How do you know how many board feet to get?
A SawmillCreeker asked this same question recently in “How to calculate board feet for project?” Like all things, it’s simple but with some serious complexities the more you dig into it. We’ll start simple, add some complexities to confuse you and end with some links to calculators and other resources that will really muck things up ;)
Here’s one way:
thickness (inches) x width (inches) x length (feet) / 12 = board feet
Here’s a variation:
thickness (inches) x width (inches) x length (inches) / 144 = board feet
Ok, now lets get a little funky…
So it’s not as simple as figuring out your cubic footage of wood.
You have to account for waste in the form of cutoffs, any planing and jointing, grain and color selection, saw kerf, resawing, splits, and even cracks you didn’t notice at the lumber yard.
Some folks from the thread add a flat 20%. Others 30%. Some as high as 40-50% just for waste. No wonder woodworkers have to be such cheapskates ;)
And now let’s further complicate things with some highly insightful quotes from SawmillCreeker Peter Quinn:
“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.”
So what were those calculations again for the cherry sawhorses you were planning to make? ;)
Getting Way Too Technical About It
This is how the Departments of Commerce of all states require vendors to measure board feet, according to Gene Wengert a technical advisor at the WoodWeb (in other words he knows his sheet…. goods.):
For hardwoods: First, you need to use the standard length (4′, 5′, 6′, etc. and not include any inches over the standard length). However, 12’11” is still 12′. Then multiply by the actual width in inches and fractions and then divide by 12. Now round to the closest whole number. This is called the surface measure (SM). If you have several pieces of the same thickness, add up all their SMs. Finally, multiply by the *standard* thickness in inches (not actual). So, a piece 1-7/32″ thick is 4/4, so the multiplier is 1. Apices 1-15/32 is 5/4, so use 1.25. The final answer is a whole number and not fractions.
For softwoods, you would use the standard width (not actual). You would also use two decimal places for SM and BF. Otherwise, it is the same as for hardwoods.
Refining Your Lumber Yard Strategy
Here’s a bit more from Mr. Quinn on how he buys wood for projects:
“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.”
He checks his parts off as he finds them in the wood at the lumberyard. He even describes changing his plans based on the wood he finds available… As he puts it, “I like to understand my plans intimately then go searching for parts in the boards, not boards for the parts.”
There is some useful software for maximizing how many parts you get out of your sheet goods – Cut List Plus. I’ve not used it, but the site sure looks good ;) Apparently it figures out your board feet for you AND helps you figure out the most efficient way to cut so you get the most out of your wood. It costs though.
More Software + Free Calculators:
Timber and Lumber Calculators from Woodweb
You’re not going to find a better collection of free calculators anywhere… they have lumber weight calculators to wood shrinkage to log volume. There’s definitely more of an industrial slant here though.
Woodbin’s Free Tabulator
You’re not going to find a more THOROUGH board foot calculator anywhere.
Board foot calculator from University of Missouri
Board Foot Calculator in MS Excel
by David Lykins
Board Foot Calculator by Ben Erickson is as simple as it gets. No frills.
There you have it folks… I hope that’s enough information on calculating board feet to keep you busy for awhile. If you have any questions be sure to check with any of the folks I linked to ;)