Lumber
Sizes
Lumber is named by its unplaned edge dimensions. In the chart below, the actual size refers to the actual dimensions of the lumber you buy from Lowes or HomeDepot or a lumber yard. The lumber supplied will have rounded edges, so if you want square edges you will need to trim it with a table saw. The cleaned edge versions are also given in the table below...
Name Actual size Cleaned edge
2 x 2 1-1/2" x 1-1/2"
1-1/2" x 1"
2 x 3
1-1/2" x 2-1/2"
1-1/2" x 2"
2 x 4
1-1/2" x 3-1/2"
1-1/2" x 3"
2 x 6
1-1/2" x 5-1/2"
1-1/2" x 5"
2 x 8
1-1/2" x 7-1/2"
1-1/2" x 7"
2 x 10 1-1/2" x
9-3/8"
1-1/2" x 9"
2 x 12 1-1/2" x
11-1/4"
1-1/2" x 10-1/2"
Internal lumber
Wood looks great inside your home. I use it in non structural applications, ie just for decoration. In non-structural applications it does not need to be structurally certified so it is fine to use lumber straight from the sawmill. I cut my own lumber as described here.
Structural lumber
Where I do need structural lumber, eg the roof structure, I select and buy lumber from either HomeDepot or Lowes. It is kiln dried and certified.
Plastic lumber
You can also buy "lumber" made entirely from recycled plastic which has the advantage in external applications of not rotting. It is supplied to the "Actual size" listed in the table above. It is significantly more expensive than actual lumber, but in applications such as framing a roof overhang its worth it to avoid rot, ie it lasts indefinitely. Available from eg http://www.american-plasticlumber.com/sizes.html