Quilt math · Calculator
On-point quilt size calculator
How big a quilt your blocks make set on point, for any block size and layout, and how much bigger that is than setting them straight.
The short answer
Multiply the blocks across and down by the block’s diagonal: its side × 1.414. 4 × 5 blocks of 9″ on point make a 50⅞″ × 63⅝″ top; with a 4″ border, 58⅞″ × 71⅝″.
Work out yours
Free · no accountEach block’s diagonal: 12.73″
20 blocks, 12 squares between, 18 triangles
With a 4″ border all around.
4 diagonals across and 5 down. Cut borders to 51″ × 63¾″: the diagonals rounded up, never short.
Side by side, with the same border.
In Quarter Inch, choose On point and the size and blocks you want: it draws the quilt, fits the borders and cuts the setting triangles.
Start designing freeSame sums as the app: @quarterinch/quilt-engine fitOf, on point · borders cut to the diagonals rounded up (cutting cuttingCenter)
On-point size chart
The width of an on-point top, before borders, for blocks across. The length works the same way: look up the blocks down.
| Block | Diagonal | 3 across | 4 across | 5 across | 6 across | 7 across | 8 across |
|---|---|---|---|---|---|---|---|
| 6″ | 8.49″ | 25½″ | 34″ | 42⅜″ | 50⅞″ | 59⅜″ | 67⅞″ |
| 8″ | 11.31″ | 34″ | 45¼″ | 56⅝″ | 67⅞″ | 79¼″ | 90½″ |
| 9″ | 12.73″ | 38⅛″ | 50⅞″ | 63⅝″ | 76⅜″ | 89⅛″ | 101⅞″ |
| 10″ | 14.14″ | 42⅜″ | 56⅝″ | 70¾″ | 84⅞″ | 99″ | 113⅛″ |
| 12″ | 16.97″ | 50⅞″ | 67⅞″ | 84⅞″ | 101⅞″ | 118¾″ | 135¾″ |
How it’s worked out
On point, each block is turned 45° to stand on a corner. Across the quilt you see its diagonal, not its side: the side × √2, about 1.414 times as wide. So a row of blocks on point is that many diagonals wide, and the top is the number of blocks across times the diagonal by the number down times the diagonal. The squares between the blocks and the setting triangles fill in around them; they don’t add to the width.
| The block, finished | 9″ |
|---|---|
| Its diagonal: 9″ × 1.414 | 12.728″ |
| Across: 4 diagonals | 50.91″ → 50⅞″ |
| Down: 5 diagonals | 63.64″ → 63⅝″ |
| A 4″ border all around | + 8″ each way |
| The quilt | 58⅞″ × 71⅝″ |
The top is rounded once to the nearest ⅛″. When you cut borders for it, Quarter Inch rounds up instead (51″ × 63¾″ here), so a border is never short; you trim the spare when you sew it on.
Bigger than you’d think. The same 20 blocks set straight make a 44″ × 53″ quilt with the same border. On point, with 12 squares between them, it’s 58⅞″ × 71⅝″.
What you need
- 20 blocks, 4 across and 5 down, standing on point.
- 12 squares between them: plain squares, or more blocks for a quilt that’s all blocks.
- 14 side triangles and 4 corners to fill the edges. See setting triangles for their sizes.
What changes the answer
- Trimming the edges. Setting triangles are cut a little large and the edges trimmed after; trim to the size above, ¼″ outside the block points.
- Borders. Every border adds twice its width each way. To plan them, see quilt borders.
- A bed size. To reach a size, change the blocks across and down first, then the border. See standard quilt sizes.
Questions
Short answers.
Something else? A real person replies.
hello@quarterinch.app
How do you find the diagonal of a quilt block?
Multiply its finished side by 1.414 (√2). A 9″ block’s diagonal is 12.73″.
How big is a quilt on point?
The blocks across times the diagonal, by the blocks down times the diagonal, plus the borders. 4 × 5 blocks of 9″: 50⅞″ × 63⅝″ before borders.
How many blocks do I need for an on-point quilt?
Blocks across × down, and the squares between them: (across − 1) × (down − 1). For 4 × 5, 20 blocks and 12 squares.
Does setting blocks on point make a quilt bigger?
Yes. 20 blocks of 9″ make 44″ × 53″ set straight but 58⅞″ × 71⅝″ on point (with a 4″ border), because of the squares and triangles between.