Explanation of Wood Dimensions

The most confusing part of using lumber is defining and understanding the relationship between board feet , nominal lumber sizes, linear feet and square feet. The term board foot is derived from the rough lumber business where one square foot of lumber 1 inch thick equals one board foot or 144 cubic inches. It also is used to price lumber at distribution yards and lumber yards after milling into “nominal sizes”. Nominal sizes are anywhere from 1/4 to 1/2 inches thinner and 1/2 to 3/4 narrower than rough sizes. So after milling into nominal sizes the board foot no longer covers a square foot. Even more confusing is the fact that most nominal sizes get milled even further into patterns commonly used in construction that have tongue and grooves or overlaps that further reduce the actual coverage. So what are you to do?

FORGET ABOUT BOARD FEET! FORGET THEY EXIST. THEY DON’T MEAN A DAMN THING WHEN IT COMES TO ACTUALLY FIGURING OUT WHAT YOU NEED IN A GIVEN SIZE OF MILLED LUMBER TO COVER A GIVEN AMOUNT OF SQUARE FEET.

Gross         Gross       Normal        Normal    Pattern       Pattern      Pattern

Thickness    Width      Thickness Width     Thickness     Width      Reveal
Gross size –   1 x 12            1           12           0.75         11.25         0.681       11     10.75
Gross        Nominal       Pattern
Coverage – one lineal foot 1   0.9375      0.895833

You now can see that in the rough form one lineal foot of 1 x 12 will cover a one square foot, a nominal 1 x 12 on foot long will cover .9375 square foot, and a milled to pattern 1 x 12 will cover .8958 square foot. To make things more confusing each width requires different math to determine the actual coverage. So the most important quantifier to know is what the repeat or the reveal is for a given width or pattern including any overlap or gaps between the boards. The following numbers are good examples of determining the actual board footage required for a job. Keep in mind that for the most part – order material in lineal feet or ask for enough material to cover a given amount of square feet – don’t convert – you will have fewer problems. Notice I have included a waste factor in all the calculations
So you can always use this formula

12 divided by the repeat times the waste factor plus 1 times square feet to cover.

Pattern                                                                                   total ln per

Repeat        ln ft / sq ft    Waste                    sq foot
1 x 6 WP4                           5            2.4            10%                   2.64

So if you are ordering 1 x 6 STK cedar in a WP4 and you are covering 1000 sq feet you will need – 2640 ln feet! That will be 1320 bd ft. But forget the bd feet part – just order the ln feet part – let the guy on the other end convert if you have to.

Full rough sizes versus nominal sizes
You probably covered this in school – but in case you didn’t or you have just forgotten here is  list of the most commonally used lumber dimensions in their full rough size, nominal size and their milled to pattern size. Milled to pattern means milled to a pattern beyond the basic S4S – like a tounge and groove or siding pattern.

milled to milled to actual ln feet
rough rough nominal nominal pattern pattern repeat per square
4/4 lumber thickness width thickness width thickness width width foot
1 x 2 1 2 3/4 1 1/2 11/16 1 1/4 1 rarely used as a milled to pattern
1 x 3 1 3 3/4 2 1/2 11/16 2 1/4 2 6.00
1 x 4 1 4 3/4 3 1/2 11/16 3 1/4 3 4.00
1 x 6 1 6 3/4 5 1/2 11/16 5 1/4 5 2.40
1 x 8 1 8 3/4 7 1/4 11/16 7 6 3/4 1.78
1 x 10 1 10 3/4 9 1/4 11/16 9 8 3/4 1.37
1 x 12 1 12 3/4 11 1/4 11/16 11 10 3/4 1.12
8/4 lumber
2 x 2 2 2 1 1/2 1 1/2 1 3/8 1 3/8 rarely used as a milled to pattern
2 x 4 2 4 1 1/2 3 1/2 1 3/8 3 1/4 3 4.00
2 x 6 2 6 1 1/2 5 1/2 1 3/8 5 1/4 5 2.40
2 x 8 2 8 1 1/2 7 1/4 1 3/8 7 6 3/4 1.78
2 x 10 2 10 1 1/2 9 1/4 1 3/8 9 8 3/4 1.37
2 x 12 2 12 1 1/2 11 1/4 1 3/8 11 10 3/4 1.12

So that about covers the basics on lumber dimensions. Don’t make some bush league mistake and design something on an 8 inch repeat and then think the 1 x 8 cedar siding will match up to what you have drawn  Last – but not least – the most versatile and energy efficient building material we have is wood! Learn about it and how to correctly specify it and install it and you will be amazed at the results you you will get.

Any questions – Feel free to call.
Wood Haven Inc.
401 Bridge Street
Perry KS 66073
785-597-5618