Two methods, fixed quarter-inch seams
Choose two at a time for smaller matching pairs, or eight at a time for larger batches of the same two fabrics. These trim-safe methods use quarter-inch seams either side of marked diagonals. They do not support a different seam width, four-at-a-time bias-edge construction, foundation papers or a no-trim promise.
Finished HST size means its seam-line size after assembly into the quilt. Unfinished trim size includes a quarter inch on each edge, so it is ½ inch larger than finished size. Starting squares are larger again to permit trimming after sewing and pressing. The additional trim margin is optional extra on top of the stated oversized formula.
Starting squares and whole-batch formulas
Unfinished trim size = finished HST size + ½ inch Two-at-a-time starting square = finished size + 1 inch + extra trim Eight-at-a-time starting square = 2 × (finished size + 1 inch + extra trim) Pairs to cut = round requested HST count UP ÷ units per pair Each pair needs one starting square from each of two fabrics
For two at a time, mark a diagonal, sew a quarter inch on each side, cut the marked diagonal, press and trim. For Magic 8, mark both diagonals, sew a quarter inch either side of both, then cut the vertical and horizontal center lines and the marked diagonals. The diagram distinguishes cut lines from sewing lines. Align the seam corner to corner when trimming.
Worked example: seventeen 3-inch finished HSTs
Trim each sewn unit to 3½ inches unfinished. Two at a time starts with 4-inch squares: nine pairs require 18 squares total, nine per fabric, and produce 18 HSTs. One unit is surplus. Magic 8 starts with 8-inch squares: three pairs require six squares total, three per fabric, and produce 24 HSTs. Seven units are surplus.
At 42 inches usable WOF, the two-at-a-time plan fits ten 4-inch squares across one row; nine required squares of each fabric consume a 4-inch bolt length per fabric, rounded to 4½ inches at an eighth-yard purchase increment. Magic 8 fits five 8-inch squares across a row; three per fabric consume 8 inches, rounded to 9 inches or ¼ yard of each fabric. The stock layout does not reuse missing squares from the other color.
Trim allowance is deliberate
The smaller conventional two-at-a-time formula often adds ⅞ inch. This planner adds a full inch before any optional extra, so it is explicitly an oversized trim method. For a geometrically ideal quarter-inch seam, the diagonal seam reduces the effective square dimension by approximately √2 × ¼ inch; the full-inch formula provides room beyond the required unfinished square. Real sewing and pressing can consume that margin, so test a pair rather than assume digital math guarantees a physical result.
Magic 8's formula doubles the entire small starting-square expression. Increasing extra trim by ¼ inch therefore increases its large square by ½ inch. Whole pairs are rounded first, cutting rows next, and aggregate purchase depth last. Displayed decimals are never used to calculate subsequent sizes.
Fabric layout and practical checks
Fabric quantity is for each of two fabrics separately. Every square in a batch is paired with the same other fabric; multiple color combinations need separate plans. The row layout assumes squared fabric and no pattern matching. Directional prints can end up pointing different ways within HSTs, and the tool does not rotate or match motifs. Shrinkage, defects and physical seam accuracy are excluded.
See Diary of a Quilter's oversized two-at-a-time tutorial and Penny Spool Quilts' Magic 8 tutorial for the sewing methods. The Fat Quarter Shop HST reference provides a separate cutting check.