How to use the nominal to actual lumber size converter
- Choose what you know: the nominal size (the name on the label, such as 2 x 4) or a measured size (the real thickness and width).
- For a nominal size, pick the thickness and width, then enter the length in feet and the number of pieces.
- Read the actual thickness and width in inches and millimeters, the board feet by the nominal size and by the real size, and how much smaller the end area is.
- For a measured size, enter the two sides of the end of the board in decimal inches. The page lists the nominal sizes that match within 1/16 inch.
Formula
board feet = thickness in inches x width in inches x length in feet / 12
end area = thickness x width
dressed thickness: 1 -> 3/4, 5/4 -> 1, 6/4 -> 1 1/4, 2 -> 1 1/2, 2 1/2 -> 2, 3 -> 2 1/2, 4 -> 3 1/2
dressed width: 2 -> 1 1/2, 3 -> 2 1/2, 4 -> 3 1/2, 5 -> 4 1/2, 6 -> 5 1/2, 8 -> 7 1/4, 10 -> 9 1/4, 12 -> 11 1/4, 14 -> 13 1/4, 16 -> 15 1/4
The sizes are for dressed, dry softwood sold in the United States. Wood is cut to the nominal size green, then dried and planed, which is why the finished piece is smaller.
A 2 x 4 that is eight feet long
A 2 x 4 is really 1 1/2 inches by 3 1/2 inches (38.1 by 88.9 mm). Its end area is 5.25 square inches, 34.4 percent less than the 8 square inches that the name suggests. For a board 8 feet long, board feet by the nominal size are 2 x 4 x 8 / 12 = 5.33, and by the real size 1.5 x 3.5 x 8 / 12 = 3.50.
A 1 x 6 that is 10 feet long is 3/4 by 5 1/2 inches, with 5.00 nominal and 3.44 actual board feet; four of them hold 13.75 actual board feet. A 2 x 10 that is 12 feet long is 1 1/2 by 9 1/4 inches, with 20.00 nominal and 13.88 actual board feet. A measured end of 1.5 by 3.5 inches matches a 2 x 4.
Why the names do not match the sizes
Lumber is named for the rough size it is cut to at the mill. It then dries, which shrinks it, and is planed smooth on all four sides. The standard for softwood lumber in the United States sets the finished sizes, so a 2 x 4 sold today is the same real size from any mill. Wider boards lose more because the planing takes the same amount off each side and the drying shrinks the whole width.
This matters when you plan framing, shelves or a project that has to fit a space, and when you price wood sold by the board foot. The board foot calculator totals a whole list of boards, the rough opening calculator uses real sizes for door and window frames, and the deck calculator plans decking boards.
Where the table does not apply
Hardwood is sold by its rough thickness in quarters of an inch, rough sawn lumber is close to its full nominal size, timbers of 5 inches and more are usually half an inch smaller than their names, and Canada, the United Kingdom and Australia use their own standards and often metric sizes. Reclaimed and old lumber can be full size. Measure the actual boards you buy. For turned rings and mitered shapes see the segmented ring calculator.
Assumptions and limits
- The sizes are for dressed, dry softwood boards and dimension lumber in the United States.
- The length is the full length of the piece, and board feet use the given length without trimming.
- The measured size match allows 1/16 inch either way and checks both orders of the two sides.
- Widths above 16 inches, timbers and metric sizes are not listed.
Frequently asked questions
Is a 2 x 4 ever really 2 by 4 inches?
Rough sawn or reclaimed lumber can be, and some specialty mills sell full-size boards. Ordinary dressed lumber from a store is 1 1/2 by 3 1/2 inches.
Why does the page show two board foot figures?
Many sellers price by the nominal board foot, while the real wood you get is the actual figure. Seeing both shows the gap, which is 34 percent less wood in a 2 x 4.
What is 5/4 lumber?
It is stock sold as five quarters of an inch thick when rough, which is a little over an inch. After planing it is about 1 inch thick, and it is often used for decking and trim.
Why does a measured 3 1/2 by 3 1/2 board match a 4 x 4?
A 4 x 4 is dressed to 3 1/2 inches on both sides. The page checks the two measurements in both orders, so a 2 x 4 measured as 3 1/2 by 1 1/2 matches too.