The maths involved in working out how much to eat is a little bit complicated if you’re not a huge fan of numbers but I’m a complete geek and love maths. So for me, this blog post is fun.

There are a few steps involved and it’s a little bit long winded but for most people it does add up really well and works effectively so it really is worth doing and sticking to. For people like me who have crazy thyroid functions, it probably isn’t very accurate and although I still stick to these numbers, my weight loss is largely dependant on my thyroid function being somewhere within a normal range!

The first step is to work out your basal metabolic rate (BMR). This is the amount of energy your body would burn if you laid totally still for a day without consuming anything.

The equations I use to do this differ a bit for men and women. And I will explain below how to do the calculations for anyone wanting to know where the numbers come from but I use the Mifflin-St Jeor equation to determine BMR which has been found to be the most accurate, and you can read a little more about it’s origins, or use an existing online calculator to skip the first part of our calculation here if you would like.

To calculate your BMR you need to know your weight in kg, your height in cm and your age.

**Men’s BMR** = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) + 5

**Women’s BMR** = 10 x weight (kg) + 6.25 x height (cm) – 5 x age (years) – 161

(Remember to use BODMAS when doing the equation)

So for me, I’m a 65kg, 170cms tall, 40 year old woman, the calculation would be….

BMR = 10 x 65 + 6.25 x 170 – 5 x 40 – 161

= 650 + 1062.5 – 200 – 161

= 1351.5

Basically the energy my body uses to lay still for 24 hours is approx 1350kcals.

The next thing to work out is your total daily energy expenditure (TDEE). This is the total number of calories you burn every 24 hours, taking into account your lifestyle and activity level. There are lots of ways of working out your TDEE.

Wrist activity trackers being the most common but also the most inaccurate. (Probably not want you want to hear when you’ve just put on the brand new Fitbit or other fitness gimmick you’ve received for Christmas, sorry). And exercise machines are shocking at over estimating your calorie expenditure during a workout.

Fitness trackers with the chest strap to monitor your heart rate are a little more accurate at estimating your TDEE but are expensive and not really required unless you have medical issues like I do and need to monitor your heart rate.

Maths is again the best and most accurate way of doing it.

In general rather than calculate a different TDEE for each day, I just work out my average. It means some days I’ll be in a calorie deficit and others in a slight surplus but it’s easier to remember and plan if I have each day the same and it averages out pretty well.

To work out your TDEE you multiply your BMR with an activity multiplier that is dependent on your activity level. Most people severely over estimate their activity level so if your aim is weight loss, choose the multiplier for your minimum weekly activity level rather than your maximum or average, you can always make adjustments later if you are genuinely struggling to maintain the calorie restrictions. 🙂

**Here’s how to work out your TDEE….**

For sedentary days, days when you are not very active, you should consume around 115% of your BMR. So TDEE = BMR x 1.15

For lightly active days, where you do about 30 minutes of vigorous physical activity or about an hour or light activity, you should consume about 125% of your BMR. So TDEE = BMR x 1.25

For moderately active days, about an hour of vigorous exercise or around two hours of light exercise, you should consume around 145% of your BMR. So TDEE = BMR x 1.45

On days when you are very active and do around 90 minutes of vigorous activity or about three hours of light exercise, you should consume about 165% of your BMR. So TDEE = BMR x 1.65

And on days when you are extremely active and do around two hours of vigorous exercise or several hours of light exercise, you should consume around 185% of your BMR. So TDEE = BMR x 1.85.

Rather than calculate my TDEE each day I average my week out. And use the multiplier for moderately active as around five days per week I do at least 90 minutes of vigorous activity and some light activity and then the other two days I do around 60 minutes of light exercise.

So my TDEE = 1351.5 x 1.45 = 1960 (I’m rounding up now rather than working with lots of decimal places).

The next part of working out how many calories you should eat is totally dependent on what your goals are! If you want to maintain your weight then you need to consume roughly the same number of calories you are burning. So your calorie intake would be the same as your TDEE. For lean bulking, you’d eat slightly more. But as almost everyone that has spoken to me about diet changes recently has expressed they want to lose fat, I’m going to just focus on that.

When I’m doing fat loss plans my recommendation is to eat around 75% of your TDEE. Despite what a lot of people say, bodies don’t go into starvation mode and gain weight when we restrict calories. If you consume less calories that you use, then you will lose weight. However if you restrict calories to much, you also lose muscle, which isn’t a good thing and can have a negative effect on the body and how it looks and performs. Reducing calorie intake by about 25% has been shown to help lose fat slowly whilst still maintaining the ability to build muscle if strength training is also undertaken.

To work out the total number of calories to consume to lose fat multiply your TDEE by 0.75.

Target total calorie intake= TDEE x 0.75.

So for me my target total calorie intake is 1960 x 0.75 = 1470.

And that’s it, that’s how much I should it to lose fat. I like food and to be able to maintain a diet, I have a break at weekends. I don’t spend the whole weekend consuming everything I possibly can, I still try to make my calorie intake less than my TDEE so I’m not counteracting my efforts throughout the week, but it allows me more scope to indulge a little and accept that my diet doesn’t have to be perfect to make progress. My progress is slower because of my weekends but it’s also more sustainable for me, which is the important part!

The next thing I would work out if I was working with personal training clients is their macronutrient ratio, the proportion of their calorie intake that should come from proteins, carbohydrates and fats. But it can get very confusing to work out as its largely dependant on what your aims are, what your lifestyle is like, and what training you are doing. So I’m not prepared to go into macro nutrient ratios on here as this blog post is already long and there’s just way to many variables to take into account.

I personally work on a 40/40/20 ratio and try to get 40% of my calorific intake from proteins, 40% from carbs and 20% from fats. I struggle to hit my protein goals but it’s a target and it’s been working for me but it isn’t a one size fits all.

Also despite the obsession dieters and trainers have with macronutrient ratios, recent research suggests that, for fat loss and body composition, macronutrient ratio doesn’t have a huge impact, providing over all calorie intake is controlled. So it’s a lot more important that you are in a calorie deficit if you want to lose fat, than it is to be obsessing over macros. **If you can’t achieve a calorie deficit, you will not lose fat regardless of what fancy macronutrient ratio you have in place.**