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Why are 2" x 4"s Not Two Inches by Four Inches?

Carpenter working with a two by four.
Tree stumps before being milled.
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  • Written By: Michael Pollick
  • Edited By: Niki Foster
  • Last Modified Date: 15 December 2014
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Many do-it-yourselfers have discovered that the two by fours (or 2 x 4s) they bring home from the lumber supply store are not actually 2 inches (5.08 cm) thick or 4 inches (10.16 cm) wide. The actual dimensions of this lumber can vary somewhat, but a true measurement of 1.5 inches (3.81 cm) by 3.5 inches (8.89 cm) is not unusual. Still, the designation is not necessarily a misnomer.

The lumber industry is nothing if not consistent with its measuring system. When harvested trees are brought to commercial sawmills for processing, the first cuts can be rough. Many of the irregular outer planks are discarded immediately. The usable lumber is often cut into easily divisible multiples of two, such as 24-foot (7.3-m), 12-foot (3.6-m), and 6-foot (1.8-m) lengths.

Indeed, at one point in the milling process, two by fours actually do measure 2 inches by 4 inches. This is the measurement of the planks just before they are run through a machine called a planer. A planer uses sharp blades to shave off all of the imperfect edges left behind by the rough sawing process. Commercial lumber mills may have to plane off as much as 0.5 inch (1.27 cm) of length and width from two by fours to provide a quality product for carpenters, roofers, and homeowners.

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Older boards recovered from 100-year-old homes and other buildings may actually be true two by fours, however. Carpenters routinely planed their own lumber onsite to create a better fit between individual boards. Sawmills did not always use planers to create perfect boards — framers and carpenters were accustomed to working with rough-sawn planks of variable thickness and appearance. Planks were generally cut to standard measurements such as eight by eights, two by eights, and one by twos. The measuring terms still remain a popular reference, even if the actual dimensions are not entirely accurate.

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anon981697
Post 20

With all the problems California has and this is what they're trying to fix?

anon981638
Post 19

A 2"X 4" rough cut piece of lumber is allowed 1/4"waste for each side for truing up the board to it's finished or dressed dimension of 1 1/2"x 3 1/2". All contractors, architects, and carpenters including their apprentices know that. Apparently that idiot of a judge does not. The sizing practice is not a result of trying to create a four inch wall.

anon976293
Post 17

People are too stupid and too many lawyers have no purpose being on earth. I may only be a kid of 52, but I have known this my whole life. Any person who has ever built anything with wood knows the size of dimensional lumber. We are becoming a nation of the most insanely idiotic creatures imaginable.

anon975379
Post 16

It also helps when you create a wall using modern 2x4 and 5/8" wall board (on each side) you get a 5-inch thick wall. It's easier for designers to deal with.

anon975295
Post 15

Most engineers plan for a 4" thick wall in modern construction. So a 3 1/2 inch thick board with a 1/2 inch piece of sheetrock is 4" Duh! Not hard to figure. And those who can't figure out that you always center a board before you place it need their collective heads examined. We are a nation of whiners. Why this judge ever decided to hear this case is beyond me. It should have been tossed long ago. Ever try buying plumbing fittings? They aren't even close to what they say they are!

anon975240
Post 14

So, it's something that has been going on for many many years and takes someone who says "I'm not smart enough to figure this out" and sues someone else because they are an idiot. And what's worse is some liberal judge finds the store guilty and fines them. Guess who pays for that? Typical of where our country has gone.

anon975238
Post 13

There aren't too many do it yourselfer types who don't already know this. People, educate yourselves!

jhale411
Post 11

Quit your moaning and crying and embrace the change. There is nothing you can do about it.

anon350522
Post 10

Why does it matter? It's been this way for years, it's the standard and it's not going to change anytime soon, if ever. What matters most in most projects and builds are the lengths and consistency of the boards. If the thickness and widths are consistent then it shouldn't matter if they are all a 1/2in off.

If you truly need exact sizes, go to a lumber yard and pay more. True contractors and builders have no problem with this size difference; it's only the inexperienced do it yourselfers who freak out when a project calls for a 2x4 and find out it's actually smaller. Or someone measured some boards they need to replace and they go to the store and can't find the 1.5 x 3.5's. If it's such an issue, have someone who knows what they are doing do it for you.

anon324458
Post 9

I think it is about money for the most part. But what I really don't understand, is the fall off of measurement in the wider lumber, like a 2x4 being 1 1/2 thick and 3 1x2 wide, and a 2x10 being 1 1/2 thick and 9 1/4 wide.

anon277328
Post 7

I agree with anon110499. If a sawmill can adjust its cutters to increments of 1/4", they can make the proper adjustments to give us all lumber dimensions as advertised and sold. The truth in advertising in America is a joke. Worse, I needed a few boards for an interior wall, and I needed 4 inches wide, not 3.75!

So I tried to buy a couple of 6" wide boards and have them ripped down to 4," but neither Loews or Home Depot would cut them. Said it was a safety hazard.

anon229337
Post 6

How does a DIY'er like myself ever figure out how much lumber I really need? The lumber was posted as a 10" wide piece, but it was actually 9 1/4" wide. Why aren't the true measurements posted, or cut the wood to begin with so it ends up being a 10" wide piece? Is it so I have to purchase more lumber than needed and cut it down? No other industry would get away with this. Buy a 2" screw ...oops it's only 1 3/4 long. False advertising.

anon178680
Post 5

When did the industry generally change from full dimensional lumber to the sizes we use today? Was it after World war II? I would like to know.

anon137053
Post 4

Try this on for size (as it is the "why" of the matter).

2" x 4" is the name (as is 2" x anything... 1" x whatever). This "sizing standard" has been the norm since sawmills began supplying wood to carpenters/builders in America. And, most wood was not planed down much at first, and the fact is that many of the truly older homes in America still show much of the rough wood that they were constructed of back then.

True, if a joint needed whittling or planing, the carpenters, being the craftsmen that they were, would do what was necessary to the rough wood (and this even includes the whittling down of wood to make pegs for the joints, etc.).

Much of the wood back then was as much as 1/4" under/over size.

Then sawmills added planers and a touch of consistency was enjoyed -- if you dealt with the same sawmill time and time again!

Then came change. As time moved on, so did peoples' care/concern for the trees that were being harvested for construction, and add to that the increase of homes being built as the country grew.

Also, as time moved on, so did consistency. So, save the trees/plant a tree became the motto of life.

And, more board-feet of lumber can be had from a tree if the lumber is cut at 1 3/4 inches rather than 2 - 2 1/4 inches. Also, lumber cut at 3 3/4 inches gives more cuts per tree than cuts at 4 - 4 1/4 inches.

anon113211
Post 3

2x4 should be 2 x 4, or say 1.5 x 3.5 or 1.5to2 x 3.5to4 or use a ~ sign.

anon111528
Post 2

Lumber today is produced to standards set forth by the grading organizations that regulate the quality of the grade. For example in the northwest fir, hemlock, western red cedar, spruce, and pine are regulated by the WCLB (West coast lumber inspection bureau) They set a standard size for mills that they govern so the buying public can go to their local hardware store and buy a 2x4 and go to another store and buy the another 2x4 and they will be the same size.

Within their rules the standard for a standard dressed (planed)green 2x4 is 1 9/16 by 3 9/16. This will allow for shrinkage. If the lumber has been kiln dried to the standard of 19% moisture the dressed size is 1 1/2 by 3 1/2. This has been the standard for a number of years. Rug green is manufactured to 1 3/4 by 3 3/4 This allows for shrinkage or saw snake.

anon110499
Post 1

Hogwash! This is the lumber industry shaving down the numbers over the years to help pad their bottom line they're giving you only about 65% of the lumber they advertise. 1/4" from a planer on each side? Why is it only an 1/8" for 1" material? My house is only about 50 years old and every board in it is dimensional, this was a cookie cutter housing project too so this isn't a matter of people on site making true boards, the downside is it makes it a royal pain in the butt to repair anything because nothing is the same size any more!

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