when buying precut fats, beware. 95% of the time you will be thrilled with your purchase. however, some of them are 18" at only one end, and less at the other. some are at least 18", but by the time you square them off on the straight of grain they are much less. some will fray and/or shrink like crazy, leaving you with less than 18" usable inches of fabric.
no kidding. i bought a batch of fats from a famous LQS in houston. when i finally went to use them i found all sorts of unpleasant surprises.
if you're buying in person, lay them out and measure again to ensure you are getting a full fat with no damage hidden inside the folded parts.
if they won't let you do that, either walk away without the fat(s) or make sure you pay no more than $1 for each. that way if you don't end up with a full fat, at least you didn't pay full (or more) of the per-whole-yard price.
if you're buying online for the first time from that vendor, order only a few. if they're good when you get them, it should be safe to order more from that same place.