Your Resting Metabolic Rate (RMR) is the estimated number of calories your body would burn if you were to do nothing but rest for 24 hours. This number can easily be found using online RMR calculators, or manually inputting your personal data into the necessary equation. Many people use their RMR to regulate caloric intake for weight loss.
The Mufflin Equation is the most accurate way to calculate your RMR. Using height (h), weight (w) and age (a), the equation estimates the number of calories you would burn in a 24-hour period with no activity. For men the equation is as follows: (10 x w) + (6.25 x h) - (5 x a) + 5. For women: (10 x w) + (6.25 x h) - (5 x a) - 161.
The RMR calculator does not take body composition into account. Your body composition is the percentage of muscle and fat that make up your body. So if you have a nontypical body composition, the calculator will be less accurate. An above average amount of muscle would yield a higher number, and a below average amount of muscle a lower number. This is because muscle burns calories and fat does not.
Keep in mind that your RMR is the number of calories your body needs to maintain your current weight at rest. To get an estimate of the number of calories burned during your active day, you would need to multiply your RMR rate by an activity factor.
Fluctuations in your height, weight and age all affect your RMR. As you age, your RMR goes down. As you become shorter, your RMR goes down. As your weight decreases, your RMR goes down. This is why periodically re-calculating your RMR can be important.
Many people interchange the phrases BMR and RMR. While similar, they are not exactly the same thing. BMR, or Basal Metabolic Rate, is usually tested in a darkened room by a reclining subject who has just slept for 8 hours, with 12 hours of fasting to ensure that his digestive system is inactive. RMR is generally tested under less restrictive conditions and is therefore more likely to be an appropriate and accurate measure for someone trying to estimate their own rate.