The layout formula
Identical rectangles on a bolt are a packing problem. Grow each finished piece by seam allowance on every edge, see how many cut widths fit in the usable fabric width, then stack enough rows for the quantity. Convert the resulting length from inches to yards by dividing by 36.
Usable width is the bolt width minus whatever you trim from each selvedge. Quilting cotton is often sold near 42 to 44 in; home-dec bolts run wider. If the cut width is larger than usable width, rotate the piece so the longer edge runs with the length of the bolt, or buy a wider fabric.
This calculator assumes grain-aligned rectangles cut in a simple grid. It does not price nap layouts that force every piece the same direction with gaps, matched plaids, or nested irregular pattern pieces from a commercial envelope.
Seam allowance and pattern repeats
A 1/2 in seam allowance adds 1 in to each dimension (1/2 in on both sides). A 5/8 in apparel seam adds 1.25 in. Enter the allowance your pattern calls for, not a guess from a different project.
When the print has a vertical repeat, each cut length must land on a whole number of repeats or the motifs will not line up from row to row. Set Vertical pattern repeat to that distance in inches; the tool rounds each cut length up before multiplying by rows. Leave it at 0 for solids or all-over prints you are not matching.
Worked default: 44 in bolt, 0.5 in selvedge trim each side (43 in usable), finished 18×22 in pieces, 0.5 in seams, quantity 4, no repeat. Cut size 19×23 in. Two pieces fit across. Two rows need 46 in of length, or about 1.28 yd. Buy a little extra if the shop only sells eighth-yard increments.
What to verify in the shop
Confirm the actual cuttable width on the bolt end, not only the marketing width. Some "44/45 in" goods leave less after selvedge and printing margins.
Directional prints, one-way designs and fabrics with a nap (velvet, corduroy) may need every piece oriented the same way, which can raise yardage above this grid estimate. Treat the result as a lower bound and add a named buffer for those cases.
Related planning: square footage when you are covering a flat area with fabric as a sheet, and box fill when the question is electrical box volume rather than cloth.