Even the hyper-sensitive princess in Hans Christian Andersen's fairy tale, The Princess and the Pea, would be comfortable on today's extra-thick thick pillow top mattresses. Mattress technology has come a long way in the last 20 years and "standard" sizes don't mean as much as they did. If you're planning a bed linen project, the best advice is to measure your mattress to be certain your finished project will actually fit your bed.
Measuring Your Mattress
Standard Mattress Dimensions
For Twin, Double, Queen, King and even California King, mattress sizing is pretty universal. Where you are most likely to run into discrepancies is with an older mattress. Crib mattresses, for example, need to fit snugly inside the crib frame so there is no "danger gap" between the mattress and the frame. Older crib mattresses were often 26 to 27 inches wide, but that is now considered unsafe. The minimum safe size is 27-1/4" x 51-5/8" and newer crib mattresses are often 28" x 52".
Mattress depths are NOT standard and vary wildly. Old mattresses were almost always 9 inches deep, however, that's more the exception now than the rule. This measurement is critical in making bed linens, since mattresses are as deep as 21 inches now, with 14 inches being closer to average.
My king mattress has a pillow top on both sides and is about 15 inches thick. A "standard" 104-inch-wide king size comforter is simply not wide enough. It needs to be a minimum of 108 inches and preferably 110 inches to properly cover the mattress. It's actually surprisingly difficult to find comforters that fit my rather ordinary mattress, so I make my own.
Drop is the amount a comforter, quilt or bedspread hangs over the top of the mattress. To look right, a comforter or quilt should drop below the base of the mattress by at least two inches. If you have a 14-inch mattress, it's obvious that a standard 12-inch drop is not enough.
If you are measuring for a bedspread, consider the depth of the box spring since box springs also vary in height. A bedspread will need approximately 18 inches added to the top measurement if you want it to cover pillows. Box spring drop is also important for determining the length required for a bed skirt.
The following are considered standard sizes. It's preferable, and I highly recommend that you measure for yourself. All measurements are in inches showing width by length.