on a building site. His age, height and physical activity levels mean his TDEE is around 3,900, way higher than the average for men of 2,200-2,400 calories...
Recommendation: Mark can eat up to an average of 3900 on his non-fast days and still lose enough weight to bring him into a healthy BMI. That*s a pretty large allowance, but to speed up weight loss, he could easily aim for 3,000 and make very rapid progress. I*d also suggest he starts Fast Days at up to 800-1000 calories as he has such active working days.

Anita, 22

is 5ft 8/68 inches/173cm tall and weighs 190lbs/13 stone 8/86.2kg 每 that makes her at the higher end of overweight. She*s started exercising intensely at least four times a week to help her weight loss regime and her height, age, weight and activity levels mean her TDEE, at 2900, is higher than the average for women of 1,800-2000 calories.

Recommendation: to help her weight loss, Anita could still aim for around 2,000-2,500 on non-fasting days which will allow her plenty to eat, without feeling hungry, but speed up weight loss. She might also try a slightly higher fast day limit if 500 is challenging.

Should I adjust my calorie limit on Fast Days?

The limit of 500 for women and 600 for men is based on around 25% of the &average* TDEE.For most of us, the averages work just fine. If you*re much taller, heavier or more active than average, it may be worth allowing yourself slightly higher limits on a Fast Day 每 just divide your TDEE by four.If your TDEE is lower than average, you can limit your Fast Day intake if you like, but it*s unlikely to make a significant difference to your weight loss if you cut your limit to 450, for example, rather than 500, and may make meal planning that bit harder.

Is the TDEE completely accurate?

It*s an estimate: in addition, there are different formulae used which may mean your estimate varies by up to 150-200 calories with different online calculators.The trick with 5:2 is not to obsess, but to use the TDEE as a guideline on how much fuel your body needs. That, and learning to listen to your body, will help you make the best choices when it comes to staying in good, healthy shape.

PS: if you*ve used the calculator before, you may notice you*ve got a different result. I*ve recently changed the calculator to a more accurate one, but don*t fret too much if you are losing weight!