Example: ten 3½-inch squares from an 18 × 10-inch remnant. Drawing not to scale.
Count what the new row actually needs
Before changing the quilt layout, work out the pieces for the extra row. Suppose you want six 6-inch finished four-patch blocks in two fabrics. With ¼-inch seams, each block needs four 3½-inch cut squares: two from each fabric. The whole row needs 12 squares from fabric A and 12 from fabric B. Check the two fabrics separately, even if the remnants look similar in size.
Measure a rectangle you can cut from
Press the remnant flat and measure between usable edges, leaving out the selvages, fraying and any awkward notch. For this example, fabric A has an 18 × 10-inch usable rectangle. Five 3½-inch squares fit across the 18-inch side, and two rows fit in the 10-inch length. That's ten complete squares. The remaining narrow bands don't make another full square.
Use the calculator to check the shortfall
Open the fabric square cutting calculator and enter 18 inches of usable width, a 3½-inch cut square and a quantity of 12. It calls for three strips and 10½ inches of fabric length. The remnant is only 10 inches long, so it is two squares short. Comparing the raw length with the measured remnant is more useful here than looking at rounded yardage.
Choose a fix before cutting either fabric
You could use a third fabric for the two missing squares, change the color placement, or make a shorter row. Joining scraps to make a larger square is another construction choice, but it adds seams and needs its own cutting plan. Don't assume the calculator has pieced those leftovers together for you. Settle on the change while the remnant is still intact.
Check the second fabric and save the cut list
Repeat the same count for fabric B, using its own usable width and length. Once both fabrics cover the row, save or print the cutting plans and label the physical remnants. After sewing the new row, measure the quilt again before planning its borders, binding or backing. The extra row changes more than the block count.