Two answers in one place: how many blocks your quilt takes, and what to cut so each block finishes true, plain squares, nine-patch patches, and the triangle oversizes, all with seam allowance applied for you.
Your Blocks
Your Quilt Takes
63blocks
7 across × 9 down · finished 70 × 90 in
Cut 63 squares at 10 ½ in: finished size plus ½ in for seams.
- Blocks63 (7 × 9)
- Finished size70 × 90 in
- Cut each square10 ½″
The Formulas
Blocks for the Quilt
across = quilt width ÷ block size, rounded · down = quilt length ÷ block size, roundedCut Sizes with Seam Allowance
plain square: finished + ½ in · HST: finished + ⅞ in · QST: finished + 1¼ inQuarter-inch seams cost ½ in on a straight-sided patch. Triangles lose more, because the seam crosses the bias corners: ⅞ in for two-triangle squares, 1¼ in for four-triangle hourglasses. Many quilters cut HSTs a full inch over and trim to size.
Nine-Patch and Four-Patch
patch cut size = finished block ÷ 3 (or 2) + ½ inWorked Example
A twin quilt, 70 × 90 in, from 10 in nine-patch blocks:
- Blocks: 70 ÷ 10 = 7 across, 90 ÷ 10 = 9 down → 63 blocks.
- Each nine-patch patch finishes at 10 ÷ 3 = 3⅓ in, so the cut is 3.83 in. No ruler has that line: cut 3⅞ and trim the sewn block square.
- Nine patches × 63 blocks = 567 squares. A 9 in finished block cuts friendlier: 3 in patches, cut 3½.
Cut Size Chart
Finished sizes quilters use most, with the seam allowance applied.
| Finished | Plain square | HST | QST | Nine-patch patch |
|---|---|---|---|---|
| 2″ | 2 ½″ | 2 ⅞″ | 3 ¼″ | , |
| 3″ | 3 ½″ | 3 ⅞″ | 4 ¼″ | 1 ½″ |
| 4″ | 4 ½″ | 4 ⅞″ | 5 ¼″ | , |
| 5″ | 5 ½″ | 5 ⅞″ | 6 ¼″ | , |
| 6″ | 6 ½″ | 6 ⅞″ | 7 ¼″ | 2 ½″ |
| 8″ | 8 ½″ | 8 ⅞″ | 9 ¼″ | , |
| 9″ | 9 ½″ | 9 ⅞″ | 10 ¼″ | 3 ½″ |
| 10″ | 10 ½″ | 10 ⅞″ | 11 ¼″ | , |
| 12″ | 12 ½″ | 12 ⅞″ | 13 ¼″ | 4 ½″ |
How to Measure Your Inputs
- Finished block size is the block inside the sewn quilt. A block that measures 10½ in on your table before joining is a 10 in finished block.
- Nine-patch sizes: pick a finished block divisible by 3, 6, 9, or 12 in, and every patch cuts on a ruler line.
- Quilt size: this page fits whole blocks edge to edge; sashing and borders change the layout, and the quilt calculator plans those.
Assumptions and Defaults
- Seam allowance is ¼ in throughout, the piecing standard.
- Block counts round to the nearest whole block and the true finished size is reported alongside.
- Awkward cuts (thirds of an inch) are flagged and the nearest ruler-friendly cut suggested, oversize-and-trim.
- HST and QST entries size the squares you start from; the HST calculator handles 2-, 4-, and 8-at-a-time methods.
Block Math Questions, Answered
How many blocks do I need for my quilt?
The number of blocks for a quilt is the quilt width ÷ the finished block size across and the length ÷ the block size down, each rounded to whole blocks. A 70 × 90 in twin in 10 in blocks takes 7 × 9 = 63 blocks, and if you add sashing, plan it in the quilt calculator, since sashing stretches the repeat.
What size do I cut quilt squares?
Quilt squares are cut at their finished size plus half an inch, so a 4 in finished square cuts at 4½ in. That half inch covers two ¼ in seam allowances, one on each side of the square, and the same rule applies to plain rectangles.
Why do triangles add ⅞ and 1¼ instead of ½?
Triangles add more than ½ in because the seam crosses their corners at an angle, so more fabric disappears into it. Two-triangle squares (HSTs) take the finished size + ⅞ in, and four-triangle hourglass units (QSTs) take the finished size + 1¼ in, while cutting slightly larger and trimming the sewn unit square is the more forgiving route.
What sizes suit nine-patch blocks?
Nine-patch blocks work best at multiples of three, such as 6, 9, or 12 in finished, which give 2, 3, or 4 in patches that cut at 2½, 3½, and 4½ in. A 10 in nine-patch forces patches of a third of an inch, which is possible, but every seam becomes harder to measure.
What seam allowance do quilt blocks use?
Quilt blocks use a ¼ in seam allowance, sewn slightly scant, about one thread narrower, so the pressed fold keeps its full width. To check yours, sew three 1½ in strips together, and the middle strip should measure exactly 1 in once sewn.