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Understanding Lumber Thickness

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?
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


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WoodZone Woodworking Retail Store
2263 Sunset Blvd. West Columbia, SC 29169.
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