Can you post a link to the pattern you are using?
If you washed your fabric, you may have "lost" a bit to shrinkage.
If it is not perfectly straight, you will lose some (assuming that being perfectly straight is important to you)
How much "usable" width do you have? Very few of my "quilting fabrics" are more than 42 inches wide - including the selvages?
Dunster's calculations are correct - assuming you have that much "usable" fabric available.