There have been a few reasonably successful methods of calculating metabolism, or rather your RMR – Resting Metabolic Rate – but the currently accepted one is known as the Mifflin-St Jeor method (Frankenfield study).
It is widely regarded as the most accurate method for anybody to calculate their RMR to within a maximum deviation of 10% from the true value (true value measured by detailed analysis).
If you want to use your calculated RMR in order to eat less calories than you burn -and thus, lose weight – then the resting metabolic rate is only part of the picture.
In addition to the RMR, there is the factor of your spontaneous physical activity (taking stairs, chewing gum, fidgeting etc.) and your physical activity (aka exercise).
The sum of these values, each presented as kcal (commonly called calories), is your Total Daily Expenditure, or, TDEE.
Again, calculating your numbers will be based on averages and estimates but it generally comes in under 10% error, which means you can even subtract a little at the end to be conservative and leave a little extra breathing room.
Calculating Resting Metabolic Rate – RMR
The RMR is also known as the BMR, or Basal Metabolic Rate. It’s basically the energy required for you to maintain all the chemical reactions sufficiently to function at rest.
It’s measured in kilocalories, which is usually just referred to as calories.
The Mifflin St-Jeor equation is also simple to use and doesn’t require any measurements beyond knowing your weight, height and age.
First, get prepared. Measure your weight in kilograms. Measure your height in centimetres. And, of course, phone your mum to make sure how old you are.
The formula for Women:
(10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161
The formula for Men:
(10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) + 5
Here’s a couple examples of the results, which will also allow me to use them throughout the article. I’m going to round up or down by a tiny amount here or there to make them easier numbers to deal with.
A woman weighing 76 kg (168 lbs), measuring 163 cm (5 ft 5 in) and 30 years of age has a 1470 calorie RMR.
A man weighing 80 kg (176 lbs), measuring 182 cm (6 ft) and 30 years of age has a 1800 calorie RMR.
At this point, if you want to, you could subtract a full 10% of the value you calculate for yourself in order to get a more conservative estimate of your RMR.
A conservative value means that when you eat in calorie deficit (to lose weight) you will likely lose weight faster.
Ultimately, the calculation assumes averages, therefore if you exercise a lot and know your RMR is probably slightly higher than this (due to muscle mass) then keep it at the value you reach from the equation.
Sedentary People
A sedentary lifestyle is one of minimal activity. You can assume it’s where someone moves, or is active, only when they need to be, and not by choice.
This means that we can do away with the physical activity aspect of the Total Daily Energy Expenditure and only modify the number for spontaneous activity, bearing in mind that stairs will not be taken if there is an elevator, and other similar instances.
A sedentary person should only really add 20% of their RMR to come up with their TDEE.
In the case of our example woman, that means her TDEE is 1763 calories if she leads a sedentary lifestyle.
In the case of our example man, it means that his TDEE is 2153 calories.
To maintain bodyweight, our sedentary woman can consume 1763 calories per day and our sedentary man can consume 2153 calories.
So in order to lose weight, these guys would want to consume somewhere between their RMR and their TDEE.
And for sedentary people, that’s not a huge margin, which is one of the reasons it is quite difficult to lose weight as an inactive person and not feel weakened by the dieting process.
Active People
Activity level is basically a sliding scale all the way from mildly active, a notch above sedentary, to heavily/extremely active, which is somewhere in the region of professional athlete and obsessive.
Occupational activity should be included if your job requires more physical exertion than the average. Someone who does physical labour all day long can put themselves in the heavy to extreme category.
For this reason it’s the most variable of the factors making up your TDEE.
The extremely active person can almost double their RMR and multiply it by about 1.9. Using our male example from above, he would have a TDEE of about 3410 calories.
Considering the sedentary person applies a multiplier of only 1.2 to their RMR, you can see that there is a factor of 0.7 x RMR between the two extremes.
With this value you can apply your own modifier to your RMR and determine a decent estimate of your TDEE. Below is in example of 3 levels of activity and the multiplier to their RMR.
TDEE based on activity level
- Sedentary = RMR x 1.2
- Moderately Active* = RMR x 1.55
- Extremely Active = RMR x 1.9
*I’ve chosen “moderately active” as the middle level as this applies to a great number of people who exercise for an intense 30 to 60 minutes about 4 times a week.
As you can see the Moderately Active group sit halfway between sedentary and extreme, simply by dividing the difference between the two extremes by half.
Of course, your activity level is highly individual, and nothing can determine the absolute number. The best you can do is make a conservative estimate and go from there.
Your Number and How To Use It
My hope is that if you have gotten this far, then you understand the basics of calculating a reasonably accurate value for your Total Daily Energy Expenditure.
It’s that end modifier based on your activity level that will cause the most confusion, but you are in the best position to determine it.
To begin with, take that moderately active example of 4 hours of hard training in the gym per week and decide whether you are above or below that. And no, walking on the treadmill doesn’t count unfortunately.
Stay on the conservative side of your estimation and you are more likely to come out with a number that is actually below your TDEE.
That’s a good thing because however many calories less that you consume per day, the margin is likely to be even bigger and you will achieve greater success.
A good starting place for a successful weight loss diet is to consume 20% less calories than your TDEE value. It’s sustainable and also provides visible results before long, which helps with motivation.
For our moderately active woman who it turns out has a TDEE of 2278 calories (non-conservative), she would want to eat about 455 calories less than TDEE, which equates to 1823 calories per day.
In order to preserve muscle mass and maximize fat burning, a high proportion of those calories consumed should be protein.