How to calculate sashing and cornerstones

Sashing is the grid of strips between blocks — and the math trips people up because the strip count depends on whether the sashing also frames the outside edge.

The quick answer, for a quilt of R rows × C columns of blocks that finish B“ square, with sashing that finishes S“ wide:

• Vertical strips (between columns): C − 1 per row, or C + 1 if the outside is framed
• Horizontal strips (between rows): R − 1 rows of sashing, or R + 1 framed
• Cut size of each short strip: (S + 1/2“) × (B + 1/2“)
• Cornerstones (if used): one at every sashing intersection, cut (S + 1/2“) square

How sashing changes your quilt size

Every column of sashing adds S inches, so:

Finished width = C × B + (C − 1) × S (add 2 × S more if framed) Finished length = R × B + (R − 1) × S (same)

Worked example: a 4 × 5 quilt of 12“ blocks with 2“ sashing, not framed:

	Formula	Result
Width	4 × 12 + 3 × 2	54“
Length	5 × 12 + 4 × 2	66“

That 6“–8“ of growth is why quilts with sashing often need one fewer row of blocks than you planned — check the size before you make all the blocks.

With cornerstones vs. without

Without cornerstones, the long horizontal strips run the full width of the quilt. You can piece them from WOF strips, but each seam is a spot to keep straight during assembly.

With cornerstones, every strip is short — (S + 1/2“) × (B + 1/2“) — and each intersection gets a contrasting square cut (S + 1/2“) × (S + 1/2“). Counts for an R × C layout, framed on all sides:

• Short strips: R × (C + 1) + C × (R + 1)
• Cornerstones: (R + 1) × (C + 1)

Cornerstones cost more cutting but pay you back at assembly: every seam has a matching point, so the grid almost aligns itself.

How wide should sashing be?

Convention says one-quarter to one-third of the block size: 2“–3“ finished for a 9“–12“ block. Narrower reads as a fine grid; wider starts to read as its own design element. Whatever you choose, cut it consistently — a 1/8“ wobble across ten strips shows up as a visibly crooked grid.

Cutting from yardage

Strips are cut across the width of fabric, then subcut to length. From a 42“ WOF strip you get ⌊42 ÷ (B + 1/2)⌋ short strips per cut — for a 12“ block, three per strip. Count your strips, divide, and round up to get the number of WOF cuts; multiply by the cut width for total yardage.

Or skip the arithmetic: the free quilt size calculator shows how sashing and borders change your finished dimensions, and the Sashing app’s sashing calculator goes further — strip counts, cornerstones, and yardage from your block count, with every formula shown.