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

This BMR Calculator is a simple tool that helps you calculate how many calories your body needs if you were only to rest for the whole day. Based on your age, height, weight, and gender, the Basal Metabolic Rate Calculator returns your BMR score

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. 


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.