I bought a 1979 split-level, twinhome. The stairs are open ended (no backing, very 70s...) The carpet wrapped around them is nasty and I want to replace it. I haven't removed the carpet yet so I don't know what the structure of the stairs are. But do I replace the carpet? How do I know how much to buy? I saw "remnant" carpet at Menards the other day which is why this got me thinking about the stairs.

I am super-super new to house-stuff, so please avoid any jargon.