Timber Sizes....... My Frustration

Good Evening

So, a long running curiosity of mine…

I’ve never got my head around Timber sizes as quoted and listed by Timber Suppliers. Not in the sense of the different sizes supplied, different grades etc. I understand all that.

But, one thing I have never understood is the background to the sizing of timber. For example, a 6 x 1 inch piece of timber is actually less than 6 x 1 Inch. Why is it listed as 6 x 1 inch if it is not 6 x 1 inch? Why is the actual size not listed? To me, it might as well be listed as 6 Bananas x 4 Sheep, as the size listed is irrelevant, as it is not what you actually get.

I’m guessing there must be some historical reason behind this, but I do find it a pain sometimes.

Am I correct in stating that PSE timber sizes are the actual sizes? There seems to be so many conventions that sometimes it confuses me, (which is not difficult to do :grinning:).

Anyway, just thought I’d ask.


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I have always thought that the stated sizes are the rough sawn ones before shrinkage during drying and dressing to final size. But yes it is quite confusing to novices why (here in the US) a so-called 1 by 8 is actually 3/4 x 7 1/4.


Generally the size before dressing is the given reason for the size discrepancy but over the years the actual timber sizes have been reduced. 2x4s used to be 2 inches by 4 inches. I remember when they were around 1-3/4 in. by 3-3/4 in. and now they are typically just barely 1-1/2 by 3-1/2.


I think it will be the first question I ask when I can get back to the Timber Yard near me, once this lockdown finishes, and I order all the stuff for my new shed.

It’s the only industry I can think off where sizing is done in this way. They might as well just use the size of the tree the piece of wood cane from.


Look at the common 2x4. You’re not far off. :slight_smile:

You mean bunker don’t you, I’ll keep an eye out for you on doomsday preppers.


After a while you memorize the numbers. Then you learn, wood doesn’t stay that size…flush frame the tops of 2x10’s with an LVL beam, and the 2x10’s shrink and the LVL doesn’t, leaving a hump in your floor at the beam.

What puzzles me more is the standard practice in mail order house plans to dimension all interior walls as 4" What’s 4"? The studs? No, they’re 3 1’2". The finishes? No, that’s at least 4 1/2" if not a little more. In this era of drawing on the computer, it does the math for you. Why not just draw it correctly and let the computer do the math?


It’s all very simple and standardized

2x4 = 1.5 x 3.5
2x6 = 1.5 x 5.5
2x8 = 1.5 x 7.25
2x10 = 1.5 x 9.25
2x12 = 1.5 x 11.25

1x4 = 0.75 x 3.5
For those of us who deal with these numbers everyday there really is no mystery.

Sometimes you do encounter rough sawn lumber which could be any size, but by and large things are pretty standard in the US.

If you need more specifics go right to the source, Table 1A (Page 13):


The question wasn’t what are the sizes, it was why are the sizes.


@MichaelSiggers I am puzzled by your use of the word “inch” as you appear to be a UK resident. I assume it is because you assume most people responding to this will be in the US and it will make sense to them.

I tend to refer to your bog standard “2x4” as 50 x 100 but in practice the timber you get supplied with (as you point out) is never a nice round number. That is no doubt partly to do with machining, and maybe also to shrinkage. Nowadays it is also affected by “regularisation”. This article may help:


My experience (when I was a full time builder) at my local timber merchant (Norwich UK) was that day to day we would colloquially say… “inch by 6”, “2 by 4”, etc and append Sawn, CLS, PSE after.

If we needed to clarify we would use metric.

If we made a formal order it would be in metric.

These would probably be the settings of the mill when sawing. In the old days, the moment of cutting the tree and sawing it could be more than (several) years. With the production forests, it is less then a few weeks if not days. That and the combination of 4-sided cutters that need to bu used to get straight timber results in smaller dimensions. So the difference would be bigger, these day’s.
And then there are the local differentations (before Napoleon, we had about 16 different sizes of a ‘thumb’ in our area, alone)

even the standard meter ( as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris , assuming an Earth’s flattening of 1/334) won’t do for some industries, but is very common in the AEC.

I was very frustrated when ordering a FAT MAX stanley from a webshop and it had some sort of 12 numeric system on it (!)
But there are stranger systems, though:

Schermafbeelding 2020-04-22 om 14.55.21

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If you’re going that way…
Does the statement, “We’ve always done it that way” ring any bells? The US standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. That is an exceedingly odd number. Why was that gauge used? Because that’s the way they built them in England, and English expatriates built the US Railroads. Why did the English build them like that? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that’s the gauge they used. Why did “they” use that gauge then? Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing. Okay! Why did the wagons have that particular odd wheel spacing? Well, if they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England, because that’s the spacing of the wheel ruts. So, who built those old rutted roads? Imperial Rome built the first long distance roads in Europe (and England) for their legions. The roads have been used ever since. And the ruts in the roads? Roman war chariots formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels. Since the chariots were made for imperial Rome, they were all alike in the matter of wheel spacing. The United States standard railroad gauge of 4’-8.5" is derived from the original specifications for an Imperial Rome war chariot. And Bureaucracies live forever. So the next time you are handed a specification and wonder what horse’s ass came up with it, you may be exactly right, because the Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war horses. Now the twist to the story…when you see a Space Shuttle sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters, or SRBs. The SRBs are made by Thikol at their factory in Utah. The engineers who designed SRBs would have preferred to make them a bit fatter to carry a bigger fuel payload, but the SRBs had to be shipped by train from the factory to the launch site. The railroad line from the factory happens to run through a tunnel in the mountains. The SRBs had to fit through that tunnel. The tunnel is slightly wider than the railroad track, and the railroad track, as you now know, is about as wide as two horses’s behinds. So, a major Space Shuttle design feature of what is arguably the world’s most advanced transportation system was determined over two thousand years ago by the width of a horse’s ass. And you thought being a horse’s ass wasn’t important!


I LOVE your last post, Box! Thank you so much.

More soberly, and reverting to the original topic thread…

In the UK, the same sort of reduction in size took place from sawn to planed dimensions for softwood PAR (Planed All Round), sometimes called PSE (Planed Square Edges) in other countries.

But the reduction is smaller (about 1/4" or 6mm for nominal sizes above 1", or 1/8" or 3mm for sizes 1" or below), not the 1/2" or 1/4" used in US and Canada.

@slbaumgartner helped me last year to create a plugin (avalaible free from SketchUcation Plugin Store) to draw Wood Framing. We had to create different finished sizes for use in US and Canada, vs. UK.

Not sure what happens in Europe or Australia/NZ.

While UK usage is still to call something (for example) 2x1 or 4x2, timber suppliers have rounded these in metric (e.g., 50 x 25mm or 100 x 50mm). They finish at 44 x 22mm or 95 x 44mm respectively (7/8" x 1 3/4" or 3 3/4" x 1 3/4") but approximately. Actual dimensions vary slightly between batches, depending on shrinkage and machine settings.

Here’s a link to the plugin.


to further add to the confusion:
I buy some of my wood by the term’ 4/4 , 5/4 3/4 8/4 . This is rough sawn lumber. Each term relating to 1/4 inch increments.

Now I can picture @Box’s interest in rockets, walking down the road … towards the pub park!

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The UK only half-went metric in 1968.

At our local amateur theatre (abbeytheatre.org.uk, founded in 1934 and in its own purpose built theatre from 1968) much of our stock scenery was bought or built in the 1960s and 70s.

To match the old flats and wooden rostra, we continue to design and build sets in imperial dimensions, mostly in multiples of a foot or 6 inches overall sizes.

We only convert to metric when ordering timber or sheet materials.

Then order timber as noted above - e.g., 25 x 50mm (timber merchants usually put the smaller dimension first) with lengths in multiples of 300mm (just shorter than a foot, by enough to matter!)

A full sized sheet of material like ply, chipboard, or MDF is 1220 x 2400 mm or (surprise?) 4ft x 8ft within a mm or two.

There has long been a difference in the US between buying from either a sawmill or a wholesale supplier vs buying from a general retailer. The standard practice for wholesale rough lumber is still to give the thickness in 1/4 inch steps as you noted. These are intended to be the actual size, though I often see variations up to +1/8 and -1/16.

Usually the other dimensions are “random width and length” (RWL). The expectation is that the user will dress the lumber to make it flat, smooth, and the required thickness, length, and width. Because 1/2" isn’t a standard size and planing 3/4 down to that wastes a lot of wood, I often buy 5/4 or 6/4, resaw it, and then plane it down to 1/2.


That was the original definition of the meter from 1799. However, it was soon discovered that the length of the original piece of iridium was based on erroneous calculations, so it was decided that the actual piece was the new normal. Since 1960 the length hasn’t been based on actual pieces of matter but on measurements performed with light. So when you go buy a measuring tape or laser it has been calibrated by comparing it with something that is a descendant of the collection of lasers and prisms that resides in the ISO laboratory.

Yes, this causes many issues with floors and beams, if the beams are at (2x30) 60cm at heart and you need to replace the solid wood for ply, you end up sawing the plywood.