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Slide your measuring tape across a 2 x 4 and you’ll discover that it doesn’t actually measure two inches by four inches. In fact, it will be ½-in. shy in both directions. In its rough state, when the lumber was originally ripped into studs, this same piece was in fact a true 2 x 4. But after drying, it shrank a little. Then it was surfaced (planed) on all four faces, and it shrank a little more.

When you buy standard softwood lumber at your home center, surfaced and jointed on all faces and edges, the industry sells it to you in finished dimensions, but still describes it in nominal dimensions—the size it was before milling.

A piece of softwood lumber with a nominal 1-in. thickness is generally referred to as a board, while nominal 2-in.-thick softwood is called framing stock (as in studs, joists and rafters), or dimension lumber. The chart below lists nominal and dimension lumber sizes for the stock you’ll find in home centers.

Softwood lumber is graded by strength and appearance as well as moisture content. For woodworking applications, the three common grades to know are Select, Finish and Common (See the chart, below). While boards in the Common grade categories may contain some blemishes and knots, Select and Finish grades are clear or nearly clear of defects. Be aware, however, that boards within any grade may exhibit some degree of natural distortion (cupping, bowing, twisting), so it’s important to examine each board carefully by sighting along its length and width before you buy.

SOFTWOOD LUMBER GRADES
GRADE	GRADING CRITERIA
B Select and BTR	Highest quality lumber with little or no defects or blemishes. Nominal sizes may be limited.
C Select	Some small defects or blemishes permissible, but still largely clear and of high quality.
D Select	One board face usually defect-free.
Superior Finish	Highest grade finish lumber with only minor defects.
Prime Finish	High quality with some defects and blemishes.
No. 1 Common	Highest grade of knotty lumber; usually available by special-order.
No. 2 Common	Pronounced knots and larger blemishes permissible.

Nominal vs. dimension softwood lumber sizes

NOMINAL	FINISHED
1 x 2	¾ x 1½
1 x 3	¾ x 2½
1 x 4	¾ x 3½
1 x 6	¾ x 5½
1 x 8	¾ x 7¼
1 x 10	¾ x 9¼
1 x 12	¾ x 11¼
DIMENSION LUMBER SIZES
2 x 2	1½ x 1½
2 x 3	1½ x 2½
2 x 4	1½ x 3½
2 x 6	1½ x 5½
2 x 8	1½ x 7¼
2 x 10	1½ x 9¼
2 x 12	1½ x 11¼