A Clever Trick!
My beautiful new LL Bean sheet set came with one fitted, one flat, and two pillowcases. I never use a flat sheet, so I converted the flat to a second fitted like this:
I laid the flat sheet on my bed and measured how much extra fabric I’d need to add in order for it to have a good, deep, elasticized tuck-in edge: Six inches, plus a half-inch for seams. I took my old Target fitted sheet (which constantly pops off the corners of my adjustable mattress because the pockets aren’t deep enough) and cut the entire elasticized edge off at 6.5″ from the elastic. This gave me a giant fabric loop with one raw edge and one elasticized edge.
I placed the loop right-sides together with my flat sheet, matching the exact centers of the edges and making sure long sides and short sides were appropriately matched. I pinned everything together well and basted. There is of course a bunch of extra sheet fabric at the corners because a fitted sheet isn’t flat (duh) but that’s fine — just tuck them under when you make your bed. If they bother you, try the sheet on your bed, wrong side up, and pin, seam, and trim away that extra corner fabric. I haven’t bothered yet but I probably will when I take the sheet off to wash it.
Now I have two fitted sheets!