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You can punch a bunch of numbers into a spread sheet and churn out another number. That tells you how much lumber is in the finished product. What's tough is walking into a lumber yard and picking boards that agree with the spread sheet. Figuring the waste factor and minimizing it its the hard part and no computer is going to do that for you.
You didn't mention species. Pattern grade mahogany? For me, add a 10% waste factor maximum. You couldn't build a bird house with the drops. Walnut? Add 40%, maybe more if things don't go well. Sap, sap, sap. Jatoba? There are lots of checks and cracks that seem to appear only in the bright light of my shop but never in the dim light of the lumber yard! White oak? If color matters you'll need to carry each board out into the day light, because they all seem to match under fluorescent light. Cherry? Good luck. Each species is a new adventure in beauty and frustration.
My point is not to be a sarcastic mule here, but that it is a little trickier than developing some number plus a waste factor. Lots of variables. I rarely develop a BF number but always go to the lumber yard armed with a cut list, sometimes drawings too. I buy 4/4 for 3/4", sometimes if the stock seems flat enough and I'm feeling brave I'll call 4/4 7/8" finished. 5/4" up to 1 1/16", and so on. Rod's argument about adding 25% waste for the planer makes sense if you rely on a spread sheet and final dimensions, but I don't ever figure things that way. If a part calls for 3/4", I translate that to 1" rough in my head and calculations. By experience I know what I can expect to get out of an actual board best case scenario, and I remember to add 1/8" for each kerf plus as much as 1/4" for jointing.
I mark up boards in the yard with a crayon or chalk, look for my parts in the stock, try to buy lengths and widths that fit my situation. I make a pile and check off parts as they are accounted for in my pile. Sometimes I change my design in the yard to suit what I find, other times that is not possible with in the scope of a project. Sometimes you need a 4/4 X 6" X 9' for a table top (say 18"X36") , but you get to the yard and the 4/4 is not flat enough over 3' to reach 7/8", the 9' stock has bad splits at the ends, all the nice boards are 7 1/2" wide and the 5/4 has better figure anyway! Now thats going to throw off your BF equation considerably.
I encourage you to use your eyes and brain to hone your stock purchases as much as possible and rely on BF calculators for rough cost estimations and little else. Given all the variables in a complicated project, or even a simple one, the BF estimate is almost as good as a random number in my eyes. I like to understand my plans intimately then go searching for parts in the boards, not boards for the parts. Make sense?
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