Fortunately, substitutions can be made. It just requires a bit of simple math.
Annie’s has put together a good article that outlines the equivalents between common pre-cuts and yardage. For example, one fat quarter contains the same amount of usable fabric as 1/4 yard of fabric.
The shapes of the two cuts are different, however, so you may need to take that into account when making substitutions.
Look the pattern over carefully to make sure the substitution works. In some cases you may need more pre-cuts, or more yardage, to produce the correctly sized cuts.
A second article, from Brown Bird Designs, outlines how to make your own pre-cuts from yardage. This info can help in visualizing how to make substitutions between pre-cuts and yardage.
Image Source: The fabric featured on this page is Boundless Cottage Market. It’s available here as yardage and pre-cuts while supplies last.