Converting Board Feet to Metric Units

      You do the math... March 26, 2010

Question
Im working on a software program and need to provide board footage and square footage calculations for cabinetmakers that use metric. I cant seem to find much on the web regarding this issue. I am wondering if the data is displayed in mm or cm. Im also wondering about the formulas, if they differ greatly from the imperial version.

Forum Responses
(Cabinetmaking Forum)
From contributor R:
One board foot would be 2359.74 cubic centimeters.



From contributor G:
Don't mean to be picky, but - how, exactly, did you look for this information on the web?
When I Googled "board footage and sq footage calculations" (pasted straight from your post), I received almost 300,000 hits.


From Gene Wengert, forum technical advisor:
The standard conversion for lumber is 424 BF per cubic meter. It would be nice if a board foot was really 1/12 of a cubic foot. But, a softwood 2x4x12' (8 BF) is really 1.5"x3.5"x12.0' (5/12 cu ft and not 8/12 cu ft). So, when converting to metric, do you use the nominal size of 2"x4"x12' or the actual size? Metric uses actual sizes, but BF uses nominal.

For hardwoods, a piece of lumber that is 1-7/32" thick and 7.49" wide and 12' long is 7BF. So is a piece that is 1.0" thick x 6.5" x 12'. The "soft" conversion above will give you greatly different cubic metric volumes. So, if you use the 424 number, appreciate that it is not accurate for converting actual volumes. As we cut up lumber into pieces, we tend to drop the BF designation and use measurements such as sq ft or cu ft or cu in.



From the original questioner:
Yes. I got the same results as you. I was looking for a metric calculator of footage, one that showed the preferred units used and what was considered a metric board foot. The calculator was just one part of my research.

Heres some info I found. The Americans, as you say, sell and buy lumber by the board foot (bd ft or bf). A board foot is 1/12 of a cubic foot. A board foot is a plank 12" long X 12" wide X 1" thick, or 2' long X 6" wide X 1" thick, etc. North Americans list their dimensions backwards to the rest of the world, such as thickness X width X length. Instead of using 1728 to convert cubic inches to a cubic foot you divide cubic inches by 144 to establish board footage. If you work in foot lengths and inch widths as they do in the kiln operations and lumber yards in the North American system, you divide the result of the foot length X inch width by 12 to establish the bf.

If you are setting up a program that calculates how much wood to buy from a timber merchant or kiln operator, don't forget that you need to include an allowance for buying the wood on either a green tally (aka wet tally) or a dry tally; there's about 10% difference between the two. One cubic meter (M3) = 35.31 cubic feet (ft3) = 423.77 board feet (BF or bf) in the USA. Useful quick and dirty close conversions from cubic meters to cubic feet are: 0.5 M3 = 18 ft3 , 0.25 M3 = 9 ft3 , 0.1 M3 = 3.5 ft3. One cubic foot = 0.028 M3 = 12 board feet (BF) in the USA.

The following calculations for converting one unit to another are close, but not perfect. I worked them out a long time ago and I've been using them for about fifteen years now and their accuracy has always been good enough for my needs.

To convert cubic millimetres to board feet in the US:
Divide mm by 2,350,000, e.g. 305 X 305 X 305 mm = 28,372,625/2,350,000 = 12.07 BF.

To convert cubic millimetres to cubic feet:
Divide mm by 28,200,000, e.g., 305 X 305 X 305= 28,372,625/28,200,000= 1.006 ft

Converting cubic metres into cubic feet:
Divide M by 0.0282, e.g., 0.305 X 0.305 X 0.305= 0.0283726/ 0.0282 = 1.006 ft

Maybe some of that might help you. I've had an Excel spreadsheet on my computer that uses some or all of these conversion factors since about 1997 or 98. The metric convention used in furniture making is the same as engineering: it's meters and millimeters.



From Gene Wengert, forum technical advisor:
The forum link above is pretty good except that a board foot is not 12" x12" x 1". As I mentioned previously, for softwoods there is a tremendous difference. For hardwoods, a piece that is 6.1" x 12" x 1" is also 1 BF and so is a piece 17.9" x 12" x 1.24". For softwoods, a 2x10x12' is actually 1.5" x 9.25" x 12'. Calculate the actual volume in cu ft or cu inch and then convert to cubic meters or cm.

I am not sure what the writer means about dry versus wet tally and that they are 10% different. Perhaps the writer means gross and net tally. The National Conference on Weights and Measures (which is the law in most states) adopted the following in 1977: "Sales of hardwood lumber measured after kiln drying shall be ... on the basis of net board footage with no addition of footage for kiln drying shrinkage."



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