Even when you are sleeping, your body requires a certain minimum amount of energy to function properly. Your heart, lungs, kidneys, liver, etc, are always active and working and this "internal machinery" burns a certain amount of calories every second, every minute, continually.

This is called Basal Metabolic Rate (BMR), Resting Metabolic Rate (RMR), or Resting Energy Expenditure (REE). Technically, BMR is slightly more accurate than RMR (REE) because it's measured more strictly in labs. However, for our purposes BMR and RMR are basically interchangeable.

For our purposes, we prefer to use the term Resting Metabolic Rate (RMR) so that's all you need to remember.

Also, even though RMR is a rate (such as X calories per minute) we'll simply refer to RMR as the number calories your body expends per day (24 hour period).

RMR does not include energy expended by your movement or activity.

Interestingly, RMR accounts for the majority (60-70% or so) of all the calories your body burns throughout the day. We'll talk about the rest of the calories in the next topic (Energy Balance Equation).

Factors That Affect RMR

Your RMR depends mainly on your body size: the more you weigh, the higher your RMR. It's important to understand this, because it means your caloric needs go down as you lose weight. We'll talk more about this in the Metabolic Adaptation page.

Greatest contributors to RMR:

Lean body mass (LBM) is the greatest factor. This makes sense because most of your body is LBM. Muscles account for about 25% of total RMR. Organs account for 60%.
Gender. Men on average have higher RMR. One reason is because men generally have higher LBM (less fat). Also, during the menstrual cycle, RMR can vary quite a bit.
Age. RMR decreases slightly with age, partly due to loss of LBM and probably also because bodily functions are slowing down.
Fat Mass. Fat only burns about 3 calories per pound per day so it contributes very little to RMR.

With those 4 factors above, we can predict your RMR with 85% accuracy. The other 15% depends on your genetics and can't be predicted.

Some people basically are just born with a bit lower (or higher) metabolic rate. It simply means that given the exact same diet, some people will lose more, other people will lose less.

What else affects (or does not affect) RMR?

The type of food you eat has no effect on RMR.
Some supplements (like caffeine) or drugs can increase RMR moderately.
More muscle mass or being super fit doesn't have a huge effect on RMR. It helps a little but you would have to have a massive amount of extra muscle to make a significant difference. However, having more muscle does have health and quality-of-life benefits -- and -- with more muscle the body can become more efficient and capable of exercise (fitness) allowing you to exercise longer and harder and therefore burn more calories.

Estimating your RMR

Unless you can go to a metabolic lab and have your RMR measured, you can only estimate. But all you need for weight loss purposes is an estimate.

NOTE: We have some formulas and a calculator to help you with this in the Setup section.

A ballpark estimate for RMR is 10-11 calories per pound (22-24 calories/kg), 10 for women and 11 for men. So for example if you are female and weigh 150 pounds, your RMR is estimated to be 1500 calories per day. That's how much your body will burn, on its own, each day -- regardless of your physical activity or what you eat.

One of the most accurate methods of estimating your BMR is the Harris-Benedict Formula. Here is the original formula:

Adult male: 66 + (6.3 x body weight in lbs.) + (12.9 x height in inches) - (6.8 x age in years)
Adult female: 655 + (4.3 x weight in lbs.) + (4.7 x height in inches) - (4.7 x age in years)

The formula was later revised to the Mifflin St Jeor Equation:

Adult male: (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5
Adult female: (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161

However, the Harris-Benedict Formula isn't perfect. It's only an estimate; when it's wrong, it typically overestimates the RMR.