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.

Are you embarking on a woodworking project with a hint of uncertainty about lumber dimensions? Many craftsmen have encountered the frustration of commencing a project without a clear grasp of lumber terminology, only to discover their creation marred by unevenness or inadequacy, necessitating a fresh start.

Fear not, let me unravel the enigma of 5/4 lumber, and tackling the intricacies of lumber dimensions, paving the way for smoother, more successful woodworking endeavors.

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 a metric used for rough-sawn lumber, indicating the thickness of the rough stock in quarter-inches.

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

The 5/4 x 4 lumber is my go-to for outdoor projects like playsets, walkways, and landscaping. Crafted by seasoned woodworkers, it’s designed to withstand the elements. Plus, it readily accepts stains and paints for a personalized touch. It’s the ideal choice for both durability and aesthetics.


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].


When you embark on your next lumber-buying journey, keep in mind that the designation “5/4” pertains to the thickness of the wood. While it may appear to be a straightforward fraction denoting thickness, it’s important to note that 5/4 isn’t an absolute measurement. 

It’s more of a rough approximation, serving as a starting point. If precision is your aim, achieving a precise 1 ¼-inch depth may require selecting lumber that’s slightly thicker than the nominal 5/4 designation. This subtle nuance in measurement can make a significant difference in your woodworking projects, ensuring a polished and professional finish.

robert headshot

Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

Leave a Reply

Your email address will not be published.

Related Articles
Join our community on facebook and get 3 woodworking plans for free!