Quilt Sashing Calculator
Sashing separates blocks and gives a quilt breathing room. Find out how many strips you need and how much fabric to buy.
Tip: use URL parameters, e.g. ?blockSize=12&sashingWidth=3&blocksAcross=5&blocksDown=6.
How sashing math works
Sashing strips run between blocks. With a grid of blocks across and down, you need:
Horizontal strips (between rows). For 6 rows, there are 5 horizontal sashing rows. Each horizontal strip spans the full width: blocks across × block size + sashing between them. For 5 blocks of 12" with 3" sashing, that's 5 × 12 + 4 × 3 = 72 inches, plus seam allowances.
Vertical strips (between columns, within each sashing row). For 5 columns, there are 4 vertical strips per sashing row, each the height of the sashing strip (about 3" finished).
Corners and edges. If you want sashing on the outer edges too (a "sashing border"), add strips around the whole quilt. This calculator assumes sashing between blocks only.
Practical cutting. Cut sashing strips across the width of the fabric at your sashing width plus seam allowance (usually 0.5" total), then piece them to length as needed.
What each input means
- Finished block size (inches) — the block size after seams, as it appears on the quilt top.
- Sashing width (finished, inches) — the visible sashing between blocks. The strips you cut are 0.5" wider to cover the seam allowances on both sides.
- Blocks across and down — your block grid. The calculator uses these to find how many horizontal sashing rows and vertical connectors you need.
- Fabric width (usable) — the cutting width after selvedges, about 42" for standard quilting cotton.
When to use it. Use this calculator when your design plan calls for sashing between blocks, whether you're adding plain strips or planning to use cornerstones later. It gives you both the fabric estimate and the finished top size, which is what you need to choose a border and check against standard mattress sizes. If you change your sashing width after seeing the result, recalculate — even one inch of sashing adds up across five or six rows. And if you're using cornerstones, add their fabric separately, since the small squares are usually cut from a contrasting fabric and belong in their own yardage line. A quick way to double-check the finished size is to count the gaps: five blocks across means four sashing strips between them.
Common mistakes to avoid
- Entering the cut width as the finished width. If you cut 3.5" strips but tell the calculator 3.5", you'll overstate the finished sashing and the fabric needed.
- Counting sashing rows wrong. Between six rows of blocks there are five sashing rows, not six — the classic off-by-one.
- Forgetting seam allowances on the strips. Add about 0.5" to the finished width when cutting; the calculator assumes 1/4" seams throughout.
- Ignoring cornerstones. If your design uses small squares at the intersections, they need fabric too — add the extra yardage or a separate calculation.
- Assuming outer sashing is included. This calculator covers sashing between blocks only; outer sashing acts like a border and needs its own estimate.
Frequently asked questions
How wide should sashing be?
Two to four inches finished is common. Too narrow looks cramped; too wide overwhelms the blocks. Match it to your block size — 3" on a 12" block is a classic proportion.
Should I use cornerstones in sashing?
Cornerstones (small squares at sashing intersections) add a secondary design element and make the sashing rows easier to assemble. They're optional and use a bit more fabric.
Does sashing make the quilt bigger?
Yes — each sashing strip adds its width between blocks. A 5 × 6 grid of 12" blocks with 3" sashing grows to about 72" × 90" finished.
Do I need sashing on the outside edges?
No. Many quilts only sashing between blocks and use borders on the outside. Add outer sashing only if you want that look.