How do you measure fitted sheets to get the right size at the store?

you trace it on sheet of paper from the big toe to your heel,preferrably in the afternoon when your feet are at its largest size.How do you measure fitted sheets to get the right size at the store?
because if you have single bed,you get single fitted sheet , and so on double bed double fitted sheet;or queen size bed get queensize,or king size kingsize fitted sheet it will have on the bedding,single double and so on hope you get what i mean its easy when you know how.How do you measure fitted sheets to get the right size at the store?
You go by the size of your bed, for a king you ask for king size fitted sheets. The size is pretty standard for each size of bed.
know what size bed you have full , queen , king , twin it should say on the package the sheets come in
I have never measured my sheets I am married 45 years, I go to the store and buy full size, because my bed is a full size..So if you have a king size bed, then you get a king size set of sheets. when they start to get too loose .which will be a long time from now, Then you buy garters for sheets, they work wonderful..