Posted on

Nominal vs Actual Lumbers Size – Why my 2×4 measures smaller

Construction lumber is often referred to as its nominal size or trade size. This size does not match the actual measurement of the wood you are buying. It is used in name only to identify the general/approximate size you are getting. For example, it is much easier to say “2×4” that it is to say “1-1/2×3-1/2”.

Referring to construction materials in their nominal sizes have become the industry standards for many of the products used. Other examples you will see nominal sizes used are windows and doors. Each product category has its own reasons and story for how the sizing reference came to be.

In the instance of dimensional lumber, the sizing refers to how the mill does its first cuts from the log. For an example, the mill may initially cut a 2×4 at 2”x4” but as the wood is further processed, it will end up at a final size of 1-1/2” x 3-1/2”. The reason being is that when a tree is freshly cut cell structure of the tree contains a lot of water. Once the boards are rough cut they are kiln dried, as the wood dries and loses that water it shrinks. Then the board is ran through a plainer and smoothed to its final size.

Below is a chart of nominal vs actual size of common boards used in the construction industry.


Nominal vs Actual Board Sizes in inches
Nominal Size Actual Size
1×2 3/4 x 1-1/2
1×3 3/4 x 2-1/2
1×4 3/4 x 3-1/2
1×6 3/4 x 5-1/2
1×8 3/4 x 7-1/4
1×10 3/4 x 9-1/4
1×12 3/4×11-1/4
5/4 x 4 1 x 3-1/2
5/4 x 6 1 x 5-1/2
2×2 1-1/2 x 1-1/2
2×4 1-1/2 x 3-1/2
2×6 1-1/2 x 5-1/2
2×8 1-1/2 x 7-1/4
2×10 1-1/2 x 9-1/4
2×12 1-1/2 x 11-1/4
4×4 3-1/2 x 3-1/2
6×6 5-1/2 x 5-1/2

7-1/4 x 7-1/4

12×12 11-1/4 x 11-1/4

The information provided here is only intended to be used for general knowledge. Before implementing into your project consult an architect/engineer, your local building department, and the manufacture of the products you are using.

Popular Plans: