According to Madaspen Home, "Bed skirts are used to cover the box spring, bed frame and underbed space as well as decoratively to complete the bed." Measuring for a bed skirt isn't as daunting as it may seem. You will need to remove the mattress or have someone with a strong back do it for you.
Remove the mattress from the bed, placing it safely out of your way.
Measure the length and width of your box spring with your tape measure. Write these measurements on your paper.
Measure the "drop" of your box spring. This is the height from the floor to the top edge of your box spring. Write this measurement on the paper. If your box spring/bed is on risers, measure all the way to the floor. You will want to conceal the risers.
Determine which size your box spring measurements fall into and purchase the closest size. The following standard bed measurements are for US and UK beds, according to Furniture Depot:
Twin (US) 39" --- 75"
Twin (UK) 36" --- 75"
Full (US & UK) 54" --- 75"
Queen (US) 60" --- 80"
Queen (UK; called King) 60" --- 78"
King (US) 76" --- 80"
King (UK; called Super King) 72" --- 78"
California King (US) 72" --- 84"
If your bed is on risers and your drop is longer than 15 to 16 inches, you might have to shop at a specialty store that supplies longer drop bed skirts or make your own.