Quilt block sizes and on-point settings

The short answer: a straight-set quilt finishes at its blocks plus any sashing plus any borders, added up across the rows and down the columns. An on-point quilt turns those same blocks on the diagonal, which changes the arithmetic and adds two extra pieces you have to cut: side setting triangles along the edges and corner setting triangles in the four corners.

Here is how both settings work.

How do I calculate a straight-set quilt size?

In a straight set, blocks sit square to the edges in neat rows and columns. Finished width is the number of columns times the finished block size, plus the sashing between and around the blocks, plus twice the border width (one border on each side). Finished height uses the same math with rows instead of columns.

Say you have a 4 by 5 grid of 12 inch blocks, 2 inch sashing framing every block, and a 4 inch border:

• Width: 4 blocks x 12 + 5 sashing lines x 2 + 2 x 4 border = 66 inches
• Height: 5 blocks x 12 + 6 sashing lines x 2 + 2 x 4 border = 80 inches

Drop the sashing and the borders and it is just blocks: 48 by 60 inches. Every layer you add is another term in the sum.

What size should a quilt be?

Aim for a finished size, then pick a block and grid that lands near it. These are common targets:

Size	Finished (inches)
Throw	52 x 65
Twin	70 x 90
Full	84 x 92
Queen	90 x 108
King	108 x 108

Mattress sizes vary and drop length is personal, so treat these as starting points. A throw meant for the couch can be smaller; a quilt you want to tuck in on both sides needs more length.

What is an on-point setting?

On-point, sometimes called a diagonal set, rotates each block 45 degrees so it reads as a diamond rather than a square. Blocks still line up in rows, but the rows run on the diagonal. Because the outer blocks now point into empty corners, you fill the gaps with two kinds of half blocks.

Setting the same block on point makes the quilt larger for the same block count, because the block’s diagonal, not its side, spans the row. A 12 inch block measures about 17 inches corner to corner, so five blocks on point cover more ground than five set straight.

How do I cut setting triangles?

Side setting triangles go along the four edges. They are cut as quarter-square triangles so the straight of grain lands on the outside edge, which keeps that edge from stretching. Corner setting triangles fill the four corners and are cut as half-square triangles for the same reason.

The formulas key off your finished block size:

• Side triangles: cut a square of block x 1.414 plus 1 1/4 inches, then cut it on both diagonals to yield four triangles.
• Corner triangles: cut a square of block x 0.707 plus 7/8 inch, then cut it on one diagonal to yield two triangles.

For a 12 inch finished block that works out to:

Piece	Formula	Cut square	Cuts	Yield
Side triangle	12 x 1.414 + 1.25	18 1/4“	both diagonals	4
Corner triangle	12 x 0.707 + 0.875	9 3/8“	one diagonal	2

You need one side triangle for each block on an outside edge and exactly four corner triangles per quilt. Cut them a hair generous and square the quilt up after piecing if you like extra room to trim.

Let the app do the math

The quilt size calculator works out finished dimensions and setting-triangle cuts for you, so you can try a few block sizes before you cut anything. Sashing on iPhone does the same offline and keeps your project measurements in one place. Get the app to plan the next quilt on the go.