Calculating your Resting metabolic rate


Let's get started on being VERY specific on how to lose weight. This article is going to be a bit scientific and mathematics driven so bear with me. The bottom line is, if you want to be successful at weight loss (and I know I do), then you need to learn how this works from the ground up. Ready? Let's go.

The first thing to realise is that all weight loss stems from a very simple formula called the Energy Balance Equation. It looks like this:

Energy Balance Equation = Calories in - Calories out = your results.

Furthermore, you need to understand that energy (in this case as a calorie) is never created or destroyed, it is simply transformed, This is the 1st Law of Thermodynamics. Why it is relevant is that the calories you consume are either metabolised (used by your body) or stored for later use. Normally in the form of fat.

Yes, it's really that simple. It's pure physics. What it means is this:

  • if you have a positive calorie balance and you will gain weight
  • if you have a negative calorie balance and you will lose weight.
  • if you have an equal calorie balance you will maintain your weight.

Note: When I refer to a calorie, I am referring to a Kilocalorie just to make it easier and follow convention.

In order to make weight loss effective you need to understand and quantify both sides of the Energy Balance Equation or both calories in (your diet) and calories out (your metabolism). This article is about your calories out. By knowing how many calories we can be expected to burn every day we can adjust our calories. Let's look at the components of metabolism.

Your metabolism consists of 3 parts, your resting metabolism, your physical activity and the thermic effect of food. It looks like this:

Now lets talk about these 3 things in order of simplest to most complicated.

Thermic effect of food (TEF)

This can account for up to 10% of your daily calorie expenditure. What it means is that the harder to digest your food is, the more calories will be spent on digestion. It is quite difficult to affect (and measure) this, but if you are eating a diet high in protein and complex carbohydrates with plenty of fiber from your veggies, you already got this covered. However there is a limit to how many calories are burned here. For my calculations, I like to disregard trying to calculate this and use it as a bonus to my equation.

Physical Activity

This one is very straightforward too. Physical activity can account for 15-35% of your daily calorie burn. I defined this previously as having two parts passive and active activity. In essence Passive is what you do every day, such as walking, housework, working, etc. and Active is when you choose to exercise, either cardio or at the gym. It would stand to reason that the more active you are, the more calories you will burn. This is referred to as your Voluntary Metabolism (VM).  This article explores the calorie expenditure of various activities including exercise so you can more accurately estimate your calorie burn. Look for that soon.

Resting Metabolic Rate

Here is where we get into the nitty gritty, and is the most calorie burn in your day. Your body needs calories for all its basic processes, such as digestion, breathing, heartbeat, nervous system and your organs. Your Resting Metabolic Rate (RMR) accounts for 60-70% of your total expenditure, so we need to understand this. Your metabolic rate is variable depending on age, gender, genetics, height, weight, lean body mass and hormone activity. In order to successfully effect the Energy Balance Equation you need to know this number. There are 2 main ways to calculate this.

Lean Mass Equation

This equation requires you know your body composition accurately in terms of fat-free mass. It is gender and age non specific so does have some accuracy issues. It look like this:

RMR = (21.6 x Fat Free Mass in Kg) + 370

For me this looks like:

RMR = (21.6 x 62.3) + 370

RMR = 1345.68 + 370

RMR = 1715 calories per day

Harris Benedict Equation

This equation is slightly more accurate in terms of using multiple data points, however fails to identify lean mass (which is more metabolically active than fat mass). Instead, it uses instead predictive norms of the general population (American by the way) when it comes to body composition. It looks like this:

Males: RMR = 66 + (5 x height in cm) + (13.8 x weight in kg) - (6.8 x age)

Females: RMR = 655 + (1.8 x height in cm) + (9.6 x weight in kg) - (4.7 x age)

For me this looks like:

RMR = 66 + 890 + 1028 - 299

RMR = 1685 calories per day

So you see there is quite a difference. of 30 calories. Let's just take an average or about `1700 calories per day, which falls within the predictive norms for my weight and age.

Note: If you want to shortcut this whole thing, a Bioelectrical Impedance Analysis using a machine like an Inbody will give you a pretty accurate evaluation of your RMR. my latest BIA gives me a value of 1715 calories per day. ( I think they are using the Lean Mass Equation) So it seems I am in the ball park..

So your Calories Out part of the Energy Equation will be:


or in english:

Daily Calorie Expenditure = Resting Metabolic Rate + Voluntary Metabolism

Do note we are going to exclude the Thermic Effect as stated above. I think it gives you a bit better margin of error. Hopefully positive. Haha.

Anyway let's say I hypothetically burn 1000 calories a day from my VM.

It would look like this:

DCE = 1700 + 1000 = 2700

That means I can expect to burn 2700 calories per day.

Figuring out how much calories to cut

Now there is a lot of different opinions here, but I like to think that the easier any change is the better you will adhere to it. So for example I would cut around 10% (around 300) of my calorie intake to make sure I was in a Negative Caloric Balance. These are the magic words for weight loss. For me this would mean restricting my diet to about 2500 calories per day. Or if I wanted to be more aggressive, I would go for cutting 500 per day or  about 2200 calories.

Now this can and should be adjusted up or down depending on your results. You need an accurate scale and need to keep track of your weight each week to see if you are losing, going up or staying the same. 

Note: There can be a significant margin of error in this calculation, mainly do to the VM part. It is hard to calculate actual calories expended. I will detail this in my upcoming as already spoken.

Now that we have an estimate of our calories out we can then start to worry about our calories in. This is the next part or the Energy Balance Equation.

Got a question about this? Shoot me a line or a comment. I am happy to answer. 

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