Quilt Math Lab

Start with your quilt size and block size. A 60" x 80" quilt made of 12" blocks needs 5 blocks across and 7 down (80 / 12 = 6.67, rounded up) = 35 blocks. As a rule of thumb, multiply total finished block area by 1.4 for piecing waste, divide by 42" of usable fabric width, and add 10% — that 60" x 80" example needs about 5⅛ yards, so buy 5¼ yards. The Yardage tab above does this math for any size, and adds optional border and sashing yardage.

Binding = perimeter ÷ usable strip length, plus one extra strip for joining seams. A 60" x 80" quilt has a 280" perimeter; with 40" usable strips (42" width of fabric minus selvage and trimming), 280 / 40 = 7 strips + 1 join strip = 8 strips. At the standard 2.5" binding width that is 20" of fabric = 0.56 yard, so buy ⅝ yard. The Binding tab also handles 2.25", 2.75" and 3" binding widths.

Cut a square of fabric whose area covers perimeter × binding width × 1.1 (seam waste). The side of the square is √(perimeter × width × 1.1). For a 60" x 80" quilt with 2.5" binding: √(280 × 2.5 × 1.1) = 27.7", so cut a 28" square — about ⅞ yard of 44" fabric gives you the square with room to spare. Bias binding stretches around curves, which is why it doubles as bias tape for garment sewing.

Quilt backing should extend about 4" beyond the quilt top on every side, so add 8" to each dimension: a 60" x 80" quilt needs a 68" x 88" backing. From 44" fabric (42" usable) that takes 2 panels × 88" = 4.89 yards (buy 5 yards). From 108" wide quilt backing (106" usable), one width covers it — you need just 1.89 yards, so buy 2 yards. The Backing tab tries both panel orientations and picks the cheaper layout.

Instead of two straight panels, you cut one length of 44" fabric and seam it diagonally, which uses less yardage for medium quilts. The shortcut length is the diagonal of your backing target: √(W² + H²). For a 68" x 88" backing that is 111.2", or 3.09 yards (buy 3⅛) — versus 4.89 yards with straight panels. It works best when the quilt is under about 72" wide; the Backing tab compares all three methods side by side.

Like backing, batting should be about 8" larger than the quilt top in each dimension (4" per side for quilting take-up). A 60" x 80" quilt needs 68" x 88" of batting. Batting comes on rolls 45", 60", 90", 96", 108" and 120" wide and is sold by the yard of length: with a 90" roll you buy 88" = 2.5 yards. If your quilt is wider than the roll, the calculator tells you to piece two panels.

Add ⅞" to the finished size: cut size = finished + 0.875". For a 6" finished HST, cut 6⅞" squares, sew ¼" on each side of the diagonal, and cut apart. Each pair of squares yields two HSTs. The rule comes from the two ¼" seam allowances plus the diagonal trim: the Block Math tab shows the exact cut with fraction display for any finished size, including pinwheel blocks (four HSTs per 12" pinwheel = cut 6⅞").

Quarter-square triangles (hourglass blocks): add 1¼" to the finished size — a 6" finished QST starts from a 7¼" square cut twice diagonally. Flying geese: add 1¼" to both finished dimensions — a 2" x 4" finished goose needs a 3¼" x 5¼" rectangle plus two 3¼" squares (cut from the finished height + 1¼"). The Block Math tab computes all three.

Divide the new finished size by the old finished size to get the scale factor, then multiply every cut measurement by it. Resizing a 12" block to 9": 9 / 12 = 0.75, so a 6⅞" HST square becomes 5⅛". Remember that seam allowances do not scale — always apply the factor to cut sizes, then verify the two classic rules (HST = finished + ⅞", QST = finished + 1¼") still hold after scaling.

An on-point block's finished diagonal is block size × 1.4142 — a 12" block measures 16.97" corner to corner. Side setting triangles: cut squares of (block × 0.7071) + ⅞" (12" block → 9⅜"), then cut once diagonally to make 2 triangles. Corner triangles: cut squares of (block × 0.7071 ÷ 2) + 1¼" (12" block → 5½"), then cut twice on the diagonal (an X) to make 4. Straight-of-grain lands on the quilt's outer edge, so the edges won't stretch. The Block Math tab computes these "magic numbers" for any block size.

A standard jelly roll has 40–42 strips, each 2.5" x 42". For strip-pieced blocks, each 2.5" strip contributes 2" of finished height, so a 12" finished block needs 6 strips, and each 42" strip yields three 12½" segments — 40 strips make 20 twelve-inch blocks, enough for a 48" x 60" quilt top (4 × 5 blocks). Enter any strip count and block size in the Precut Lab to plan rail-fence, strip-pieced, or log-cabin layouts.

A fat quarter is 18" x 22". Cutting 6" finished squares (6½" with seam allowance), each fat quarter yields 2 × 3 = 6 squares, so 10 fat quarters give you 60 squares — enough for a 6 × 10 layout, a 36" x 60" quilt top before borders. The Precut Lab computes the yield for any cut size: 5" squares give 12 per fat quarter (120 total from 10), 10" squares give 4 per fat quarter.

A charm pack (42 squares, 5") makes ten 4-patch blocks (9" finished each) with 2 squares left over, or 21 HST pairs yielding 42 half-square triangles at 4⅛" finished. A layer cake (42 squares, 10") makes four disappearing nine-patch sets = 16 blocks at 14¼" finished — a 4 × 4 layout gives a 57" square quilt top. The Precut Lab works with any pack count, not just full packs.

Rag quilts use a 1" seam allowance on every side (the clipped, frayed seams are the feature), so cut size = finished size + 2". For an 8" finished block, cut 10" squares. A 60" x 80" rag quilt needs 8 × 10 = 80 blocks per layer; with the usual 3 layers (top, batting or flannel, backing) that is 80 × 100 sq in × 3 = 24,000 sq in ≈ 17.46 yards — buy 17½ yards total, split across your fabrics. Flannel is the classic choice because it frays beautifully.

Common finished sizes: crib/baby 36" x 52", throw/lap 50" x 65", twin 66" x 90", double/full 80" x 96", queen 90" x 108", king 104" x 108". These are the quilt tops before quilting shrinkage (expect 1–3% draw-up). The Yardage tab includes a size picker that fills in these dimensions, and every result (binding, backing, batting) updates automatically.

Measure hexagons flat-to-flat (the finished size across the middle). Add ½" for the ¼" seam allowance on each side to get the cut size — a 2" finished hexie is cut 2½" across the flats. A flat-to-flat hexagon covers (√3/2) × size² ≈ 3.46 sq in at 2", so a 60" x 80" quilt needs about 1,594 two-inch hexies plus 15% for honeycomb-packing waste. For English paper piecing, cut the same size and baste over paper templates; the Hexie mode in the Precut Lab runs the numbers for any size and quilt dimension.