Enter the cut size of the square
Use the square size you'll mark on the fabric, including any seam allowance. If your pattern asks for a 4½-inch cut square, enter 4½ inches. You don't need to add the seam allowance again. Measure the fabric width between the edges you can actually use, leaving out the selvages.
Count whole squares across the width
Forty inches of usable fabric fits eight 4½-inch squares across, with 4 inches left at the edge. To cut 40 squares, you need five strips, each 4½ inches deep. That's 22½ inches of raw fabric length. The calculator also shows extra complete squares when the final strip produces more pieces than you need.
Use the strip and square rows together
The strip row tells you what to cut from the fabric; the square row tells you how to crosscut those same strips. They aren't two separate fabric requirements. For remnants, check that the available length is at least the raw length shown. The calculation assumes a clean rectangle with no extra allowance for flaws, shrinking or matching a print.