Updated: Sep 26, 2020
Your Basal Metabolic Rate (BMR) is the amount of energy that your body uses at rest to sustain its normal functions - to breathe, beat the heart, circulate blood, maintain body temperature, digest food, build and protect tissues (muscle, fat and bone) and perform other vital functions. Think of it as your body's minimum fuel requirement to maintain business as usual.
Your BMR accounts for about 50 to 80 per cent of your daily calorie needs. It ranges from around 1100 to 1800 calories, depending on your age, gender and build.
In addition to BMR, your body uses at least another 500 to 700 calories to fuel normal, light activities, more if you are highly active. The precise amount of calories you need each day will also depend on your individual metabolism - some people have a faster metabolism than others.
How useful are BMR calculators?
There are ways to actually measure a person's BMR, but it's far more common to use a calculator that follows certain formulas to estimate it.
Think of it as your body's minimum fuel requirement to maintain business as usual.
However, your BMR can vary dramatically from another person who is the same age, height and weight as you. This is because there are many variables that affect how your body uses energy.
In fact, BMR calculators can be far from accurate. Scientists have compared BMR estimates based on mathematical equations to the actual, measured BMR of particular subjects and found that the calculations show a 60 to 65 per cent margin of error.
That means if a PT has plugged your data into a calculator and told you that 'based on your body composition, your BMR is X, therefore your calorie requirement per day is Y,' and then used this to put you in either a calorie deficit (for weight loss) or calorie surplus (for weight gain), there’s a good chance that the calculations are far from exact. That's a lot of room for error if BMR is your primary tool to assess your caloric requirements.
Your BMR can vary dramatically from another person who is the same age, height and weight as you. This is because there are many variables that affect how your body uses energy.
Should you dismiss BMR entirely? Not necessarily. It can be useful to consider your estimated BMR, but keep in mind that the result is just that - an estimate only.
Treat it as just one tool in your body recomposition tool box. Don't blindly abide by it.
Sure, there are plenty of people who see fantastic results based on the use of BMR (and TDEE) calculations. But to me, that's beside the point. Even if it works wonders, a diet based on a BMR formula won't help you to understand your body. In fact, it is often a distraction from what really matters - your unique metabolism, body and circumstances. An equation can't account for the many factors in your life that have created the body that you have today.
When should I use a BMR calculator?
That said, it can sometimes help to refer to a person's estimated BMR for a couple of reasons.
First, a BMR estimate is an indicator of under eating. If you subsist on a 1000 calorie a day diet for a decent chunk of the year, then it is a sobering reality check to realise that you deny your body the amount of fuel that it needs as a bare minimum. If a general formula indicates that your body can't properly fuel its normal functions, can you really expect it to look and feel fantastic?
Treat it as just one tool in your body recomposition tool box.
Second, it's a general clue about the amount you need to eat to put on muscle. If you want a lean and sculpted physique, you can't live on a rabbit's diet. At some stage in the year, you'll need to dedicate a reasonable amount of time to training for hypertrophy and sustaining a caloric surplus. For many people, this isn't as much fun as it sounds. You can't miss a meal because you're busy. You might lose your appetite sometimes. And you can't just rely on empty calories from ultra processed foods either - your body needs a sufficient supply of nutrient-dense foods to build and sustain muscle tissue. If you have been a serial dieter, it can be confronting to embrace the quantities of food you may need to eat to build the shape and size that you're after. Your dream boulder shoulders and sculpted glutes aren't going to build themselves.
If a general formula indicates that your body can't properly fuel its normal functions, can you really expect it to look and feel fantastic?
Take home message
The only way to really identify and assess your unique caloric needs is to listen to your body. BMR is useful as a guide to help you find a baseline. But ultimately, you'll need to put in the time and effort to assess your energy requirements. It may be a lot more than you think!
The resting metabolic rate (RMR) is a different formula that also measures how many calories the body burns at rest. It is usually a slightly higher reading because it also takes into account the caloric requirement of light activities (like walking around the house). It is said to be less accurate than BMR due to looser test conditions. Ultimately, if you don't actually measure your BMR or RMR, the distinction doesn't really matter.
 For more information about this research and the metabolism, listen to the ATP Science podcast episode on Boosting Your Metabolic Rate.