How Do I Calculate My BMR? Understanding Basal Metabolic Rate
Losing weight is challenging for many people because they can be misinformed about how weight loss truly works.
In its simplest form, losing weight comes down to a mathematical equation of subtracting calories consumed from calories burnt.
The biggest factor influencing our ability to burn calories and lose weight is our metabolism.
In many ways, your metabolism is a like a computer; program the computer to work for you and not against you, and you’ll lose weight naturally.
That’s why the key to understanding calories and weight loss comes down to calculating our basal metabolic rate, or the rate at which we burn calories at rest.
In this article, you’ll learn everything you need to know about your basal metabolic rate, how to calculate it using a BMR calculator, and some factors that influence your BMR.
What is Basal Metabolic Rate (BMR)?
At its most fundamental principle, if you want to lose weight, you need to burn more calories than you consume. Therefore, calculating how many calories you burn at rest with or without exercise can establish a benchmark for how many calories you need to eat each day under your BMR.
Once your body exhausts all of its fuel through food and water, it will break down fat stores in the body to convert them to energy. However, consuming too few calories can actually be counterproductive, as we’ll explain.
Dr. Matthew Kulka recommends consuming between 200-300 calories less than your BMR daily to hit that sweet spot for weight loss and not risk lowering your metabolism too low.
How Does BMR Factor into Weight Loss?
The human body transforms food and water into energy through the process of metabolism. While some people think of metabolism as a muscle in your stomach, it’s a core body function carried out by each cell in the body.
Your basal metabolic rate (BMR) calculates the number of calories your body needs to run while at rest.
In scientific terms, your basal metabolic rate measures the number of calories your body burns at rest when the digestive system is inactive in a neutral climate.
Our bodies need energy to run basic core functions, such as pumping blood, breathing, and all of the other unconscious processes we perform. As a result, it’s estimated that 60-70% of the calories you burn daily come from basic bodily functions.
For example, your body will burn more calories from regulating ion levels in the blood than it will from flexing a muscle.
Since most of your calories are burned through metabolism at rest, calculating your BMR significantly impacts how efficiently your body can lose weight.
How to Calculate Basal Metabolic Rate
To calculate your BMR, we use a specific formula that gives us the best estimate for your BMR. Unlike measuring your blood pressure, there is no instrument other than a calculator you can use to calculate your BMR.
Most calculators use the Mifflin St. Jeor formula or the Harris-Benedict equation (Seriously Simple Steps allows for both) to calculate a person’s BMR accurately.
Generally, we divide the BMR formula by sex, so men and women will have different equations to properly calculate their BMR.
Male Equation: Mifflin St. Jeor
(4.536 × weight in pounds) + (15.88 × height in inches) − (5 × age) + 5 = BMR
Example: 6’ male, age 38, weighs 245 lbs.
(4.536 x 245) + (15.88 x 72) – (5 x 38) + 5 = 2,069 calories per day
Female Equation: Mifflin St. Jeor
(4.536 × weight in pounds) + (15.88 × height in inches) − (5 × age) − 161 = BMR
Example: 5’ 5” female, age 52, 175 lbs.
(4.536 x 175) + (15.88 x 65) – (5 x 52) = 1,405 calories per day
The Mifflin St. Joer formula is considered the most accurate to date. However, you are welcome to use the Katch-McArdle Formula if you know your body fat percentage.
However, your BMR may not factor in calories burnt through exercise or your daily routine, which is why we often use total daily energy expenditure (TDEE) in tandem with BMR.
TDEE, RMR, and BMR: What’s the Difference?
Before we touch on TDEE, we need to discuss resting metabolic rate (RMR), otherwise known as resting energy expenditure (REE). This measurement is virtually the same as BMR and tells you how many calories your body consumes at rest.
The only difference between RMR and BMR is that RMR accounts for food digestion, increasing energy expenditure by up to 10%. For the purposes of weight loss, calculating your RMR will not be necessary.
On the other hand, if you want to factor in how many calories you burn through exercise and physical activity, you’ll need to calculate your TDEE.
Fortunately, your TDEE uses the same formula as your BMR, except with a multiplier to account for exercise levels.
In short, you will take your BMR and multiply it by the following values based on your estimated level of exercise:
- Low exercise or only one day per week = BMR x 1.2
- Semi-moderate exercise of 1-3 days per week = BMR x 1.375
- Moderate exercise between 3-5 days per week = BMR x 1.55
- Heavy exercise between 6-7 days per week = BMR x 1.725
- Extreme exercise daily = BMR x 1.9
So if we take the BMR from the male in the previous section and say he has been hitting the gym about 2-3 times per week, we can calculate his TDEE at roughly 2,882 calories (2,069 BMR x 1.375).
Your TDEE is a rough estimate, which is why we use BMR as a better indicator of hitting your caloric goals.
Factors that Impact BMR
- Once you calculate your BMR, the question becomes: is BMR static?
Your BMR will fluctuate based on weight and other factors that impact our BMR formula.
In general, the following factors will impact your BMR calculation:
- Weight: BMR will change with your weight. As you lose weight, your metabolism will shrink so your body can consume enough energy to stay active. This is why many people may have rebound results after losing a lot of weight.
- Age: While conventional wisdom has told us that our metabolism slows with age, a recent study published in Science has challenged this notion. However, age may impact our activity levels, which will alter our metabolism over time.
- Fat-free mass and fat mass: The ratio of fat-free mass to fat mass directly impacts your BMR. Since muscles require more energy to function, having a higher muscle mass will boost your BMR.
- Exercise Level: Exercise can improve muscle mass, which boosts BMR and your TDEE.
- Genetics: Genetics slightly influences your BMR, but this can be altered through exercise and diet.
- Sex/gender: Women tend to have much lower metabolisms than men.
- Hormones: Hormone regulation from your thyroid gland can impact your metabolism.
- Diet/Drugs: Different foods require greater energy to digest and store, directly impacting your BMR. In addition, drugs can also impact your hormones, insulin levels, and other factors that contribute to your BMR.
- With these factors in mind, you can discover ways to boost your metabolism to work for you and burn more calories at rest.
How to Boost Your Metabolism
- Exercise: Engaging in moderate exercise can significantly boost your metabolism and TDEE so you can eat more and still lose weight. Even walking for a half hour a day can make a big difference.
- Drink water: Unlike sugary drinks, which increase your calorie intake, water doesn’t add extra calories, and it forces your body to burn more through digestion. In turn, water is a natural way to boost your REE.
- Get good rest: Sleep regulates hormone levels, organ function, blood pressure, and several other factors that impact our metabolism. For example, studies show that sleep reduces hunger, regulates insulin, and is when you burn most of your calories.
- Eat high-protein foods: High-protein foods that fuel muscle mass require more energy to break down and store, thus improving your BMR.
Is it Harmful to Eat Below Your BMR to Lose Weight?
While severe calorie deficits can lead to malnutrition and extreme thermogenesis, eating below your BMR is not dangerous. In fact, your body will respond by burning more fat naturally.
Unless you have a lean body type, eating below your BMR is not a cause for concern.
However, extreme caloric deficits can often be counterintuitive, and we’ll explain why.
Does Losing Weight Slow Your Metabolism
Unfortunately, extreme calorie deficits can lead to a condition known as adaptive thermogenesis. In essence, the more weight you lose, the more your body will slow down your metabolism to keep itself functioning.
In turn, it can be more difficult to keep weight off the more rapidly you lose it. For example, in a study conducted on contestants of The Biggest Loser, 13 out of 14 contestants put the weight they lost back on.
That’s why Seriously Simple Steps takes a progressive approach to weight loss, only encouraging a mild caloric deficit under your BMR, paired with other nutritional steps.
To recap, your BMR is a calculation of the number of calories you burn at rest. Since you burn most of your calories at rest, calculating your BMR can help you establish daily calorie goals to lose weight.
There are several ways to boost your BMR through exercise, sleep, and other helpful factors. However, it’s essential to go about dieting and caloric deficits in a slow and controlled manner, lest you risk slowing down your metabolism and regaining all of the weight you lost.
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