4 Steps to Understand Nominal and Actual Sizes

From simple things like specie to milling options like straight-line ripping or sanding, buying lumber can be tricky. One of the biggest catches to lumber is the size. If you are buying lumber, you might notice that the size that the lumber is advertised isn’t the actual size of the product.

1. What is Actual and Nominal Size?

Let’s take a 2×4 for an example. 2×4’s are advertised and are sometimes understood as measuring 2” thick by 4” wide. This type of standard size board is often called dimensional lumber. This 2×4 measurement is the nominal size, but that’s not the actual size. If you measure a 2×4, you’ll find that it measures 1 ½” thick by 3 ½” wide. The finished size of a product is the actual size. The actual size is always smaller than the nominal size.

2. Importance of Knowing the Difference

The most important reason you need to know the difference between actual and nominal size is proper communication between yourself and your lumber supplier. What I mean by this is if buy a 2×4 from a home improvement store, expect the 2×4 to measure 1 ½” x 3 ½”. You won’t find a construction 2×4 to measure 2” x 4”. This means if your project requires the parts to be an actual size of 2” x 4”, you’ll have to purchase a different size material. The other solution is to design your project based on the actual standard sizes. 

If you specialty order dimensional lumber, make sure to properly communicate with the lumber supply the actual/finished sized product you need/expect.

3. Hardwoods and Softwoods are Different

The last tricky part of nominal and actual sizes is softwoods and hardwoods are a little different. What I’m talking about most construction softwoods sold as 1×4, 1×6, 2×4, 2×6, etc. As you might know from previous blogs, hardwoods aren’t typically sold as dimensional lumber.

Hardwood lumber is usually sold in random widths, random lengths, and surfaced to a specific thickness. Thickness is the only tricky part of hardwood sizes. The nominal thickness of hardwood lumber is based on the quarter scale, 4/4, 5/4, 6/4, 8/4, etc. The rough size can be found by dividing the numerator by the denominator. For example, 8/4 lumber is roughly 2” thick (8 divided by 4 equals 2).  But once again, the quarter-scale is the nominal thickness. Most lumber is surfaced 1/16” less than the nominal thickness. For example, 4/4 lumber is surfaced to 15/16”. Some suppliers surface lumber 3/16” less than the nominal. That would make 4/4 lumber 13/16”. Just make sure to talk with your hardwood supplier about the thickness you need.

4. Calculating the Actual Size

Now that we know what nominal size and actual size is, we can calculate the actual size of a nominal size board.  Nominal sizes are usually in even widths and even lengths. This means you can expect to see widths and lengths in increments of 2. The thickness always increases in 1 step intervals. Sizes might look like 1×4, 1×6, 1×8, 1×10, 2×4, 2×6, 2×8, 2×10, etc. Lengths typically start at 6’ and increase in 2-foot increments up to 24’.

From these nominal sizes, we can start calculating what the actual size would be. It’s pretty simple. Any nominal width or thickness 6” or less actually measures 1/2” less. Take a 2×4, for example.  The 2×4 measures 1 ½” x 3 ½”.  Any nominal width or thickness 7” and greater measures ¾” less. This means the actual size of an 8×8 is 7 ¼” x 7 ¼”. These rules still apply if you have a width 7” and wider with a thickness of 6” or less. Following these rules, the actual size of a 2×8 is 1 ½” x 7 ¼”. There’s only one exception to these rules, which applies to 1”. The actual size of the nominal 1” is ¾”. This chart goes into lays out all the numbers:

Nominal SizeActual Size
2”1 ½”
3”2 ½”
4”3 ½”
5”4 ½”
6”5 ½”
7”6 ¼”
8”7 ¼”
9”8 ¼”
10”9 ¼”
11”10 ¼”
12”11 ¼”

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