Good news! You burn calories just by being alive… so even in your sleep, your body is working away to use those tacos you love for fuel. The baseline amount of calories that your body burns at rest (aka existing) is called your Basal Metabolic Rate (BMR). If you are actively trying to lose, gain, or maintain your weight you should know what your BMR is – that way the amount of calories you eat in a day can be customized to you, rather than the 800 calories your coworker swears works for them.
Your BMR can be calculated in a variety of ways.
There’s a very basic calculation of:
24 X Bodyweight (kg)
To more complex versions using your height, weight, and age:
Harris- Benedict Equation
BMR = 66.47 + (13.75 x weight in kg) + (5.003 x height in cm) – (6.755 x age in years)
BMR = 655.1 + (9.563 x weight in kg) + (1.85 x height in cm) – (4.676 x age in years)
Mifflin – St Jeor
BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) +5
BMR = (10 x weight in kg) + (6.25 x height in cm) – (5 x age in years) – 161
And a formula that uses your lean mass, but doesn’t consider gender or age:
BMR = 500 + (22 x kg of Lean Body Mass)
Each equation has proven been proven accurate to some degree – there is a margin of error of course. Because the equations don’t use all of the factors that can affect your BMR, I prefer to calculate my BMR using at least two of the equations and then taking the average.
After you’ve calculated your BMR, you need to add in any activity you do during the day – because like it or not, you can’t lie in bed all day <sigh>. Daily activity is measured on a scale:
- Sedentary — Desk job and little to no exercise = BMR x 1.2
- Light Activity — You may stand up during the day and lightly exercise a few days/week = BMR x 1.375
- Moderate Activity — Active day job (retail, sales, waitress, teacher/daycare) and you have moderate exercise 3 to 5 days/week = BMR x 1.55
- Very Active — Labor intense work and hard exercise 6-7 days/week = BMR x 1.725
- Super Active — Very intense exercise or training (triathlon, marathon or intense two a day sessions) plus active job = BMR x 1.9
Keep in mind that if you are trying to lose weight, it is better to err on the side of underestimating your calorie burn, rather than over estimating. The trick in figuring out your activity level is the combination of your job plus exercise – what if you have a desk job, but have very intense workouts? Since you spend most of your day at your desk job and only a few hours exercising, I would lean more towards the Moderate multiplier.
Knowing how many calories your body needs to function on a very basic level is extremely important. Losing weight is accomplished by burning more calories than you consume (barring some health conditions), but you also need to eat a minimum amount of calories to keep your body nourished and healthy. If you are ever “prescribed” calories by an app, friend, trainer, or even Doctor, make sure that your BMR was calculated first. One, it will confirm that your diet is actually tailored to you, and two, it may show that this person is at least somewhat knowledgeable (I make no guarantees about anything else they say). It is much safer to add extra activity to burn more calories than it is to eat less than your BMR – consuming less than your BMR long term can lead to decreased metabolism *, reduced reproductive function in females (amenorrhea)**, and even bone loss/bone weakness ***. Why not take a walk during your lunch or break, take the stairs instead of an elevator or do some body-weight exercises in your living room to burn a few more calories rather than risk your health?