Basic Rules of Dimensional Lumber: Nominal vs Actual

​Why is my 2x4 lumber not 2 inches by 4 inches?

Many decades ago, lumber purchased from a sawmill physically matched the common dimensions we use today. So why does that pine or spruce 2x4 from your local lumber yard now come up short? The short answer is modern surfacing and shrinkage.

Over time we've come to accept these nominal (approximate) dimensions as standard. But there's still a catch. Shrinkage can be highly variable. This makes it very difficult to manage and regulate dimensional accuracy.

Dimensional lumber is a term given to boards cut to standard sizes (specified in inches).

Rules for Calculating Dimensions

​So now you know that 1x3, 2x4 are 4x8 are really just names or nominal dimensions. But... it gets even more complicated. Dimensions change based on the width or the thickness. Here are the general "rules" for calculating softwood dimensions:
 
Nominal vs Actual Dimension Rules

Nominal Dimension
Less than 2 inches​
​Less than 8 inches
​8 inches or greater

Actual Size
​​Reduce by 1/4 inch
Reduce by 1/2 inch
Reduce by 3/4 inch

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​Dimensional Lumber

Softwoods (pine, spruce, fir, etc.) are generally known as framing or dimensional lumber. This wood is commonly used for framing walls and floors in our homes and regularly used for building wooded crates and pallets. Since no one likes unexpected dimensional errors, the handy chart below outlines the changes in dimensions you can expect:
​
Dimensional vs Actual Lumber Chart for Dried Softwoods

​Nominal Size
thickness x width
​1 x 1
​1 x 2
​1 x 3
​​1 x 4
​1 x 6
​1 x 8
​1 x 10
​1 x 12
​
​2 x 2
​2 x 3
​2 x 4
​2 x 6
​2 x 8
​2 x 10
​2 x 12
2 x 14

Actual Size
thickness x width
​3/4 x 3/4
3/4 x 1 1/2
3/4 x  2 1/2
3/4 x 3 1/2
3/4 x 5 1/2
3/4 x 7 1/4
3/4 x 9 1/4
3/4 x 11 1/4

​1 1/2 x 1 1/2
1 1/2 x 2 1/2
1 1/2 x 3 1/2
1 1/2 x 5 1/2
1 1/2 x 7 1/4
1 1/2 x 9 1/4
1 1/2 x 11 1/4
1 1/2 x 13 1/4

​​Nominal Size
thickness x width
3 x 3
​3 x 4
​3 x 6
​3 x 8
​3 x 10
​3 x 12
​3 x 14
​3 x 16
​
​4 x 4
4 x 6
​​4 x 8

​​Actual Size
thickness x width
2 1/2 x 2 1/2
2 1/2 x 3 1/2
2 1/2 x 5 1/2
2 1/2 x 7 1/4
2 1/2 x 9 1/4
2 1/2 x 11 1/4
2 1/2 x 13 1/4
2 1/2 x 15 1/4

​3 1/2 x 3 1/2
3 1/2 x 5 1/2
3 1/2 x 7 1/4

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Regulating Lumber Dimensions

​The US Department of Commerce standardized softwood dimensions for green and finished lumber back in 1969.

Today, regulating dimensional accuracy in the US falls under the responsibility of the National Institute of Standards and Technology (NIST). The current standards are published by the government in a document titled the American Softwood Lumber Standard (PS 20). The latest version is labeled the Voluntary Product Standard PS 20-15.

Why is regulation so difficult?

State and local inspectors spot check lumber dimensions at mills and retail locations but determining if softwood boards are within limits is complicated. Here's why:​
​  

1. Shrinkage
   Green lumber has higher moisture content. As the wood dries (either in an oven or air dried) the dimensions reduce. Depending on wood species - and specifically wood grain direction - shrinkage can range from 2% to 15%. This equals anywhere from 0.012” to 0.6" for a 4 inch board.
    
2. Surfacing
   Boards with smooth sides are said to have a dressed surfaced. The process of smoothing out board surfaces further reduces the original width or thickness. Dimensional lumber is commonly labeled S4S. The designation stands for "surfaced 4 sides".
     
3. Inconsistent Testing
   Inspectors do not have nationally agreed-upon testing procedures. Everything from an individual inspector's sampling protocol to the device used for measuring can muddy the results. That's about to change.

 
If you're wondering about hardwoods, thickness measurements are slightly different, but most hardwood widths follow the softwood standard. As an example, a 6 inch hardwood board is actually 5.5".


New Testing Procedures Under Review

The Office of Weights and Measures (OWM) at the NIST has been working with the American Lumber Standards Committee (ALSC) since 2016 to develop new testing procedures for dimensional accuracy. The next big step in moving the new procedures forward is expected by summer 2018 when the proposal goes to a vote.

Have you ever wondered about about the impact of lumber's dimensional accuracy on wooden pallets and crates? Have your own wrong-size lumber story? Talk to me in the comments below.

Post by: Kent Longardner, Technical Sales Manager