That's why most house framing material is based on multiples of 300mm.
e.g.: 1200x2400mm (see http://www.boral.com.au/plaste... for more sizes)
300mm, 450mm or 600mm centers
1800mm, 2400mm or 2700mm lengths of timber
And 300mm is pretty close to 12 in
1 1/2 x 3 1/12 is more like 38x90 mm