BMR Calculator (Basal Metabolic Rate, Mifflin St Jeor Equation)

BMR Calculator (Basal Metabolic Rate, Mifflin St Jeor Equation)

Understanding Basal Metabolic Rate (BMR)

Basal Metabolic Rate (BMR) is the amount of energy the human body uses at rest. This energy supports various vital functions, including:

• Breathing
• Blood circulation
• Controlling body temperature
• Brain and nerve functions

The organs consuming the most energy at rest include the brain, central nervous system, and liver. Interestingly, regulating fluid volumes and ion levels throughout the day consumes more energy than the mechanical work of muscle contractions, like breathing.

What is BMR? – BMR definition

Basal Metabolic Rate is the amount of energy a human body uses when it is completely at rest. It’s the amount of energy your body needs to support its vital functions: breathing, blood circulation, controlling body temperature, brain and nerve functions to name a few. The organs that use the most energy at rest are the brain, the central nervous system and the liver. What’s interesting is that, throughout the day, more energy is consumed by the regulation of fluid volumes and ion levels than in the actual mechanical work of contracting muscles (e.g., breathing). We automatically correct concentrations and the amounts of different substances in various areas of our body to preserve homeostasis (a state of the steady internal conditions). Sometimes this requires transporting substances through barriers (e.g., cell membranes) and against a concentration (or molarity) gradient. This means that particles are transported from space, with their low concentration, to space with their higher concentration – a process which requires energy. On a whole body scale, this amounts to a lot of energy. It also explains why our central nervous system consumes so much energy in terms of Basal Metabolic Rate. When a neural impulse is conducted, a lot of different ions change their location. Afterward, they need to be transported back to their original place.

People regularly use more energy than their Basal Metabolic Rate. It is because most people do not spend all day in bed without moving! Walking, running, working, talking, and even digesting are actions that require some extra energy greater than the Basal Metabolic Rate. To achieve such a low expenditure of energy, you have to be physically and psychologically inactive. In other words, you cannot use any muscles or think intensively. You need to be as relaxed as humanly possible. Other necessary criteria include staying in an environment with thermal comfort and not eating for a certain period. The latter condition assures that you will not be using energy to digest food. In a scientific setting, BMR is often measured during a period of sleep.

Your BMR accounts for about 60% to 75% of your Total Energy Expenditure (TTE), depending on your lifestyle and activity level. The Total Energy Expenditure is the total number of calories you burn every day. The rest of your TTE comes from the physical activities (walking, talking, eating, etc.) and food digestion. Physical activities account for about 20% of your Total Energy Expenditure but can vary a bit depending on how often you exercise for and its intensity. Food digestions, or as some say postprandial (after-meal) thermogenesis, uses around 10% of your TTE. BMR tends to decrease with age and with a lower lean body mass. On the other hand, increasing your muscle mass will most likely increase your BMR. We have written more about the different things that influence the Basal Metabolic Rate in the factors affecting BMR paragraph.

Basal Metabolic Rate Calculator

BMR calculator is short for Basal Metabolic Rate calculator. Basal metabolism is a set of chemical reactions that occur in everyone’s body that maintain its living state. We have covered this topic thoroughly in the previous paragraph (What is BMR? – BMR definition). If you know what your Basal Metabolic Rate is, you can estimate the minimal amount of calories you need to take in to live, which helps you to evaluate the total number of calories you should provide to your body daily. Remember, you need to add your energy expenditure (in calories) from other activities like walking, talking, etc, to the number of calories calculated by our BMR calculator. If you add another 10% for digestion you will get your daily Total Energy Expenditure. You may use this information for the basis for your weight loss plan.

But how to use the Basal Metabolic Rate calculator?

1. Firstly, measure your weight and type it into the Weight field of our calculator.
2. Secondly, check your height and input it into the Height field.
3. Now, input your age into the Age field. Please note, that the age value has to be in years.
4. The last thing to do is to select your sex. In the next paragraph, you will how it slightly changes the Basal Metabolic Rate formula.
5. Well done! You have correctly followed all the steps, and now our Basal Metabolic Rate calculator will show your minimal caloric intake!

If you are wondering what your ideal weight should be, visit our ideal weight and body mass index calculators. Would you like to change your present weight? Our macro and protein calculators will tell you what to eat and in what quantity. Finally, our calories burned calculator will keep you motivated during a heavy training session.

BMR and Energy Consumption

• Energy for Homeostasis: To preserve homeostasis (steady internal conditions), substances often must be transported through barriers and against concentration gradients, requiring energy. This explains the significant energy consumption by our central nervous system in terms of BMR.

• BMR and Physical Activities: People typically use more energy than their BMR since most don’t spend the day motionless. Actions such as walking, working, and digestion require extra energy. BMR is often measured in a scientific setting during sleep.

• BMR and Total Energy Expenditure (TTE): BMR accounts for about 60-75% of the TTE, depending on lifestyle and activity level. TTE is the total number of calories burned daily. The remainder of TTE comes from physical activities (about 20%) and food digestion (around 10%).

What Calories You Need Daily to Get Ideal Weight? 

Remember that safe and sustainable weight loss is generally considered 0.5-1 kg (1-2 lbs) per week, which usually requires a calorie deficit of 500-1000 calories daily. A calorie surplus of around 500 calories per day is often recommended for weight gain. Always consult a healthcare professional before starting a new diet or exercise program.

Food Examples to Achieve Caloric Goals for Ideal Weight

Meeting your daily calorie needs can be accomplished with a balanced diet of various foods. Below is a table with examples of foods, their calorie content, and how much you could eat of each to meet different caloric requirements.

How to calculate BMR – the BMR formula

There are multiple formulas used to calculate BMR. Nowadays, the Mifflin-St Jeor equation is believed to give the most accurate result and, is therefore what we used in this calculator. This BMR formula is as follows:

BMR (kcal / day) = 10 * weight (kg) + 6.25 * height (cm) – 5 * age (y) + s (kcal / day),

where s is +5 for males and -161 for females.

We also have calculators that determine your Basal Metabolic Rate based on other formulas. For a long time, the most common way to calculate your Basal Metabolic Rate was the Harris-Benedict equation. It was initially published back in 1919, and for over 70 years it was considered to be the best Basal Metabolic Rate formula available. It was later replaced with another Basal Metabolic Rate formula that turned out to be even more accurate – the Mifflin St Jeor Equation. Nevertheless, you can still find many BMR calculators that use the Harris-Benedict Equation, but they are slowly but surely being replaced with the new formula.

The third known equation is Katch-McArdle formula, used to calculate the Resting Daily Energy Expenditure (RDEE).

In our Basal Metabolic Rate Calculator, we use the Mifflin St Jeor Equation to give you the most accurate BMR score.

BMR for man calculation – an example

In this paragraph, we will go through an example of a BMR calculation for a man. We will have to use the BMR for man formula (Miffin and St Jeor BMR equation for a man):

BMR (kcal / day) = 10 * weight (kg) + 6.25 * height (cm) – 5 * age (y) + 5 (kcal / day)

Let’s suppose that you want to calculate BMR for a 60 year old man, who is 5 foot, 4 inches tall and weighs 150 pounds.

1. To start, we have to convert the values from imperial to metric. We have to do that because the BMR formula has been designed to be used with the metric system only. Five feet and four inches is equal to around 162.56 centimeters, while 150 pounds weighs the same as 68.04 kilograms. Luckily, we measure age the same way in both systems. Please note that you can type pick a certain unit (e.g., feet), and then click on the unit to change the unit (e.g., into meters). Our calculator will perform the transformation for you. This option works in the majority of our calculators, and sometimes it may come very handy.
2. Now, we can input all the data into the BMR for man equation:10 * 68.04 + 6.25 * 162.56 – 5 * 60 + 5
3. The last thing to do is to solve the equation:10 * 68.04 + 6.25 * 162.56 – 5 * 60 + 5 = 680.4 + 1016 – 300 + 5 = 1401.4 (kcal / day)
4. And it’s done! That wasn’t so bad. You now know how to calculate BMR by hand (but it’s probably still easier to just input the data into our BMR calculator)!

BMR for woman calculation – an example

This time we will try to calculate BMR for a woman. We are going to use a slightly different formula – Miffin and St Jeor BMR equation for women.

BMR (kcal / day)= 10 x weight (kg) + 6.25 x height (cm) – 5 x age (y) – 161 (kcal / day)

As you may have already noticed, the only difference between these two formulas is the last part. We add 5 kcal per day for every man and subtract 161 kcal per day for every woman.

Now, let’s focus on an exemplary woman that is 25 years old and is 5 foot, 8 inches tall. She weights 132 pounds. We can proceed with the calculations.

1. Firstly, we need to transform the values from the imperial system to the metric system. You can use the same method we have mentioned in the last paragraph. Five feet and eight inches is 172.72 centimeters and 132 pounds is 59.87 kilograms. Age, again, stays the same.
2. Input the acquired values into the BMR for woman equation:10 * 59.87 + 6.25 * 172.72 – 5 * 25 - 161
3. Solve the equation itself:10 * 59.87 + 6.25 * 172.72 – 5 * 25 - 161 = 598.7 + 1079.5 – 125 – 161 = 1392.2 (kcal / day)
4. As always, we encourage you to check the result with our BMR calculator. As we have made a few approximations to make the calculations by hand easier, there may be a small difference between the results. But don’t worry! It is so small that it will not affect any diet plan.

BMR vs. RMR

When researching Basal Metabolic Rate, you may have also encounter the term RMR. It stands for the Resting Metabolic Rate. As these terms sound very similar and have a very similar meaning, it is very easy to get confused. But don’t worry, we will explain it to you shortly. Resting Metabolic Rate, much like BMR, is also a measure of a human body energy expenditure without performing any additional activities (so at rest). However, there is a slight difference. Resting Metabolic Rate also includes the energy used for the digestion of food.

Our body has to use some energy to transform the food in our gut into substances that can be used by it. As we have already mentioned in the what is BMR? – BMR definition paragraph, food digestion accounts for around 10% percent of your Total Energy Expenditure. Because of that, RMR has a higher value than BMR. We use the modified Harris-Benedict formula, instead of Mifflin St Jeor equation, to calculate Resting Metabolic Rate. For more information on this topic, and an automatic estimation of your RMR visit our Resting Metabolic Rate calculator.

Factors affecting BMR

Each cell, tissue, and organ in your body requires a constant supply of energy to survive. It logically follows that the more your body weighs (and the more cells it has), and the more kilocalories it needs. With increasing weight, your Basal Metabolic Rate will rise as well. However, BMR doesn’t depend solely on weight. In many different studies, researchers found other factors affecting our Basal Metabolic Rate value. Here, we mention some of them:

1. FFM – standing for fat-free mass. It is the weight of your body without the weight of any fat tissue. If you think about it, it makes sense – muscles, even at rest, use up a lot more of energy than fat. In fact, the main function of fat tissue is to preserve energy in the form of fatty acids. A big ratio of fat-free mass to fat-mass increases BMR. Two men may weigh 220 pounds, but if one of them has more muscle due to working out, he will have a significantly higher Basal Metabolic Rate.
2. FM – shortcut for fat-mass. As the name suggests, it is the weight of all the fat in our body. Although much lower, fat-mass also has an influence on our Basal Metabolic Rate.
3. Age – your Basal Metabolic Rate increases with each year as you grow up and as your weight increases. For adults, BMR tends to decrease as they get older. Researchers mainly attribute this to the change in compositions of the human body as it ages; the fat-free mass drops, and thus the Basal Metabolic Rate becomes lower.
4. Sex – men have a statistically higher BMR than women. The reason is that their fat-free mass is, on average, higher. This is why the creators of this equation added a special adjustment to the equation for each sex (plus 5 kcal/day for man, and minus 161 kcal/day for woman).
5. Genetics – every person is slightly different regarding their BMR. Some of these differences can be explained by the rate of your metabolism encrypted into your DNA.
6. Exercise – the amount you exercise affects your BMR, especially body-building exercises (e.g., bench press). When you develop your muscles, you increase the weight of your fat free mass. Remember not to overextend when exercising. Our max heart rate calculator may be useful here since it shows you your maximum healthy heart rate.
7. Body temperature – your BMR increases as the temperature of your body rises. The higher the temperature (e.g. during a fever), the quicker various chemical processes in your body happen. Because of that, there is a higher demand for energy, and thus your BMR increases.
8. Temperature of environment – in colder temperatures your body needs to create more heat to preserve its proper temperature. This leads to an increase in BMR.
9. Hormones – hormones are substances produced by the many glands of the body. Their main task is to regulate the functions of other organs and tissues. The most interesting gland in terms of BMR modification is, without a doubt, the thyroid gland. Hormones excreted by this gland are responsible for regulating the rate of your metabolism. When they are in high concentration in blood, your BMR increases and vice versa.
10. Pregnancy – when a woman is pregnant, there is another organism developing inside her body. Naturally, we are talking about the fetus (the baby). It has its own metabolism and BMR. Because it receives all the nutrition from its mother, we need to add its BMR to BMR of its mother when calculating the basic required calorie supply. If you are pregnant, visit our due date calculator to find out when to expect your child to be born.