Will it hang over the bed and by how much?
Will the quilt have to cover pillows at the top?

After you have decided on the questions above,
you’ll need to do a little measuring.

Measure the size of your mattress, not only the width and length, but also the thickness of your mattress. Standard mattresses are 8-10" thick, but some are thicker.

Are you going to cover your pillows with the quilt, and tuck an inch or two of your quilt underneath your pillow? If so, you’ll need to add 2 to 4 more inches to the length’s measurement.

These measurements will affect how much of your quilt
will drop over the sides of your bed.

Example, if you owned a queen size bed and wanted your quilt to hang at least 15 inches down each side, you would need to buy a quilt that is at least 90 inches wide - 60 inches mattress width, plus 15" drop on one side and 15" drop on the another side = 90" inches total width.