Understanding Lumber Thickness

Woodmaster,
When I go to my local wood supply store, I see that the wood is marked in a manner other than the common 1x4, etc. It's more like 4/4 or 6/4... is there a chart that explains this method?
-Fred

Fred,
The lumber you buy at your local lumber yard is usually referred to by the "¼" system". The quarter system calls out the boards rough thickness in ¼" increments. The rough thickness refers to the thickness of the board before it has been planed smooth. For example a board that has a rough thickness of 4/4" was 1" thick before it was planed. After the board is planed the thickness should be 13/16", or close to ¾". The table below details this system for rough and surfaced (S2S)* lumber thickness in inches.

Standard Thickness for Rough and Surfaced Hardwood Lumber

1/4" System	Rough Thickness	Planed Thickness
4/4	1"	13/16"
5/4	1-1/4"	1-1/16"
6/4	1-1/2"	1-5/16"
8/4	2"	1-3/4"
9/4	2-1/4"	2-1/16"
10/4	2-1/2"	2-5/16"
12/4	3"	2-3/4"

* S2S = Surfaced Two Sides – both sides of the board have been planed; the edges are usually left rough