Quick answer: how to calculate a rag quilt
Use the Rag Quilt Calculator with your own blocks across, blocks down, cut block size, seam allowance and layers per block, to check finished block size, finished quilt size, total blocks and total squares to cut before you cut.
A rag quilt is made from square blocks sewn with a wide exposed seam allowance, usually 1 inch, which is then clipped and frayed to make the ragged edge. Each block is a stack of layers, usually 3: top, batting, backing.
Finished block size equals cut block size minus 2 times seam allowance, because each side of a block loses one seam allowance where it joins the next block. Finished quilt width equals blocks across times finished block size. Finished quilt length equals blocks down times finished block size. Total blocks equals blocks across times blocks down. Total squares to cut equals total blocks times layers per block. With the batting middle layer method, fabric squares equal total blocks times layers minus 1, at cut size, and batting squares equal total blocks, at finished size. The batting is cut smaller on purpose, so it stays inside the seams and does not show in the rag edge. If your method cuts every layer the same size, choose "All layers at cut size" in the calculator instead.
For example, blocks across 4, blocks down 5, cut block size 12 inches, seam allowance 1 inch, layers per block 2, give finished block size 10 inches, finished quilt 40 by 50 inches, total blocks 20, and total squares to cut 40 squares.
Finished size is after sewing, before clipping and washing, because clipping and fraying do not change the sewn finished size in this calculation.
How to calculate a rag quilt
What you need to measure
You need five inputs: blocks across, blocks down, cut block size, seam allowance and layers per block. Blocks across and blocks down are whole numbers, greater than zero. Cut block size is the size of one square block before sewing, not the finished size. Seam allowance is the wide exposed seam allowance used for a rag quilt, usually 1 inch. Layers per block is the number of layers in each block stack, usually 3: top, batting, backing. Finished block size and finished quilt size are sizes after sewing, before clipping and washing.
Formula and calculation rule
Finished block size equals cut block size minus 2 times seam allowance, because each side of a block loses one seam allowance where it joins the next block. Reverse formula: cut block size equals finished block size plus 2 times seam allowance. Finished quilt width equals blocks across times finished block size. Finished quilt length equals blocks down times finished block size. Total blocks equals blocks across times blocks down. Total squares to cut equals total blocks times layers per block.
Step by step example
Take blocks across 3, blocks down 4, cut block size 8 inches, seam allowance 0.5 inch, layers per block 3.
Finished block size: 8 minus 2 times 0.5 equals 8 minus 1, which is 7 inches.
Finished quilt width: 3 times 7 equals 21 inches. Finished quilt length: 4 times 7 equals 28 inches.
Total blocks: 3 times 4 equals 12 blocks. Total squares to cut: 12 times 3 equals 36 squares. With the batting middle layer method, those 36 squares split into 24 fabric squares at 8 inches cut size and 12 batting squares at 7 inches finished size, because one layer per block is batting and the other two layers are fabric.
Rag quilt cut size chart
This chart uses seam allowance 1 inch, the usual wide exposed seam allowance for a rag quilt. Finished block size equals cut block size minus 2 times seam allowance, so with seam allowance 1 inch, finished block size equals cut block size minus 2 inches.
| Cut block size | Finished block size |
|---|---|
| 8 inches | 6 inches |
| 10 inches | 8 inches |
| 12 inches | 10 inches |
Use this chart for seam allowance 1 inch only. For another seam allowance, use the calculator above, with the same formula: finished block size equals cut block size minus 2 times seam allowance.
Important variations and exceptions
- Seam allowance larger than 1 inch. A seam allowance larger than 1 inch makes finished blocks smaller and fringe wider, because finished block size equals cut block size minus 2 times seam allowance, so a larger seam allowance subtracts more from the cut block size. Seam allowance must stay less than half the cut block size, otherwise finished block size is zero or negative, which cannot be sewn as a block.
- Layers change squares to cut, not finished quilt size. Layers per block change total squares to cut, because total squares to cut equals total blocks times layers per block. Layers do not change finished block size or finished quilt size, because finished block size comes from cut block size and seam allowance only, and finished quilt size comes from blocks across, blocks down and finished block size only.
- Clipping and washing make the fringe. Clipping and washing make the ragged edge, after sewing. They do not change the sewn finished size in this calculation, because finished size is after sewing, before clipping and washing.
- Non-square blocks need separate width and length calculations. This guide uses square blocks, so one cut block size gives one finished block size for both width and length. If your blocks are not square, work finished width from cut width and seam allowance separately, and finished length from cut length and seam allowance separately, and work finished quilt width and length from those separate finished sizes.
Two counts, never one unlabeled total
Most counting mistakes in rag quilts come from one number doing two jobs. Keep two counts apart, and label both every time. Blocks, also called grid positions or sandwiches, are blocks across times blocks down. Total pieces are blocks times layers per block. For an 8 by 10 grid, blocks are 80. With 2 layers, total pieces are 160 fabric squares. With 3 layers, total pieces are 240, but only if every layer is cut at the same size, which brings in the middle layer warning below. A page that headlines 80 for an 8 by 10 grid is counting blocks, not total pieces, and copying 80 as the total for a two layer quilt undercounts by half.
The same warning applies in reverse. If a page gives a front plus back total, that total already includes both outer layers. Do not multiply it by 2 again. Read the label first, blocks, per layer, front plus back, or total pieces, and only then use the number.
The middle layer is not cut like the outer layers
When the middle layer is batting, every page that addresses it cuts it at the finished size, not at the cut size, so it stays inside the seams and does not show in the rag edge. For the guide step by step example, cut 8 inches with a 0.5 inch seam, the finished block is 7 inches, so a batting middle square is cut at 7 inches, not 8 inches. The calculator total, by contrast, counts every layer at the cut size, because its formula is total squares equals total blocks times layers per block, with no separate middle layer size. Use the calculator total for outer fabric layers, and count a batting middle layer separately, one finished size square per block, with its own yardage. If the middle layer is flannel fabric rather than batting, it is cut like the outer layers, and the calculator total does apply to it. State which middle layer you mean before you cut.
Layers need the same honesty. The sources read for this guide use 2 layers, front and back, or 3 layers with a middle layer. No page read gives a worked example for 1 layer, or for 4 or 5 layers. The calculator accepts 1 to 5, but 1 layer is not the usual front and back sandwich, and 4 or 5 layers have no source example and would be very heavy. Treat 2 or 3 as the normal range, and treat anything outside it as an experiment of your own, not a standard rag quilt.
Seam allowance is a choice, and it shrinks the block
There is no single agreed seam allowance. A 0.5 inch allowance is the most common value in the worked examples read, while one calculator calls 0.75 inch the most common and offers up to 1 inch, and another page usual range stops at 0.625 inch. Pick from 0.5 to 1 inch, and show what the choice does to the block. A larger allowance gives a fuller fringe, but a smaller finished block, so the same finished quilt needs more blocks and more fabric.
| Cut block size | Seam allowance 0.5 inch | Seam allowance 0.625 inch | Seam allowance 0.75 inch | Seam allowance 1 inch |
|---|---|---|---|---|
| 10 inches | 9 inches finished | 8.75 inches finished | 8.5 inches finished | 8 inches finished |
For clipping, pages range from about every 0.25 inch to about every 0.5 inch apart, as close to the seam as possible without cutting it. Clip the exposed allowances after sewing with the raw edges facing up, then wash and dry to bloom the fringe, and clean the lint trap during the first wash, because flannel sheds heavily. Flannel frays best and gets softer with washing. Regular quilting cotton rags less dramatically, denim makes a heavy quilt, and fleece or minky stay flat instead of blooming, so they should not be used for exposed seams, although minky is popular as a backing layer. No binding is needed, because the outer edge is stitched at the seam allowance distance and clipped like the interior seams.
Prewashing is genuinely unsettled in the pages read. One project page says do not prewash, while other pages say prewash flannel because it shrinks more than quilting cotton, or buy about 10 percent extra, and prewash all fabrics if different fabrics are mixed, or the quilt may pucker. This guide does not settle that argument for you. If your fabric may shrink, prewash or buy extra, and do not mix unwashed shrinking fabric with unwashed stable fabric in one quilt.
Finished size means seam line size here, fringe is extra
The calculator finished size is measured between the outer seam lines, after sewing and before clipping and washing. The clipped raw fringe extends beyond those lines by about one seam allowance past each finished edge. That means the raw edge span is the seam line size plus twice the seam allowance in total, one allowance on each side. For the worked example below, a seam line quilt of 40 by 50 inches with a 1 inch seam has a raw edge span of about 42 by 52 inches. Other pages that include the fringe in their finished size are not disagreeing about the quilt, they are measuring to a different edge, so always state which edge your size is measured to.
Starting from a target size needs one more rule. Round squares across and down up to whole squares, because a partial square cannot be used, so the actual quilt meets or exceeds the target instead of matching it exactly. For example, a target of 55 by 47 inches with a 5 inch finished block needs 11 squares across, since 55 divided by 5 is 11, and 10 squares down, since 47 divided by 5 is 9.2, rounded up to 10, giving an actual quilt of 55 by 50 inches, larger than the target in one direction. The calculator starts from a grid, not from a target, so do this rounding yourself before you enter blocks across and blocks down.
How to make sure your result is correct
- Check cut size, not finished size, as the input. Cut block size is the size before sewing. Using finished block size as the input subtracts the seam allowance twice, once in your input and once in the formula, and gives a finished block size too small by 2 times seam allowance.
- Check seam allowance on both sides. Finished block size equals cut block size minus 2 times seam allowance, not minus seam allowance once, because each side of a block loses one seam allowance where it joins the next block.
- Check layers in the square count. Total squares to cut equals total blocks times layers per block. Forgetting layers undercounts squares to cut, because each block needs one square per layer, not one square total.
- Check units. Blocks across, blocks down, cut block size and seam allowance must be in the same unit, inches or centimetres. In centimetre mode, 1 inch equals 2.54 centimetres, so a seam allowance of 1 inch equals 2.54 centimetres, and a cut block size of 12 inches equals 30.48 centimetres.
- Check finished size timing. Finished size is after sewing, before clipping and washing. Clipping and washing do not change the sewn finished size in this calculation, so do not add or subtract anything for clipping or washing when you check finished quilt size.
Try it with the calculator: Use the Rag Quilt Calculator with your own blocks across, blocks down, cut block size, seam allowance and layers per block. It uses the same steps as this guide, and it shows finished block size, finished quilt size, total blocks and total squares to cut, together.
Worked example: 4 by 5 layout, cut block 12 inches, seam allowance 1 inch, layers 2
This worked example is different from the tool page example, which uses blocks across 5, blocks down 6, cut block size 10 inches, seam allowance 1 inch, layers per block 3.
Finished block size: 12 minus 2 times 1 equals 12 minus 2, which is 10 inches.
Finished quilt width: 4 times 10 equals 40 inches. Finished quilt length: 5 times 10 equals 50 inches.
Total blocks: 4 times 5 equals 20 blocks. Total squares to cut: 20 times 2 equals 40 squares.
Frequently asked questions
A rag quilt is made from square blocks sewn with a wide exposed seam allowance, usually 1 inch, which is then clipped and frayed to make the ragged edge. Each block is a stack of layers, usually 3: top, batting, backing.
Finished block size equals cut block size minus 2 times seam allowance, because each side of a block loses one seam allowance where it joins the next block. For example, cut block size 12 inches with seam allowance 1 inch gives finished block size 12 minus 2, which is 10 inches.
Work out finished block size first, then finished quilt width equals blocks across times finished block size, and finished quilt length equals blocks down times finished block size. For example, blocks across 4 and blocks down 5, with finished block size 10 inches, give a finished quilt 40 by 50 inches.
Total blocks equals blocks across times blocks down, and total squares to cut equals total blocks times layers per block. For example, blocks across 4 and blocks down 5 give 20 blocks, and with 2 layers per block, total squares to cut equals 20 times 2, which is 40 squares. With the batting middle layer method, those 40 squares split into 20 fabric squares at cut size and 20 batting squares at finished size.
No, not in this calculation. Finished size is after sewing, before clipping and washing, because clipping and fraying make the ragged edge, and they do not change the sewn finished size in this calculation.
Related guides and calculators
Use the Rag Quilt Calculator for your own blocks across, blocks down, cut block size, seam allowance and layers per block. For quilt size by bed, use the Quilt Size Calculator, for fabric yardage, use the Fabric Calculator, for counting pieces from yardage you already own, use the Piece Count Calculator, for batting size and yardage, use the Batting Calculator, and for backing size and yardage, use the Backing Calculator.