If your fitted sheets have not been fitting correctly and riding up at the sides, the most common reason is they are not deep enough.
If you are buying a fitted sheet or mattress protector we will state the depth separately to the width and length of the mattress.
Super King extra deep fitted sheet 6ft x 6ft 6" (183 x 200cm) + 15" (38cm) depth
Super King extra deep mattress protector 6ft x 6ft 6" (183 x 200cm) + 15" (38cm) depthIt is best to buy your fitted sheet or mattress protector at least 2 - 3 inch (8cm) deeper than your mattress to allow for a good tuck in and avoid the common problem of your sheets coming off in the night.
For example you would not put a 12" (30cm) fitted sheet on a 12" (30cm) mattress it would not be deep enough, you would need the extra deep 15" (38cm).
If your mattress is only 8" (20cm) deep then a 10" (25cm), 11" (28cm) or 12" (30cm) fitted sheet will all work perfectly, but you can use an Extra Deep 15" (38cm) if you wish.
Never buy the wrong size sheet, for example a super king fitted sheet for a king size bed, just buy a king size fitted sheet that is deep enough for your mattress.
As long as you are buying the correct size of width and length fitted sheet, then it doesn't really matter if the sheet is too deep because the excess just tucks underneath the mattress. The best rule is the deeper the better, then even if you add a mattress topper later your fitted sheet is still deep enough.