What is 5/4 Lumber? Wood Thickness + More Explained

If you buy something through our posts, we may get a small commission. Read more here.

Share It

Have you started a project without knowing what lumber dimensions are? Then, chances are you’ll end up with an uneven craft and might have to start the project all over again. Fortunately, our pro woodworkers have the answer to what 5/4 lumber is in this guide.

What is the Actual Size and Thickness of 5/4 Lumber?

For calculating lumber, nominal measurements are sold rather than actual measurements. Furthermore, the standards for hardwoods and softwoods are different.

As a common pre-cut of lumber, the actual nominal size of 5/4 is 1 ¼ inch. This is the rough size or actual thickness, and 1 ⅛ inch is the exact size when surfaced on 1 side. When surfaced on 2 sides, the actual size is 1 1/16 inches.

Explaining the Quarter System

The quarter system is the metric for rough-sawn lumber. It simply indicates how many quarter-inches thick the rough stock is.

5/4 lumber

For instance, 4/4 or four-quarters lumber is 1 inch thick in the rough. 5/4 is 1 1/4 inches thick, whereas 12/4 would be 3 inches thick.

Most of the time, you will lose about ¼ inch of material when you smooth out rough lumber. So, as a general rule, 4/4 stock surfaces down to 3/4-inch-thick wood and 5/4 surfaces down to 1-inch-thick wood. 

What Lengths are 5/4 Boards Available?

The length of the boards is usually precisely what you want to order, though there are generally only stock pieces of even lengths available.

The most common sizes are 8′, 10′, 12′, 14′, and 16′. Longer lengths up to 24 feet are available in larger timbers, such as 2x8s, 2x10s, and 2x12s.

A 5/4 lumber’s actual dimension is 1” x 5.5”. Thus, you only get two-thirds of a 2×6 board if you use a 5/4 decking.

5/4 lumber for construction

Technically, dimension lumber is sawn softwood between 2 and 5 inches thick and 4 to 12 inches wide.  The length of lumber can be anywhere from 6 to 24 feet long. And 5/4 boards are usually available in 6 feet lengths.

What is 5/4 Lumber Used For?

Decking is one of the most popular projects for 5/4 boards because it is generally a cost-effective and environmentally friendly option.

For example, if building two similar-sized decks, one with 5/4 boards and one with 2×6 boards, the 5/4 board deck will require fewer trees to be cut down to get the same amount of wood as the 2×6 board deck.

Read NextActual Size of 2×6 Lumber 

5/4-Inch vs 1-Inch

A 5/4 board is roughly 1 1/4 inches thick because each quarter refers to 1/4 inches thickness. If your project requires lumber that is exactly 1 inch thick, buy a 5/4 board and mill it down to the required size with a jointer or planer.

marine grade plywood

5/4 x 4 Lumber

A 5/4 x 4 lumber is ideal for playsets, walkways, landscaping, stair support, and other outdoor projects where lumber is getting exposed to the elements. Our professional woodworkers commonly stain and paint this wood size. 


Why is it called 5/4 board?

It is called 5/4 board because it is a measurement woodworkers use to determine lumber dimensions in the quarter system. Some professions, trades, and regions will use different terminology based on what works best for them. You can also say five-quarters lumber instead of five-fourth.

What are the actual dimensions of 5/4 decking?

The actual dimension of a 5/4 decking is 1’ x 5.5”. The length will vary depending on what you want to order. However, 5/4 lumber usually comes in 6 feet lengths and may range up to 24 feet [1].


Our resident woodworkers are often asked what 5/4 lumber is, and the answer is simple as it seems. When you buy lumber next time, remember that 5/4 simply refers to the thickness of the wood. 

And although 5/4 appears to be a fraction of the thickness, it is not an exact measurement. This is only the rough size, and if you need to smooth it out, you might need to get something that’s thicker to get the exact 1 ¼ depth.

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
Robert Johnson
Related Articles
Get 3 Free Woodworking Plans

We’ll send you free plans and more cool stuff. No spam.

Get 3 woodworking plans for free

Instant Access. Free. Beginner-Friendly.