a bit about measurements and dimensions
Wood used for building is referred to differently from place to place. In North America the wood used for building is referred to as lumber, where as in Australasia it is referred to as timber.
So in America you go off to the lumber store to buy a piece of lumber, and in Australasia you go off to the timber yard to buy a piece of timber. When the wood is 1 inch thick (25mm) or less, it is then referred to as a board.
When using the metric system for building, millimetres (mm) are the common unit to use.
One millimetre is about the thickness of a pin. There are 10 millimetres in a centimetre and 1000 millimetres in a metre.
When using the standard system for building, inches are the common unit to use.
One inch is about the thickness of a pack of playing cards. There are 12 inches in a foot and 3 feet (36 inches) in a yard.
use of " and ' symbols
1' = 1 foot
1" = 1 inch
1 x 3 = 3/4" x 2 1/2"
1" x 3" = true 1" x 3"
both standard and metric
1 inch = approx 25 millimetres
1 foot = approx 305 millimetres
1 yard = approx 915 millimetres
actual and nominal (what's that mean)
It is easy to see why some people get confused when purchasing wood, as when you go to the supplier and ask for a particular stock size, sometimes what you end up with is a different width and thickness than what you asked for.
Why is this?
That's because most wood is identified by it's sawn size (nominal size) rather than actual size which is smaller due to dressing (planing) and/or drying.
NOMINAL SIZE (also called sawn or rough sawn) is the size of the wood when it is first sawn and is usually in 1" (25mm) or 2" (50mm)
multiples, such as 100mm x 50mm which is also 2x4 (2 inches by 4 inches in imperial or standard measurements)
When the sawn wood is seasoned, dressed or planed the size becomes smaller which is the ACTUAL SIZE.
Therefore..... A piece of wood 100x50 (nominal, sawn size) may become approx 90x45 (actual size)
Or the same thing in standard (inches)
A piece of timber 2x4 (nominal, sawn size) may become approx 1-1/2" x 3-1/2" (actual size)
Countries that use the metric system put the bigger number first (100x50) and countries that use the imperial or standard system
put the smaller number first (2x4).
100x50 is the same as 2x4, the first is in millimetres and the second is in inches.