Converting BF to cubic feet

      How to convert board feet to cubic feet. March 28, 2001

Q.
What is the correct way to convert board feet to cubic feet? Specifically board feet of logs, using International log scale.

Forum Responses
Can't you just take your bd ft and divide it by 12? A log 8' x 16" is 85 bf on International scale, divide that by 12 to get 7.08 cu ft.



That sounds right to me. There are 144 cubic inches of wood in a board foot (12x12x1), and 1728 cubic inches in a cubic foot (12x12x12) so 144 divided by 1728 gives you a conversion factor of 0.083333, or 1 over 12.
(85bf x 0.08333)=7.08


Measuring cubic foot volume in a log is different than measuring bd ft volume.

The Smalian formula for 16' logs is Volume=.021817*(4D^2+*D+5.5). D is the dib at the small end and assumes a 1/2" taper per 4'.

When looking at the comparison of International to cubic foot volume, a 10" log will give 6.11 bd ft/cu ft. A 20" log will yield 7.53 bd ft/cu ft. In logs, there is no simple relationship where a certain factor can be used to convert from one scale to the other.



A board foot is not 1/12 of a cubic foot. In hardwoods, the lumber is typically 1/8 inch thicker than 1 inch for 4/4, so that means that there are about 10.67 bd ft per cubic foot. This assumes that the width of lumber is exactly on the inch, but hardwoods are also sold random width and often have about 2 inches of over-length (for example, 10' 2" and not 10'). So this further reduces the bd ft per cubic foot down to around 10.5.

For softwoods, you get about 18 bd ft per cubic foot, because 8/4 is not 2 inches, but is much less (1.5 inches after drying and planing), and 6 inches wide is about 5.5 inches (after drying and planing). It will vary quite a bit--especially with wider pieces as a 10" is actually 9.25, etc.

Gene Wengert, forum technical advisor



What correction does one make for saw kerf losses?


What happened to a board ft = 1" x 12" x 12" not 1 1/8 x 12 x 12 or 1.5 x 12 x 12? We weren't looking at kiln dried and planed, we were looking at cubic feet in a log. Does the International scale take into consideration kiln dried and planed?


From the original questioner:
When I figure board foot of boards I use the actual dimensions. This is when I'm custom sawing and being paid by the board foot of lumber produced. I understand all that about actual vs. nominal. But my question was because someone asked me for the weight by cubic foot of fresh cut green red oak logs. I use International scale and wanted to know how to convert log tally board foot to cubic feet. Someone answered with the formula and that's what I thought had to be done with each log. I have a scale put out by the North Eastern Loggers Association and The Northern Logger Magazine, and it gives weights of woods in log form based on board foot volume, and weights of lumber based on MC. I was trying to convert these weights to cubic foot volume weights.


The International scale uses 1/4 inch kerf (unless you use the 1/8" International scale) and assumes that 1 bd ft = 1/12 cu ft approximately.

When cutting softwood logs, where the actual size is 15%-20% smaller than the nominal when green, we will get more footage than is actually in the log--15% to as much as 50% with narrow kerf and good thickness control. So, two identical logs (except one is hardwood and the other softwood) will give considerably different actual BF per CF of log. The International 1/4" scale would be the same for both.

Of course, then we have to consider the conversion of scale BF to scale cubic footage--you might use 1/12 BF per CF here.

This is more than academic, however. What if you are selling some timber and want to know what it will produce? You can scale the trees in BF, but if softwood, you will be underestimating by quite a lot. As a result, we used to see federal timber go for prices that were much higher than the market price. So, the government switched to cubic volume for sales. The government still gets the same price, but the purchaser has to figure all the conversion now.

With hardwoods, we cut the green size larger, so we get only about 10.6 bf per 12 bf of log scale; for softwoods green, it is about 14 to 15 bf per cu ft. You must then use these conversions.

Gene Wengert, forum technical adivsor



I've been aware of the softwood overrun. When estimating a job, I add a 30 percent overrun to the log board foot tally, and that usually is close to actual lumber produced.

Am I right in thinking you wanted the cube foot info to calculate the weight of whole logs? First, you can not accurately go back and forth from log weight to scale BF. The weight of logs per thousand BF scale varies with stem diameter, due to scale estimating losses (slabbing mostly, the kerf estimate per BF is fairly constant, the slabbing loss is higher for a smaller log, which throws off your desired calculation). The only easy way I know for calculating the green raw log weights is this "Timber!" program. I ran these scenarios through the " Timber!" program, and got these results. I used 60lbs/ft3 and form class 78 for lack of better numbers, but you get the idea.

one each 36"x20' = 1225 BF and 13,935 lbs
BUT 20 each 11"x20' = 1225 BF and 26,020 lbs

Small logs weigh more per BF of scale than do large logs. So unfortunately, the number of BFof lumber a log will scale out at does not correlate directly with its weight.



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