The Claim: Your metabolic rate is determined by your fat free mass
Your metabolic rate is most closely correlated with your fat free mass (lean body mass). Your fat free mass is all the parts of your body that aren’t fat (rather obvious explanation)…this includes organs, bones, skeletal muscles and extracellular water.
The most metabolically active tissue of your lean body mass is your internal organs which account for approx 80% of your BMR.
The metabolic activity of your organs is tightly regulated and predictable and there isn’t much you can do to change it. Don’t bother comparing yourself to others (as someone else’s ability to burn calories will never help you burn them)
There are some pretty good metabolic rate calculators online that do a decent job of estimating metabolic rate.
I’m 6’0 tall and about 183lbs and my BMR comes out to around 1900 calories per day which is pretty accurate.
If you use a metabolic rate calculator online don’t bother with the activity factor calculation because they tend to waaay overestimate calories burned from activity. If anything just use the BMR calculated as a general guideline if what you need…oh, and probably go with the lowest estimate you can find as some of the calculators also overestimate quite a bit.