Find finished quilt dimensions from your mattress size, drop and pillow tuck.
Finished width = mattress width + 2\u00d7drop. Finished length = mattress length + drop + tuck.
A bed quilt should cover the top of the mattress and hang down the sides by the drop, and it needs extra length at the head for the pillows to tuck under. The finished width is the mattress width plus two drops, and the finished length is the mattress length plus one drop at the foot plus the pillow tuck. A twin mattress of 39 by 75 inches with a 10 inch drop and a 10 inch tuck gives a quilt of 59 by 95 inches.
Standard mattress sizes help you get started. A twin is about 39 by 75 inches, a full is 54 by 75, a queen is 60 by 80, and a king is 76 by 80, though thicker mattresses are deeper and need a bigger drop to look right.
A drop of eight to twelve inches covers the sides of the mattress, while a drop that reaches the floor needs fifteen inches or more. Quilts shrink a little when washed, commonly three to five percent, so you may want to make it slightly larger.
If you want a quilt for decoration or for a lap, use smaller sizes, and measure your bed before you begin, since mattress depths and bed frames differ. Add borders to reach the finished size if the blocks do not divide evenly.