I've heard these terms mentioned on the boards a few times but not really sure what it is or how to calculate it?

I weigh 150lbs and have 16% body fat does that mean that

lean muscle mass = 150*.84
= 126

or have i got the wrong end of the stick?

Lean muscle mass is all of your muscle. Lean muscle mass is everything but your fat.

Lean body mass = 150*.84
Fat mass = 150*.16

And I don't think there is a way to calculate how much lean muscle mass you have with only those numbers. I'll look into it though.

Yeah you can't figure straight lean muscle mass easily. The proper term is Lean BODY Mass...which is how much you weigh (skin, bones, internal organs, blood, and muscle tissue), minus the fat. they have averages based on height and frame to determine how much bones weigh...its 10-20 pounds. you only have so much blood, so that's not hard to remove.

but internal organ weight will always be an estimate unless we have a volunteer for an autopsy...

really, LBM is good enough. you can gauge how much lean mass you have gained from a bulking cycle by knowing weight and bodyfat...and vice versa for a leaning out to figure fat loss (and possible muscle loss if you lean too quickly)