How to Calculate Quilt Sashing: A Complete Guide

Quick answer: how to calculate quilt sashing

Quilt sashing is the strips of fabric between quilt blocks, which frame each block and separate it from the next block. Sashing finished width is the width of the strip in the finished quilt, after seams are sewn, for example 3 inches.

Finished quilt width equals blocks across times block finished size, plus blocks across minus 1 times sashing finished width. Finished quilt length equals blocks down times block finished size, plus blocks down minus 1 times sashing finished width.

With continuous vertical sashing strips and no cornerstones, vertical sashing strips needed equals blocks across minus 1, each running the full finished quilt length. Horizontal sashing segments needed equals blocks down minus 1 times blocks across, each one block finished size long, because each horizontal segment runs between two vertical strips.

For example, 4 blocks across and 5 blocks down, block 10 inches, sashing 3 inches, give a finished quilt 49 by 62 inches, with 3 vertical strips and 16 horizontal segments, 19 sashing pieces total.

Use the Sashing Calculator with your own blocks across, blocks down, block finished size and sashing finished width, to check finished quilt size, strip counts and total sashing length before you cut.

How to calculate quilt sashing

What you need to measure

You need four inputs: blocks across, blocks down, block finished size and sashing finished width. Blocks across and blocks down are whole numbers, greater than zero. Block finished size is the size of one square block in the finished quilt, after seams are sewn, not the cut size. Sashing finished width is the width of the sashing strip in the finished quilt, after seams are sewn, not the cut width. This guide assumes square blocks only, sashing between blocks only, with no sashing around the outside, and no cornerstones, with continuous vertical sashing strips.

Formula and calculation rule

Finished quilt width equals blocks across times block finished size, plus blocks across minus 1 times sashing finished width, because sashing strips sit between blocks, not around the outside, and there is one fewer sashing strip than blocks in each row. Finished quilt length equals blocks down times block finished size, plus blocks down minus 1 times sashing finished width. Vertical sashing strips needed equals blocks across minus 1, each running the full finished quilt length. Horizontal sashing segments needed equals blocks down minus 1 times blocks across, because each horizontal segment runs between two vertical strips, and each row of sashing between block rows is cut into one segment per block. Cut width for every sashing strip and segment equals sashing finished width plus 0.5 inch, for seam allowance 0.25 inch on each side. Cut length for each vertical strip equals finished quilt length plus 0.5 inch, for seam allowance 0.25 inch on each end. Cut length for each horizontal segment equals block finished size plus 0.5 inch, for seam allowance 0.25 inch on each end. Total sashing pieces equals vertical sashing strips plus horizontal sashing segments. Total sashing length, cut, equals vertical strips times vertical cut length, plus horizontal segments times horizontal cut length.

Step by step example

Take blocks across 4, blocks down 5, block finished size 10 inches, sashing finished width 3 inches.

Finished quilt width: 4 times 10 equals 40, plus 3 times 3 equals 9, which is 49 inches. Finished quilt length: 5 times 10 equals 50, plus 4 times 3 equals 12, which is 62 inches.

Vertical sashing strips: 4 minus 1 equals 3 strips, each cut 3.5 by 62.5 inches.

Horizontal sashing segments: 5 minus 1 times 4 equals 16 segments, each cut 3.5 by 10.5 inches.

Total sashing pieces: 3 plus 16 equals 19 pieces. Total sashing length, cut: 3 times 62.5 equals 187.5, plus 16 times 10.5 equals 168, which is 355.5 inches.

Sashing layout chart

Layout, blocks across by blocks down	Finished quilt size	Vertical strips	Horizontal segments	Total pieces
3 by 4, block 12 inches, sashing 2 inches	40 by 54 inches	2	9	11
4 by 5, block 12 inches, sashing 2 inches	54 by 68 inches	3	16	19
4 by 5, block 10 inches, sashing 3 inches	49 by 62 inches	3	16	19
5 by 6, block 12 inches, sashing 2 inches	68 by 82 inches	4	25	29
6 by 7, block 12 inches, sashing 2 inches	82 by 96 inches	5	36	41

Use this chart for common layouts. For other layouts, block sizes or sashing widths, use the calculator above, with the same formulas: finished width equals blocks across times block size, plus blocks across minus 1 times sashing width, and finished length equals blocks down times block size, plus blocks down minus 1 times sashing width.

Important variations and exceptions

• Cornerstones change the counts. If you want cornerstones, the layout is different, because horizontal sashing strips run full width and cornerstones sit where vertical and horizontal sashing strips cross, so do not use these strip counts for a cornerstone layout. With cornerstones, cornerstones needed equals blocks across minus 1 times blocks down minus 1, each one sashing finished width square, cut sashing finished width plus 0.5 inch square, but horizontal strips and vertical strips are counted differently, so work a cornerstone layout separately.
• Sashing around the outside is a border, not sashing. This guide counts sashing between blocks only, with no sashing around the outside of the quilt top. If you want sashing around the outside, use the Border Calculator for that outer frame, after sashing is worked out, because outer sashing follows border rules, not between-block rules.
• Non-square blocks need separate widths and lengths. This guide assumes square blocks only, so block finished size is used for both width and length. If your blocks are rectangles, work finished width with block finished width and finished length with block finished height separately, and work horizontal segment cut length with block finished width, not with a single block size.
• Sashing width changes finished size. A wider sashing strip makes the finished quilt larger, even with the same blocks, because finished quilt width and length both grow with sashing finished width. For example, the same 4 by 5 layout with block 12 inches gives finished 54 by 68 inches with sashing 2 inches, but finished 57 by 72 inches with sashing 3 inches, because width grows by 3 times 1 inch and length grows by 4 times 1 inch.
• Directional prints. If your sashing fabric has a directional print and strips must all run one way, check strip orientation before cutting, because long vertical strips and short horizontal segments may need different cutting directions. This calculator counts strips and segments, it does not check print direction.

Three questions first, or every count will be wrong

There is no single sashing count. Every strip count below depends on three layout choices, and mixing counts from different choices is the main error in sashing math. Answer these three questions before any number in this guide is used.

1. Between blocks only, or around the outside too? This guide and calculator count sashing between blocks only, with no sashing around the outside edge. An outside frame is a border, worked out separately.
2. Continuous strips, or cornerstones at every intersection? This guide and calculator use continuous vertical strips with no cornerstones. A cornerstone layout counts differently and must not use these strip counts.
3. Finished size or cut size? Cut width for every sashing strip equals finished sashing width plus 0.5 inch, for 0.25 inch seam allowance on each side. A cornerstone square, where used, is cut at finished sashing width plus 0.5 inch square.

Three counting models, kept separate

Model A, around the outside, no cornerstones. Short pieces sit beside every block in each row, rows times columns plus 1 pieces, each block size plus 0.5 inch. Long strips run between and around the rows, rows plus 1 strips, each finished quilt width plus 0.5 inch. For example, 5 blocks across and 4 blocks down, block 12 inches, sashing 2 inches, finish at 72 by 58 inches counting the outside edge, and need 1 and 1/4 yards, 18 strips cut 2.5 inches wide at 40 inches usable width.

Model B, between blocks only, no cornerstones. Long strips run between rows only, rows minus 1 strips, and short strips run between blocks in each row. For example, 3 rows and 3 columns, block 14 inches, sashing 4 inches finished, need 2 long strips of 50 inches and 6 short strips of 14 inches, plus 0.5 inch per strip for seam allowance, 188 inches of total sashing length, 5 width-of-fabric strips at 41 inches usable width, and 5/8 yard before any extra fabric for cutting error.

Model C, with cornerstones. All sashing strips are cut to the unfinished block size, and block rows alternate with sashing rows made of cornerstone, strip, cornerstone, strip. Cornerstone cut size equals finished sashing width plus 0.5 inch square. For the around-the-outside version, cornerstone count equals rows plus 1 times columns plus 1, for example 5 by 4 blocks need 30 squares at 2.5 inches for 2 inch sashing. The pages read give no between-only cornerstone count, so this guide publishes none for that layout. Do not apply the around-the-outside cornerstone count to a between-only layout.

From total length to yardage, using this guide's own example. This calculator gives total sashing length, cut, not yardage, because yardage depends on fabric width. To convert, divide total length by usable width, round up to whole width-of-fabric strips, multiply by cut width, and divide by 36. For this guide's worked example, 355.5 inches divided by 40 inches usable gives 9 strips after rounding up, 9 times 3.5 inches is 31.5 inches, divided by 36 is 0.875 yard, rounded up to the next quarter yard is 1.00 yard. State the usable width used, because sources differ: 40 inches, 41 inches after a 1 inch join allowance on 42 inch fabric, 42 inches, and 43 inches all appear.

Choosing a sashing width, and mistakes that show

Finished sashing 1.5 inches is narrow, 2 inches is the balanced default, 2.5 inches is moderately open, and 3 to 4 inches is wide and airy. A good starting rule is about 1/4 to 1/3 of the finished block size, so 2 to 3 inches for 12 inch blocks. Repeating a measurement already in the block is a safe choice, and auditioning widths on a design wall, with a strip between two finished blocks, beats guessing from numbers alone.

Square all blocks to the same size first. Sashing makes size differences obvious, and a block even 1/4 inch out will wave and pucker once sashed. Cut all strips in one batch, because even 1/16 inch variation in strip width adds up across rows. If a strip does not reach, recut it, never stretch it. Press seam allowances toward the sashing, and mark long strips where each block should land, so columns stay lined up.

How to make sure your result is correct

• Check finished size, not cut size. Measure block finished size and sashing finished width in the finished quilt, after seams are sewn, not cut sizes before seams are sewn. Using cut sizes in the formulas gives a finished quilt too large by 0.5 inch per block and per sashing strip.
• Check units. Blocks across, blocks down, block finished size and sashing finished width must be in the same unit, inches or centimetres. In centimetre mode, 1 inch equals 2.54 centimetres, so cut width equals sashing finished width plus 1.27 centimetres, which is 0.5 inch times 2.54.
• Check strip counts by layout, not by quilt. Count vertical strips and horizontal segments from the layout, blocks across and blocks down, not from a guess for the whole quilt. Adding vertical strips and horizontal segments from different layouts into one calculation gives the wrong piece count.
• Check total pieces against strips and segments. Total sashing pieces equals vertical sashing strips plus horizontal sashing segments. If total pieces is less than vertical strips plus horizontal segments, you have too few pieces. If total pieces is more, you have counted a strip or a segment twice.

Worked example: 4 by 5 layout, block 10 inches, sashing 3 inches

This worked example is different from the tool page example, which uses 5 by 6 layout, block 12 inches, sashing 2 inches.

Finished quilt width: 4 times 10 equals 40, plus 3 times 3 equals 9, which is 49 inches. Finished quilt length: 5 times 10 equals 50, plus 4 times 3 equals 12, which is 62 inches.

Vertical sashing strips: 4 minus 1 equals 3 strips, each cut 3.5 by 62.5 inches.

Horizontal sashing segments: 5 minus 1 times 4 equals 16 segments, each cut 3.5 by 10.5 inches.

Total sashing pieces: 3 plus 16 equals 19 pieces. Total sashing length, cut: 3 times 62.5 equals 187.5, plus 16 times 10.5 equals 168, which is 355.5 inches.

Frequently asked questions

Related guides and calculators

Next, read How to Calculate Quilt Block Size and Layout for the block layout that sashing joins, and How to Calculate a Quilt Border for the outer frame after sashing. For sashing itself, use the Sashing Calculator.