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Scale any block to any size — with the seam-allowance math done right.
To resize a quilt block, never scale the cutting measurements directly — seam allowances are a fixed ¼ inch at every size, so only the finished fabric scales. The correct method: subtract the piece's built-in seam extra from its cut size (½″ for squares, ⅞″ for half-square triangles, 1¼″ for quarter-square triangles), multiply the finished size by the ratio of new block to old, then add the same extra back. Scaling a 12″ block's 4⅞″ HST squares down to a 9″ block gives 3⅞″ — not the 3.65″ you'd get by scaling the cut size itself.
Cut at 3⅞″ for the 9″ block
The piece finishes at 4″ in your 12″ block → 3″ in the 9″ block, plus the ⅞″ the HST method adds back.
HST squares: cut = finished + ⅞″. Rule of thumb: seam allowances never scale — only the finished fabric does.
For classic blocks, we've already done every size: the block guides carry any-size cutting charts for 17 traditional blocks — churn dash, Ohio star, bear paw, log cabin and more — and the Pattern Designer writes a complete chart for anything you design yourself, at any block size from 4″ to 18″.
Never scale the cut sizes directly. For each piece: subtract the built-in seam extra from the cut size to get the finished size (½ inch for squares and rectangles, ⅞ inch for half-square triangle squares, 1¼ inches for quarter-square triangle squares), multiply the finished size by the ratio of new block to old block, then add the same seam extra back.
Almost always because the seam allowances were scaled along with the fabric. Seam allowances are a fixed ¼ inch no matter the block size — only the finished fabric scales. Scaling a 4⅞-inch HST cut by 75% gives 3.65 inches, which is wrong; the correct 75% resize is 3⅞ inches.
Mathematically yes, practically no. A resize is only rotary-cutter friendly when the new finished pieces land on ⅛-inch ruler marks. A 12-inch nine-patch resizes cleanly to 6, 9, or 18 inches (cells stay on the ruler) but awkwardly to 10 inches (3⅓-inch cells). When a resize goes off-ruler, cut generously, then trim the finished unit to size.
For a finished size F: plain squares and rectangles cut at F + ½ inch; half-square triangle squares cut at F + ⅞ inch (two-at-a-time method); quarter-square (hourglass) squares cut at F + 1¼ inches; flying geese rectangles cut at width + ½ by height + ½ with sky squares at height + ½.