By Chuck Coker
December 29, 2024. Updated March 28, 2026.

Question: The museum is built with actual 2×4 lumber. When did 11/2×31/2 lumber become the standard?

If you’ve ever bought a 2×4, you probably discovered that it’s not really 2×4 inches, it’s actually 11/2×31/2 inches. You’ve discovered the distinction between the nominal size (2×4) and the real size (11/2×31/2). “Nominal” means existing or being something in name or form only.

Stacks of 2x4 lumber.
Stacks of 2×4 lumber.

A 2×4 starts out at 2×4 inches, but then it gets trimmed down. Prior to the standardization efforts of the 1920s, lumber really was cut to 2×4 inches and sold green (not dried) and unfinished straight from the mill. The problem was that freshly cut wood is rough, full of saw marks, and not very consistent—sizes could be off by about 1/8 inch depending on the saw, wear on the equipment, or just the wood itself.

To make things more uniform, mills in the mid-20th century started planing the boards smooth on two sides. That made the lumber more consistent, but also smaller—about 13/4×33/4 inches. At the time, saws could only cut in 1-inch increments, so they couldn’t just cut the boards slightly bigger to keep them at a true 2×4 after planing.

Later on, people wanted boards that were smooth on all sides, not just two, partly to avoid splinters. That meant shaving off even more wood, which is how a 2×4 ended up as the familiar 11/2×31/2.

Modern sawmills can cut wood to very precise sizes now, so they could technically make a true 2×4 again. But by the time that technology existed, the smaller size had already become the standard. Since buildings, tools, and plans are all designed around today’s dimensions, changing it now would cause a lot of confusion, mistakes, and extra costs.

So the name stuck—even if the size didn’t.

Answering related questions…

Question: What is the difference between hardwoods and softwoods?

One often hears about hardwoods and softwoods. The difference doesn’t have anything to do with how hard or soft the wood is; it has to do with the type of tree the wood comes from. Deciduous trees—flowering trees which lose their leaves each year, called angiosperms by botanists—are hardwoods. Evergreen trees—those which have needles and cones and keep their greenery year-round, called gymnosperms by botanists—are softwoods.

Oddly enough, Balsa (Ochroma pyramidale), the world’s softest wood, is considered a hardwood. Common uses for Balsa include buoys, rafts, surfboards, model airplanes, musical instruments, packing/transport cases, core stock in sandwich laminations, and fishing lures. On the other hand, Pacific Yew (Taxus brevifolia), a wood from the Pacific Northwest known for its hardness, is a softwood. Common uses for Pacific Yew include archery bows, veneer, cabinetry, furniture, carvings, musical instruments (e.g., lutes), and turned objects.

Question: Do they have 2×4s outside the US, in metric countries like the UK?

While the 2×4 is a North American standard, many metric countries (such as the United Kingdom and Australia) use a nearly identical size known as the 100×50. Much like its American counterpart, the name refers to the “green” or unplaned state.

RegionNominal SizeActual (Finished) Size
US / Canada2 × 4 inches11/2 × 31/2 inches3.50 × 1.50 inches
UK (Regularized)100 × 50 mm94 × 44 mm3.70 × 1.73 inches
UK / Canada (CLS)100 × 50 mm89 × 38 mm3.50 × 1.50 inches

Note: CLS (Canadian Lumber Standard) is popular in UK home-building because it is surfaced on all four sides and has rounded edges, making it easier to handle.

Did You Know?

The word stud (as in a wall stud) comes from the Middle English stode, from the Old English studu, meaning “pillar, prop, post, upright timber used as a support.”

One reason the industry resisted shrinking the 2×4 even further is fire safety. Thicker wood chars on the outside, which actually creates an insulating layer that protects the core of the beam, giving people more time to exit a burning building than thinner materials would.

Before Herbert Hoover became President, he was the Secretary of Commerce. On April 4, 1922, at the annual meeting of the National Lumber Manufacturers Association in Chicago, he proposed the first American lumber standards to simplify and standardize sizes, to promote fairness and equity to the producers of lumber, and to eliminate the waste incident to excess variety and confusion of sizes. These standards helped make the post-World War I housing boom possible.

Sources

“American Softwood Lumber Standard, Voluntary Product Standard PS 20-20, Revision 1”. [PDF]. National Institute of Standards and Technology. October 2021. https://www.nist.gov/system/files/documents/2021/10/26/PS 20-20 Revsion 1 October 2021.pdf

“Balsa”. The Wood Database. Undated. Accessed December 15, 2025. https://www.wood-database.com/balsa/

“Calculating the Fire Resistance of Wood Members and Assemblies. Technical Report No. 10”. [PDF]. American Wood Council. February 2021. https://web-media.awc.org/wp-content/uploads/2021/12/17210355/AWC_TR10_20210427_AWCWebsite.pdf

Curtis, Oliver J. “Nominal Versus Actual: A History of the 2×4”. Harvard Design Magazine. Undated. Accessed December 29, 2024. https://www.harvarddesignmagazine.org/articles/nominal-versus-actual-a-history-of-the-2×4/

“Hardwood vs. Softwood: Understanding the Difference”. American International Forest Products. April 11, 2022. https://www.lumber.com/blog/hardwood-vs-softwood-understanding-the-difference

“Lumber Dimensions: Nominal vs Actual”. Archtoolbox. March 18, 2023. https://www.archtoolbox.com/lumber-dimensions/

“Lumber Grade-Marking History: 1922”. Southern Pine Inspection Bureau. Undated. Accessed March 28, 2026. https://blog.spib.org/lumber-grade-marking-history-1922/

“Pacific Yew”. The Wood Database. Undated. Accessed December 15, 2025. https://www.wood-database.com/pacific-yew/

“Understanding Timber Sizes for Various Garden Projects”. Travis Perkins Trading Company Ltd. Undated. Accessed December 15, 2025. https://www.travisperkins.co.uk/content/understanding-timber-sizes