Three sizes you will see in a quilt pattern
Cut size is the size of a piece before you sew it. Unfinished size is the size of a sewn unit or block before its outer edges are joined to something else. Finished size is the part that remains visible after those outer seams are sewn. A pattern may use all three, so check the label beside a measurement before reaching for your ruler.
Follow one square from cutting to assembly
A square that finishes at 3 inches needs to be cut at 3½ inches when you use ¼-inch seams. A four-patch made from four of those squares finishes at 6 inches, but the assembled block measures 6½ inches before it goes into the quilt. Don't trim that extra ½ inch away; it belongs to the outside seams.
Match the measurement to the task
The block and border calculators ask for finished sizes and add seam allowance for the cuts. The square cutting calculator asks for the cut size, so its input already includes the seams. For backing, enter the measured size of your pieced quilt top. That measurement is used as it stands, before quilting and the final trim.
Check the label before adding more
If a pattern already gives a cut size, use it directly. Adding another seam allowance makes the pieces too large. If the pattern gives a finished size, work out the allowance before cutting. Keeping those two numbers beside each other on your cut list is often enough to prevent a costly mix-up.