The following information was on Blank Quilting e-mail. To see all that Blank Quilting has to offer, go to http://www.blankquilting.com/. I love their website because they have a lot of free patterns you can download.
Measuring Your Bed for a Quilt
When making quilt for a bed we often forget to consider the size of the bed, resulting in a quilt that is too short or hangs down to the floor.
Measure the mattress width (A) and add twice the length (B) of the quilt that hangs over the mattress. This is called the "drop." (C)
For coverlets: measure from the top of the mattress to slightly past the bottom of the bed rail.
If you want the quilt to cover your pillows, measure the mattress length, then add one drop length plus 10" to 20".
Keep on Quilting!