It's based on weighing samples in my shop over the years. We have a digital scale with a 2000# capacity. We used to have a lot of fun betting on how much any given timber weighed. Over the years I have sort of developed a feel for it. Based on handling a given order of timbers, I can usually come up with a pretty good #/bd ft number. At least it's good enough for my purposes -- YMMV. It usually gets me within 10% of the weight of any given crane pick -- verified by the crane's load cell. And since I have a lot more than 10% of extra capacity built in to the rigging, I'm ok.
If you need numbers that are scientifically verifiable by published data, then it gets to be a bit of effort. I haven't done those calculations since college, but the formulas for moisture content adjusted weights of timber should be in the wood handbook or Hoadley's book. I seem to remember needing to know only 2 things -- specific gravity of the wood and moisture content.
Bear in mind that wood can vary significantly in it's density within a species group, so any published numbers are potentially not very accurate for your situation.
best of luck
