Basal metabolic rate (BMR) measures how many calories your body requires in order to function properly. It represents the energy needed to maintain all your bodily functions (even breathing, circulation, digestion, and the like) while you are at rest. What does “at rest” mean exactly? It’s when you are not engaged in any physical activity, like when you are sleeping, seated on the sofa, or simply lying in bed.
It’s always good to know your BMR—and it’s best measured in the lab, especially when accuracy is concerned. Still, you could compute this manually or using a BMR calculator. Unless you are a whiz, it’s suggested that you go with the latter. If you are wondering about the difference in accuracy, well, it’s usually at around 20 percent. Age, body fat percentage, level of activity, and other factors affect the resulting value.
If you are simply tracking your fitness in regular (and rather frequent) intervals, relying on a BMR calculator will definitely suffice. Later you can adjust these results to your body composition. Keep in mind though, that an understanding of BMR doesn’t end with mere numbers. It’s also important to know how it’s different from resting metabolic rate (RMR).
Both terms are often used interchangeably. However, there’s a slight difference between them. In the lab, BMR measurements are obtained shortly after a full night’s sleep, following an overnight stay in a metabolic chamber or a research facility. This limits activity or movement prior to measurement. In contrast, RMR is measured right after you arrive at the testing facility, and is generally a bit higher compared to BMR.
Due to stricter requirements, BMR provides more accurate results. Also, with that in mind, it’s safe to say that online calculators don’t really provide you with BMR—given their limitations, what they mostly show you is RMR.