Rough lumber dimensions

Understanding basic lumber dimensions and how to calculate board footage. April 20, 2001

I don't understand the dimensions of lumber. What do 4/4, 5/4, 6/4, 8/4, etc. mean? How do you figure board footage?

Forum Responses
4/4 and the others are thickness measured in quarter inch. A board foot is 12 inches square by 1 inch thick or any other way you get 144 square inches. Somewhere in the archives you can get board feet measurements of logs. These are called log scales--3 different types.

Continuing with the above--if a piece of lumber is 1-1/2 inches thick, that is six 1/4 inches thick of 6/4.

The definition of board foot is quite tricky, as with hardwoods, the thickness is 1-1/8 inch (often) and not 1-inch (or 4/4). Actually, 4/4 goes from 1.00 to 1.24 inches thick. Also, because hardwoods only use BF to the closest foot, 1 BF is the size of a piece of hardwood lumber ranging from 6" to 18" wide and 12" (1 foot) long by 1.00 to 1.24 inches thick.

I think if you search the archives, you will find a short paper that I wrote and we put here for people that are confused.

Gene Wengert, forum technical advisor

Isn't a board foot 144 cubic inches? 12"x12"x1"

No, a board foot is not 144 cubic inches.

With unplaned hardwoods it can be quite a bit different. Take a piece of lumber that is 1.00 inch thick x 5.51 inches wide x 12.00 feet long. It is 6BF. Now look at a piece that is 1.24 inches thick x 6.49 inches wide and 12.99 feet long. It is also 6BF. The first one is 793 cubic inches, or 132 cubic inches per BF. The second one is 1254 cubic inches or 209 cubic inches per board foot. Of course, you could add a little wane or a beaver tail to the end of the first piece and get even less than 132 cubic inches of wood per BF!

The calculations with softwoods and planed hardwoods are even worse, as softwoods are seldom sold rough, but are sold as planed lumber. For example, a 2x4x8' is really 1.5" x 3.5" x 8.00'. So, 5.33 BF (softwoods use 2 decimals, hardwoods do not) is actually 504 cubic inches, or 95 cubic inches per BF.

Gene Wengert, forum technical advisor

You're confusing old practices with basic definitions. One BF = 144in3. The reason those figures you quote have any bearing is because of scale issues. If wood were truly sold by the BF, we would be measuring every stick with a pair of calipers, but that wouldn't be practical, so we use a scale stick, and round up and down a lot.

If a customer asks for 100BF of lumber in 1"x12"x10' and we sell him 10 each 1 x 11.51 x 10'5", as soon as he pulls a tape over the lumber he is going to come back and demand he get what he asked for.

The way things are done on the small scale and the way they are done on the large scale are totally different. The reason all the books say it must be done the other way is because they were not written for the little guys out there.

Sorry to take exception, but the weights and measures department of every state adopted the technique of measuring board footage many years ago (in 1978?). So, within every state, when measuring lumber, the rules are the same--for small or big mills. (Parts would be different.) Also, the measure for softwoods is based legally on the nominal size and not the actual size. This is the standard used and is legally done that way in every state (and probably Canada too). There is no exception for small-scale people.

Lumber is indeed sold by the board foot. Hardwoods are sold by the "whole number" or integer board foot and not fractions. It does not have to do with scale, it has to do with the measure, which for hardwoods does not have fractions. For softwoods, the definition is for board feet measured to two decimals based on nominal size, not actual.

Incidentally, softwood pieces 1x12x10's are 0.75 x 11.25 x 10.0' actual size. This size is the size when the piece was graded, so if it dries out it will be a little smaller; if it gains moisture, a little bigger. But the BF stays the same 10 BF per piece.

Also, if you sell a dry, planed piece of hardwood lumber that is 1.00 x 11.51 x 10.5', this is actually considered as 5/4, so it is 12 BF (the first piece) and 13 BF the second piece, etc.

Gene Wengert, forum technical advisor

Like I said, the book and the laws are written to cater to big business.

Planing doesn't straighten lumber appreciably (not more than 1/2" in the length of the board (if you were to joint it, which most planers can't do). The biggest reason for the large scale planing of lumber is to eliminate the poor accuracy of old sawmills, so why should small, accurate mills be penalized for this?

Now it is a profit item. Those shavings are worth more than the cost to make them, or as in a lot of new mills, lumber is sawn very close to finish size--1 3/4 for a 2X and then dressed down to finish size.

Current terminology is all skewed way out of line. The reason for it being that way is because it has been that way, but that doesn't make it right.

The system is set up for big business and volume purchases. The new guys should learn what an actual board foot is before you start teaching them the exceptions, and how the definition of a board foot has been twisted to suit the preferences of giant corporations, and hungry wobbly circle saw blades.

It is sure fun to cut a full 2X4 and use a 30 p nail. Now do I have a 2X4 or a 2X6?
If 1.5 X 3.5 = 5.25" and 2X4 = 8 sq"
then a 2x6 is 1.5 x 5.5 = 8.25 sq "

Yes, this is correct, but I am puzzled by your use of cross sections. But it does make it obvious that in purchasing planed structural lumber, as much as 1/3 of the board footage you are paying for is being used to print your receipt on!

A full dimensioned 2x4 has almost as much cross sectional area as a "dressed" 2x6. The explanation for why we should accept "dressed" lumber it that it doesn't weaken it. Well, the facts are simple--more wood, more load carrying capacity, modern structural engineering has integrated these sizes into their tables, and it doesn't cause any loss of strength in the structure, because the structures are designed for the smaller lumber.

A lot of people crab about how you should cut your lumber to dressed sizes because it is so much more difficult to do the windows and doors in a home framed with full dimension lumber. Well, every time I see an article about installing or trimming windows, they are having to build up the casing to make up for the difference between the stock 3 1/2" casing, and the 5 1/2" wall bay, so if you are doing this already, then what is another 1/2"?

And if you are building your own home, why buy pre-hung doors? So what if pre-hung doors are setup for an undersized wall? In a custom built home, what is stopping you from building your own doorjambs?

I tire of seeing people being beat up for wanting to use full dimension lumber.

How about you go metric - cubic meters. I've always found base 10 easier to calculate than base 12 or...

Cubic measure is cool--that is coolly received by the sawmill industry. The reason is that when used to measure logs, the conversion of cubic meters (or cubic feet) to board feet of lumber produced will vary tremendously with log diameter. You may convert 45% at 6" diameter (oops--I mean 15 cm) while at 16 inches you may convert 58% to lumber. So, you need to know both cubic volume and diameter when buying logs (or trees). It is easier to have a scaling system that converts to BF directly and also adds in a fudge factor for the extra handling involved in smaller logs.

But does it really matter with lumber if you sell a 2x4x8' by the piece, or convert to BF (5.33 BF), or sell it by the actual size (500 cubic inches or 8259 cubic cm)? The price will be the same.

Gene Wengert, forum technical advisor

I understand that there is a difference in measuring hardwood lumber, surface measure (SM), and softwood lumber. But out here in the real world, the rough saw lumber world, a 2x4 is 2"x4". And a 6x6 is 6"x6", period. If you or any customer comes to my mill yard and asks for 2x4s, that's what they get.

If I sell hardwood lumber to a customer, I measure the piece and sell it to him based on the actual size of the piece. Be it 1 1/8" thick x 7 1/4" wide by 8' long. I don't round up or down. That way it's the fairest to me and to him. And none of my customers have ever complained to me that how I'm measuring board foot is wrong. Maybe I am measuring it wrong or figuring it wrong, or selling it wrong, but it works for my customers and me.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
4/4 is 1" thick rough or milled. 5/4 is 1.25" and so on. A board foot is thickness X width X length divided by 144 = bf. Figure out what you need, find out what they are selling and tell them what you want.