Note: Not all sheets are created equally! Read more about bed sheets at Before You Buy Bed Sheets. Mattress sizes also vary! Always check the mattress measurements before cutting your sheet.Measure the sides and width of the mattress
Measure the ends and length of the bed.
ADD 4" - 6" inches to each measurement to allow for sheet to tuck under the mattress. (I like a lot to tuck under but if your fabric isn't wide enough, go with a scant amount to tuck under but keep it even on all edges.)
If you are using 45" wide fabric, you will need to join lengths of fabric to have the fabric wide enough. Add the fabric to each side to avoid having seams on the area some one will be sleeping.