Nominal vs actual sizes
Timber sizes are usually purchased in “nominal” measurements. The nominal measurements are a board’s size before it has been planed smooth on all 4 sides. The actual measurements are the final size.
Why is a 4 x 2 not really 4 inches by 2 inches?
Around 100 years ago, in the US timber size standards came about to achieve a common understanding between mills and markets separated by distance.
Historically US mills cut a “2 x 4”, rough green-sawn board 2 x 4 inches in size – this was the “nominal size”. In New Zealand, before metric conversion, we referred to it the other way around – a “4 x 2” – in metric size this is a 100 x 50mm. As time went on the boards were dried and planed to achieve size consistency, the “finished” or actual size changed. Now standard “4 x 2” framing timber has a finished size of 90x45mm in New Zealand. Over time the nominal sizes (4 x 2 or 100 x 50) have become the standard way we reference timber.
New Zealand measurements
In New Zealand we express our measurements as width by thickness. Historically New Zealand used imperial measurements up to 1976, when we cut 3×1, 4×2 and 8×2 inches for example. When we changed to metric, sawmills started referring to 75×25, 100×50 and 200×50 millimetres.
New Zealand timber yards typically size timber in millimeters (100×50) and sell in metres therefore it becomes 5.4m rather than 5400mm. Some places even sell by centimetres but that is not as common.
Timber sizes on the chart below are some of the standard New Zealand sizing, yet some timber processors, such as MLC produce sizes outside this range for products that are unique to them. They may vary from this chart by a few millimetres, for example a 50 x 25 can be machined to a commonly stocked 45 x 19, but in some applications some end users request a 44 x 20 and 45 x 22mm. Our garden stakes finish at 22 x 22mm.
In New Zealand we get a mixture of people referring to 4×2, 100 x 50 and 90×45 yet they are talking about the same thing. Even people that are born after the change in 1976 refer to a 4×2. Some people still walk into timber yards and ask for some 4×2 in 2.4m lengths. This could end up being confusing for people that have just started working at a timber retailer. Therefore, it is important when you order timber to let people know if you are referring to nominal or finished sizes and millimeters or inches.