Lumber sizes · NIST PS 20-25
2x8 Actual Dimensions
The actual dimensions of a dry 2x8 are 1-1/2 by 7-1/4 inches (38.1 by 184.2 mm) under NIST PS 20-25. It is the narrowest 2x that loses 3/4 inch of width; green, it is 1-9/16 by 7-1/2 inches.
Sources checked
- Dry
- 1-1/2 × 7-1/4 in38.1 × 184.2 mm
- Green
- 1-9/16 × 7-1/2 in39.7 × 190.5 mm
- Green redwood, cedar
- 1-9/16 × 7-3/8 in39.7 × 187.3 mm
- Board feet a foot
- 1.33from nominal 2 × 8
End view of a 2x8, to scale
- Dry 1-1/2 × 7-1/4 in
- Green 1-9/16 × 7-1/2 in
- Nominal 2 × 8 in
- One square = 1/2 in
Nominal to actual converter
Where the width loss steps up on a 2x8
Up to nominal 7 inches the standard trims half an inch off a dry width. From 8 inches on it trims three quarters, so the 2x8 comes out at 7-1/4 rather than the 7-1/2 a half-inch rule would predict.
Green, the same board is 7-1/2 inches wide. Redwood and the two cedars sit between the two figures at 7-3/8 inches green, which is 187.3 mm.
The dry section is 10.88 square inches out of a nominal 16, 32% below the name.
A 2x8 as a deck joist
Stood on edge under deck boards, the 7-1/4 inch width becomes the joist's depth. Wolf's Perspective decking guide sets 2 by 8 inches as the smallest joist its boards go on, spaced 16 inches on center when the boards run square to the frame.
That makes the 2x8 the entry size for that maker. The Deck Calculator lays joists out on a plan; how far a 2x8 may span is a question for your local code.
Lengths and board feet
Master Halco's wood book lists rough western red cedar 2x8 stock as a rail in 8, 10, 12, 14, 16, 18, 20, 22 ft lengths.
By the standard's tally a 2x8 holds 1.33 board feet per foot of length, so a 16 ft joist counts 21.33 board feet; the board foot calculator does the sum for a longer list.
A 2x8 as a slab form
Slabs thicker than 5-1/2 inches and up to 7-1/4 inches are boarded with 2x8s, the first size whose real width clears the pour. The concrete slab calculator applies the rule to the slab you enter; every size beside this one is in the lumber sizes table.
2x8 questions
What is the actual width of a 2x8: 7-1/4 or 7-1/2 inches?
Both figures are in the standard: 7-1/4 in is the dry minimum and 7-1/2 in the green one. Green redwood and cedar fall in between at 7-3/8 in.
What does the nominal 2x8 size mean?
It names the piece as sawn: 2 by 8 inches. Dressing and drying leave a dry board 1-1/2 thick, and the actual thickness is the same 1-1/2 inches whatever the width.
How deep is a 2x8 joist?
7-1/4 in, or 184.2 mm, when the joist is dry stock. A joist bought green starts out at 7-1/2 in deep.
Does a 2x8 meet deck board makers’ joist rules?
For Wolf Perspective decking it is exactly the minimum, since that guide names joists of 2 in by 8 in or larger. It spaces them 16 in on center for square-laid boards.
How many board feet are in a 16 ft 2x8?
21.33 board feet. Each foot of a 2x8 counts 1.33, figured from the nominal 2 in by 8 in.
More lumber sizes
Sources
- NIST PS 20-25, American Softwood Lumber Standard (Jan 2025)
- USDA FPL Wood Handbook, FPL-GTR-282 (2021)
- Wolf Perspective decking installation guide (0620)
- Master Halco PostMaster wood product book (2018)
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