With my luck my stair width ends up being 37 inches. Am I going to have to buy 48" planks and waste the extra material not to mention the money just to gain one inch? This is what my installer is telling me and it seems like such a waste. If I don't have any choice I might have to tackle this myself to save $$$. Any tips would be appreciated!

If you're having solid treads installed (as opposed to using stair nosing and flooring), they come in 42" widths, which would save a few $. Typical sizes for stair treads are 36", 42", 48", 52", 60" and so on. If your using flooring, you can have shorter pieces laid with end butts but on 37" wide steps in may look kinda funky. You be the judge. End butts in the middle of your steps or spending the moola to have them look good.

It's not the actual planks that will be the big waste factor. I consider it mild. Just wait until you find out how much waste is invloved on the stair nosing! Considering many prefinished nosings only come on lengths of 72" you'll only be able to use 1/2 of each one if the step width is 37"

Most of the prefinished stair nosings I have gotten are 78" (6&1/2 ft. long). If the tread width is 37", you will be able to get two steps per nosing piece and waste only 4" minus the saw blade width.