Why is a 2×4 not 2 Inches by 4 Inches? (Solved!)

Why is a 2x4 not 2 Inches by 4 Inches

Have you ever purchased a 2×4 piece of lumber at the hardware store and started working on the project to realize that the 2×4 is not 2 inches by 4 inches. Are lumber companies trying to fool us into thinking we’re buying more lumber than we actually are?

The simple reason why 2×4 is not 2 inches by 4 inches is that lumber mills trim off rough or warped surfaces of a 2×4 to give it a more polished and finished look. By planning the lumber on all four sides, the original 2×4 is now reduced to 1 ½ inches by 3 1/2 inches. Once the lumber is planned to actual size, then the edges are rounded to reduce injury.

All other dimensional lumber, from 1×2 boards all the way up to 2×12 boards, are planed as well, and thus are smaller than the name they are given suggests. In the construction industry, the actual dimensions don’t actually matter anymore because everything has been sized to fit the new, smaller pieces of lumber that are today being produced.

Nominal vs Actual Lumber Size

Nominal Size – Actual SizeNominal Size – Actual SizeNominal Size – Actual Size
1 x 2 — 3/4″ x 1 1/2″1 x 12 — 3/4″ x 11 1/4″2 x 10 — 1 1/2″ x 9 1/4″
1 x 3 — 3/4″ x 2 1/2″2 x 2 — 1 1/2″ x 1 1/2″2 x 12 — 1 1/2″ x 11 1/4″
1 x 4 — 3/4″ x 3 1/2″2 x 3 — 1 1/2″ x 2 1/2″4 x 4 — 3 1/2″ x 3 1/2″
1 x 6 — 3/4″ x 5 1/2″2 x 4 — 1 1/2″ x 3 1/2″4 x 6 — 3 1/2″ x 5 1/2″
1 x 8 — 3/4″ x 7 1/4″2 x 6 — 1 1/2″ x 5 1/2″6 x 6 — 5 1/2″ x 5 1/2″
1 x 10 — 3/4″ x 9 1/4″2 x 8 — 1 1/2″ x 7 1/4″

What Is the History Of Lumber Sizes?

The history of how lumber sizes were determined is quite fascinating if you go back far enough. Of course, wood has been used for centuries all over the world in the construction of homes and other buildings. We won’t go back that far but instead to the middle of the 19th century.

It was at this time that the lumber used in construction was almost all produced in lumber yards and sawmills that were very close to the town where they would be used. There were lumber mills all across the United States producing lumber for towns and cities from coast to coast. In most cases, the lumber that they produced was cut to the sizes that builders in the specific area were using and asking for, and so they were different from one lumber yard to the next.

Keep in mind that, at this time in America’s history, most houses, barns, and other buildings were made by hand-fitting the lumber piece-by-piece, which was extremely time-consuming but made the buildings much more durable and strong. If a board wasn’t cut to the correct size it was simply made to fit by the builder, usually with an ax) and uniformity of lumber from one lumber yard to the next was practically unheard of.

As the 19th century drew to a close, however, forests were cut back further and further from the towns and cities where it was needed, and thus it had to be shipped longer and longer distances to the end-user. Locally made lumber became more of a rarity and, as this happened, builders began to notice that the lumber from 1 lumberyard was different from another in terms of the sizes that they were cutting.

Lumber From Old Homes Tells the Tale

If you’ve ever watched “This Old House“, a show about remodeling houses, you might have seen episodes where they removed boards from very old homes. The evidence that this gives us about what was used at the time is very interesting.

For example, at the turn of the century, the common board thickness for studs, rafters, joists and other framing boards was 2 inches. Regular boards for siding and flooring were more or less 1 inch thick. As before, there wasn’t a big problem with size variations because most regions of the country got their lumber from specific regional sources. But that all changed as the railroads started to be built and started to connect the country from one end to the other.

The railroads made a big difference because now lumber from lumber mills hundreds of miles away could be shipped all over the country. When that started happening, builders really started noticing the big differences in the sizes of the lumber they were receiving. Rough lumber wasn’t really a big problem because it was assumed that it would need to be cut to a specific dimension, but the framing and building lumber was a different story.

Sizing and Cutting and Weight

With builders having a problem because the lumber they were using wasn’t uniform sizes, a number of lumber yards started cutting and planing dimensional lumber to a uniform size to appease them and sell more of their product. Some lumber yards even offered 2 different types of dimensional lumber, S1S and S2S, which stood for ‘surfaced one side’ and ‘surfaced two sides’. There was even an option (if a builder was willing to pay extra) to get S4S lumber, or ‘surfaced 4 sides’, which is what you will normally find today in places like Lowe’s and Home Depot.

The result of all of this extra cutting and planing was that the actual wood weighed less, which led a few enterprising individuals to realize that they could make more money selling uniformly cut wood boards than they could by selling raw, uncut wood. A new industry was born!

It wasn’t long until lumber associations started to form all across the country, setting size standards that the members of their association would then follow.

These were all actual, functioning associations that determined what the specific sizes and grades, of lumber, would be. Even Kelloggs’s got in on the act with their 1914 lumber standards guide Lumber and Its Uses.

The End of Word War 1 Marks the Beginning of Lumber Standardization Nationwide

After the end of World War 1, there was a huge surge in construction across the entire United States. At this point, standardization was a regional phenomenon and thus lumber from one region of the country could, and usually was, still different than another region. Retail lumber dealers at the time were not very happy with this situation and it all came to a head at the American Lumber Congress in April of 1919.

As most congresses love to do, they formed a committee to investigate whether uniform lumber sizes should be adopted nationwide and, in June of the same year, they decided that yes, it was necessary. That included standardization in sizes of lumber as well as grades, nomenclature, forms, and moldings.

Fast forward to 1921 when national standardization of lumber size and grading actually began. In fact, throughout the entire year several groups of lumber manufacturers, wholesalers, and retail dealers spent a lot of time considering what the size standards for lumber should be. It was in 1922 however, as far as records tell, that the 2-inch board we still refer to today was born, at a meeting of eastern lumber associations in the great city of New York.

Things have changed in the lumber industry quite a bit since those heady days of the 1920s, and the way homes and buildings are being built today has changed even more (most would say for the worse). Whatever the case may be, however, the reason a 2×4 wood today isn’t actually 2 inches by 4 inches stretches way back to the beginnings of the 20th century.

What are the Different Types of Wood Used for Lumber?

The word ‘lumber‘ is a generic term that applies to all wood that is cut and planed to be used in construction projects. Lumber can be produced using either ‘soft’ or ‘hard’ wood, and the difference between the 2 is quite interesting.

Hardwood such as oak, walnut, cherry, maple, birch and even balsa are made from trees that have leaves that, usually, are shed in wintertime. Softwood, including fir, pine, spruce, hemlock and redwood don’t have eaves but instead have needles, which they don’t shed in winter. You might know them by their nickname, evergreen trees.

In the construction industry, the use of softwoods is widespread. They are used in beams, rafters, stringers, posts, decking, sheathing, planks, subflooring and for making the forms that are used when concrete is poured. The reason that softwood is used so extensively is simple; softwood costs less than hardwood.

Why is Lumber not Actual Size?

Lumber is not actual size because it is typically cut before it is dried and shrinks as it dries. The boards are then planed (surfaced) to the finished thickness, which takes off more wood, making the lumber smaller than the original dimensions. In addition, when lumber is first cut from the tree, it is in what is called the “green” state and contains a lot of water. As the lumber dries, it shrinks both in length and width. The amount of shrinkage depends on the type of wood and its initial moisture content. For example, oak shrinks more than pine.

There are two important things to remember about shrinkage: 1) it happens to all wood, and 2) it can’t be stopped. You just have to account for it in your projects. The best way to do this is to buy lumber that is “dry” or “kiln-dried.” This lumber has been dried in a kiln, which controls the amount of shrinkage that occurs. Once the lumber is dry, it is much less likely to shrink any further.

Of course, even dry lumber will shrink a little bit as it ages and adjusts to its new environment. So, if you are building something that needs to be an exact size (like cabinets or built-ins), it’s always best to allow for a little bit of shrinkage by making your pieces


Why is a 2X4 wood not 2 inches by 4 inches? Because standardization at the turn of the century, and later modernization, took the rough 2X4 wood boards that were being produced and planed them to be more finished. (Plus a whole lot of stuff that happened in between!)

We hope you enjoyed this article and that it shed some light on your questions. You might be interested in DIY projects that I’ve built using construction lumber. Best of luck with your construction project!

Frequently Asked Questions

What is a 2×4 Board?

A 2×4 is a board that is two inches by four inches. It is a common size of lumber used in construction.

When did 2x4s Stop Being 2×4?

The answer to this question is not entirely clear, but it seems that 2x4s stopped being two inches by four inches sometime in the late 20th century. This change is likely due to the fact that lumber sizes were changed to metric measurements, making 2x4s slightly smaller than they used to be. However, it is still possible to find 2x4s that are two inches by four inches in some stores.

6 thoughts on “Why is a 2×4 not 2 Inches by 4 Inches? (Solved!)”


  2. What in the 19th century gaslighting is this?
    If. You take a 2×4 and plane it smaller, it’s not longer a 2×4. It’s SMALLER.
    And blaming metric for this is the cherry on the cake. Then just use metric and call the actual metric dimensions in millimeters like the rest of the world!


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