What Do Nominal Lumber Sizes Mean? Explained With Examples

What Do Nominal Lumber Sizes Mean? Explained With Examples
What Do Nominal Lumber Sizes Mean? Explained With Examples

Lumber is an essential building material used in various gardening construction projects. When purchasing lumber, you may notice that it is often referred to by its nominal dimensions rather than its actual.

In this article, we will explain what nominal lumber dimensions mean and how they differ from the actual size of the lumber.

Lumber Dimensions: Nominal vs. Actual

When purchasing lumber for any gardening project, it is essential to understand the difference between nominal and actual dimensions. While lumber is often referred to by its nominal size, this size does not reflect the exact dimensions of the wood.

By understanding these differences, you can ensure that you purchase the correct size lumber for your project.

What Is the Real Size of 2×4?

2×4 boards are never truly 2×4 inches in size. If you place three 2×4 boards side by side, they will not measure a foot in width. This is because a nominal 2×4 board is actually 1-1/2 inches by 3-1/2 inches.

The reason for this difference is that the nominal dimensions reflect the original size of the timber before it has been dried and planed smooth on all four sides.

The actual dimensions exclude the timber that was lost during the drying and planing stages, while the nominal dimensions do not.

Nominal vs. Actual Lumber Dimension Charts

The National Institute of Standards American Softwood Lumber Standard governs softwood lumber. The below charts present the difference between nominal and actual softwood lumber sizes. [1]

Nominal vs. Actual Lumber Size Chart For Softwood

Nominal SizeActual Size
(Inches)
Actual Size
(mm)
1 x 23/4 x 1 1/219 x 38
1 x 33/4 x 2 1/219 x 64
1 x 43/4 x 3 1/219 x 89
1 x 53/4 x 4 1/219 x 114
1 x 63/4 x 5 1/219 x 140
1 x 83/4 x 7 1/419 x 184
1 x 103/4 x 9 1/419 x 235
1 x 123/4 x 11 1/419 x 286
2 x 21 1/2 x 1 1/238 x 38
2 x 31 1/2 x 2 1/238 x 64
2 x 41 1/2 x 3 1/238 x 89
2 x 61 1/2 x 5 1/238 x 140
2 x 81 1/2 x 7 1/438 x 184
2 x 101 1/2 x 9 1/438 x 235
2 x 121 1/2 x 11 1/438 x 286
4 x 43 1/2 x 3 1/289 x 89
4 x 63 1/2 x 5 1/289 x 140
6 x 65 1/2 x 5 12140 x 140

Softwood Lumber Lengths

Length
(feet)
Length
(inches)
Length
(meters)
6721.8
8962.4
101203.0
121443.7
141684.3
161924.9
182165.5
202406.1
222646.7
242887.3

The Difference in Dimensions Between Softwood and Hardwood

The Difference in Dimensions Between Softwood and Hardwood
The Difference in Dimensions Between Softwood and Hardwood

Actual sizes for softwood and hardwood lumber differ, with different standards for each type of wood. For softwood lumber, nominal dimensions include both length and thickness, while for hardwood lumber, nominal dimensions are only for thickness.

The actual thickness of a board also depends on whether it has been planed on one (S1S) or both (S2S) sides. [2] [3]

Nominal vs. Actual Lumber Size Chart For Hardwood

Hardwood lumber is characterized by the number of sides that have been smoothed through a finishing process to remove saw marks.

Hardwood lumber is usually referred to using the quarter system, which denotes the thickness of the lumber in quarters of an inch.

For example, a hardwood board that is just over 1 inch thick and has been surfaced on both sides is known as a five-quarter (5/4) board.

Nominal Thickness
(Inches)
Rough Size
(Inches)
Actual Size
(Surfaced 1 Side)
Actual Size
(Surfaced 2 Side)
1/23/8 inch
(9.5 mm)
5/16 inch
(7.9 mm)
5/81/2 inch
(13 mm)
7/16 inch
(11 mm)
3/45/8 inch
(16 mm)
9/16 inch
(14 mm)
4/417/8 inch
(22 mm)
13/16 inch
(21 mm)
5/41 1/41 1/8 inch
(29 mm)
1 1/16 inch
(27 mm)
6/41 1/21 3/8 inch
(35 mm)
1 5/16 inch
(33 mm)
8/421 13/16 inch
(46 mm)
1 3/4 inch
(44 mm)
12/432 13/16 inch
(71 mm)
2 3/4 inch
(70 mm)
16/443 13/16 inch
(97 mm)
3 3/4 inch
(95 mm)

Glue Laminated Lumber Dimensions

The American Institute of Timber Construction’s publication “Standard Specification for Structural Glued Laminated Timber of Hardwood Species” includes the following table.

The ANSI/AITC A190.1-1992 standard allows for any width and depth of glued laminated timber to be used. While the above publication specifies standard widths, custom sizes of a wide range of depths are available.

The lengths of glue-laminated lumber can be customized for each specific job. [4]

Nominal Width
(Inches)
Actual Finished Width
(Inches)
Actual Finished Width
(mm)
32 1/264
43 1/8 or 3 1/279 or 89
65 1/8 or 5 1/2130 or 140
86 3/4171
108 3/4222
1210 3/4273
1412 1/4311
1614 1/4362

Plywood Sheet Sizes

Just like other lumber mentioned, the nominal sizes of plywood sheets do not match their actual dimensions.

These are usually sold in nominal thicknesses of 1/4 inch, 1/2 inch, 5/8 inch, or 3/4 inch. But it’s important to understand that these sizes are not the actual dimensions of the plywood.

Nominal Thickness
(Inches)
Actual Thickness
(Inches)
Actual Thickness
(mm)
1/411/329
1/215/3212
5/819/3215
3/423/3218

Is This Legal?

You may be concerned about the use of nominal sizes, but this is a legal practice.

The American Wood Council and the National Institute of Standards and Technology allow the use of nominal lumber sizes as long as it is clearly stated that the size given is nominal. The actual or minimum measurements are also required to be provided.

How To Avoid Buying The Wrong Size Lumber?

How To Avoid Buying The Wrong Size Lumber
How To Avoid Buying The Wrong Size Lumber

To avoid surprises when working on any of your projects, take a tape measure with you when purchasing lumber. This will allow you to accurately check the widths and thicknesses of the boards before making a purchase.

This will ensure that the wood you buy is the right size for your project.

Conclusion

Nominal lumber sizes refer to the rough dimensions of a piece of lumber before it is dried and planed. These rough dimensions are typically larger than the actual finished size of the lumber.

Hopefully, you got the answer to what nominal lumber sizes mean. Check out other articles in our “Garden Tools” section.

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