@StarkRG @katemorley Lengths of timber are usually “metric feet” - multiples of 300mm. E.g, 2400, 2700, 3000 or 4200mm, just as often written in metres so 4.2m, etc. Building sites these days are usually laid out on a 600mm grid. Joists and rafters are typically spaced at 400mm or 600mm centres.

Dimensions in inches (like 2-by4) are the rough-sawn pre-planed dimensions. Planed dimensions are given in millimetres so 2x4 might be something like 47x100mm. I've been confused for a while about why some is 45mm and some 47mm.

Sheet goods (like plywood) are referred to colloquially as “8x4” but that can be either 2400x1200 mm or actual 8'x4': 2440x1220mm. If you really care you have to be specific.

Thicknesses of ply, OSB, etc, are done in mm but they're rounded versions of the old inch dimensions so typically 6, 11, 15, 19mm and so on.

As with timber dimensions, plumbing goods use inches and mm in different ways. E.g., a ½" pipe is 15mm because the ½" references the inside diameter whereas the 15mm is the outside diameter. Plastic pipe (with its thicker wall thickness) breaks the assumptions here so «½"» plastic pipe has an outside diameter of 15mm but an inside diameter considerably less than ½".

@wyliecoyoteuk Oh, TIL, ta. I think 15 mm pipe is still colloquially called ½", though?

This is mildly relevant to me as I'll need to replace my kitchen taps at some point, though the short flexible pipes they come with should deal with this?

@StarkRG @katemorley