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About lumber sizes
First a timber log will be sawn into common lumber sizes such as 100mm x 50mm (2″x4″). This is usually called the nominal size, but can also be known as the rough size, the sawn size, or the rough sawn size.
Rough sawn lumber can be used for many things, for example any project that requires a rustic look.
However some jobs require a lumber that has a smoother finish. Therefore the rough sawn lumber has to be dressed, which means it is put through a planer that makes the sides smooth. It will then be called dressed lumber or surfaced lumber. Making lumber smooth also lessens the size (width and thickness). This new finished size is called the actual size.
When a piece of rough sawn lumber 100mm x 50mm (2″x4″) is dressed on all four sides, the actual finished size will be approximately 90×45 (1 1/2 x 3 1/2).
So the nominal size of the lumber is 100mm x 50mm (2″x4″) but the actual size is 90×45 (1 1/2 x 3 1/2).
If you pop along to the local lumber store and ask for a length of 100×50 (2×4) lumber that is surfaced/dressed on all sides (S4S), the actual finished size will be approximately 90×45 (1 1/2 x 3 1/2).
S4S means lumber that is dressed/surfaced on all four sides (USA)
S2S means lumber that is dressed/surfaced on two sides (USA)
D4S means lumber that is dressed/surfaced on all four sides (Australasia)
The table below shows the approximate differences between the nominal and actual sizes of framing lumber. Also note that actual sizes can vary.