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I was always curious about this. In the states we have:

  • Lumber - Comes most commonly in 2 inch (2x4,6,8,10,12,16) and 4 inch (4x4,4x6) varieties.
  • Plywood - Almost always 4x8 feet
  • Drywall/Plasterboard - Various 4 foot varieties (4x8,10,12) and sometimes 5 foot.

Walls are typically framed with studs 16 inches on center, rafters and joists usually the same, but sometimes 24 inches on center. Ceilings are usually 8 feet high, or 10 feet in lots of newer construction.

How does this work in the metric world?

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In Canada, while officially we are metric, all the construction work I've been around is done almost entirely in imperial. Everything comes in the same dimensions, and is referred to in feet and inches. Framing is done 16 or 24" on center, standard ceilings are 8' high.

Part of this may just be a hold-over from years of everyone doing that way, and then passing that on as they teach younger tradespeople. In school, I grew up learning the metric system, with the most exposure to feet/inches while doing fractions in math class.

I think part of it has to do with convenience and standardization as well: a "1.22 x 2.44m plywood" is awkward to say, and changing the dimensions slightly to 1.25 x 2.5 m would be strange. 16" (0.406m) centers is about the right distance for drywall, and so if you had 1.25m sheets of drywall, your centers would have to be 0.416m (or 16.4") to work out.. that's awkward to deal with.

Personally, I use a tape measure that has both centimeters and feet/inches on it, and often use cm if I'm measuring any more accurately than 1/8". One thing imperial has going for it, is the foot is a very handy size. There is no equivalent in metric (the decimeter (10cm / 0.1m / 3.9") is too small (plus no one ever says the word "decimeter"), while the meter (3.28ft) is too big).

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The dimensions stay the same so you can do work on old houses. I just extended an 80 year old drywall (plasterboard) wall, the studs needed to be the same dimension as what was there before - no problem. –  Jeremy McGee Jul 24 '10 at 6:19
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@Jeremy: The problem with some old houses is that they used dimensional lumber. A 2x4 really was 2" by 4" rough cut so, in some cases, you have to use spacers to extend walls or do other similar work since a modern 2x4 has been milled to 1.5" by 3.5". –  Dennis Williamson Jul 28 '10 at 20:40
    
The main reason why Canadian dimensions are still imperial is that something like 75% of the lumber manufactured here goes to the US. –  Chris Cudmore Jun 11 '13 at 13:08

The following information is from Norway, but I would expect that the same dimensions are used elsewhere in Europe. The system used there parallels what we have here in the U.S., but the actual dimensions are different. Colloquially, terms like “two-by-four” are used, but the actual millimeter sizes are used in price lists, etc.

The dimensions referred to by the colloquial terms are quite a bit bigger than in the U.S. For instance, a U.S. milled four-by-four is actually 3.5 inches (89 mm). In contrast, a Norwegian milled “four-by-four” is actually 3.86 in (98 mm).

Here is a table I compiled with some common dimensions:

Milled structural lumber dimensions

Actual size (Europe) Actual size (U.S.) Nominal size (U.S.) 30 mm 1.18 in 36 mm 1.42 in 48 mm 1.89 in 38 mm 1.50 in 2 in 61 mm 2.40 in 73 mm 2.87 in 64 mm 2.50 in 3 in 98 mm 3.86 in 89 mm 3.50 in 4 in 123 mm 4.84 in 114 mm 4.50 in 5 in 148 mm 5.83 in 140 mm 5.50 in 6 in 173 mm 6.81 in 159 mm 6.25 in 7 in 198 mm 7.80 in 184 mm 7.25 in 8 in 223 mm 8.78 in

Unmilled lumber is sold in slightly larger dimensions: 100 mm, 125 mm, 150 mm, and so on.

Studs, joists, and rafters are always spaced 60 cm (23.62 in) on center. The dimensions of OBS and other sheets are multiples of 60 cm in both dimensions so they can be placed in either direction, e.g., 60 x 240, 60 x 300, or 120 x 300 cm.

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I'm from all metric country, but lot of stuff in constructing busyness are in "cols" that is inches. Again things like 2" or 4" are around 5cm and 10cm, so it's all round numbers.

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Depends on the country. Most English-speaking countries use imperial system, but convert all the dimensions into m/cm/mm and kg. For example, this is a 2x4

However, it's usually not an issue outside of US/Canada, as most of the other world doesn't build houses out of dimensional lumber. They use bricks, cement blocks, cement panels, etc. and the dimensions are almost always communicated in millimeters

Most of the plumbing/windows-doors/etc. uses metric system.

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(Un)fortuantely in Australia the dimensions are exactly the same. Which always throws me when I measure in MM and I go to the hardware store and they start talking in feet and inches.

It also answers questions about why some things are such incredibly odd dimensions. When you convert them from fractions-of-a-cm to feet/inches they usually fit some arcane measuring standard from the 1920s.

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What's more, 2x4's aren't 2 inches by 4 inches, that's the measurement before they get planed. –  Brad Gilbert Jul 24 '10 at 1:26

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