Quilt Calculator

Every quilt asks the same six questions about fabric, and each one is a different sum. This calculator covers all six on one page rather than sending you to six separate ones, because the numbers feed each other: the backing size depends on the top, the top size depends on the borders, and the binding depends on the perimeter that comes out the other end.

Two conventions run through everything here, and mixing them up is the single most common source of a wrong cut. Finished is the size a piece measures once it is sewn in. Cut is finished plus its seam allowance. Every field asks for one or the other explicitly, and every number the calculator hands back for cutting has the allowance already in it.

Which seam allowance applies, and why it is not always ¼"

Piecing is ¼" per piece, so ½" is lost per seam. That is the standard, it is what every block pattern assumes, and it is what this calculator uses for blocks, sashing, borders and shapes.

Backing seams are the exception, and the Backing mode lets you set the figure because quilters do not all use the same one. The default here is ½" per piece — 1" lost per seam — which is what the calculator this mode was modelled on uses for joining full widths behind a quilt. It is a wider seam than piecing, and it is a different kind of seam: you are joining two selvage edges to make one large panel, not building a block. If you back at ¼", change the field and the panel arithmetic follows.

The judgement call is the useful part: which seam you are sewing decides which allowance applies. A number quoted without that context is how a backing figure ends up applied to a block.

Backing: always check it both ways round

A backing wider than your fabric has to be pieced, and there are two ways to do it — panels side by side with the seams running down the quilt, or panels stacked with the seams running across it. They are almost never the same amount of fabric, and which one wins is not something you can tell by looking at the numbers. It depends entirely on the leftovers in each direction.

A 60 × 80 quilt with 4" of overhang needs a 68 × 88 backing. Off 42" fabric that is two panels of 88", or 4 yards 32 inches — 5 yards to order — with the seams running down the quilt. Turn it and you need three panels of 68", which is 204": nearly a whole extra yard, plus a second seam, for the identical quilt. Reverse the quilt's dimensions to 80 × 60 and the answer reverses with it.

The seam matters to the panel count too, and this is where a naive division goes wrong. Two 42" widths joined do not give you 84" — the seam takes ½" off each piece, so you get 83". A backing 83" across is two panels; a backing 84" across is three.

Piecing the backing on the diagonal

There is a third option, and it is the one people are usually looking for when they ask how to use less backing fabric. Cut the backing rectangle along its own diagonal and you have two right triangles. Their long edge is the rectangle's diagonal, and the height of each triangle measured from that edge is the rectangle's area divided by the diagonal. Nest the two triangles point to point along a bolt of fabric and they fill a strip exactly that height, exactly one diagonal long — one seam, and the area is unchanged.

So the fabric you need is a single length equal to the backing's diagonal, and the method works only if that nesting height fits inside one width of your fabric. That condition is what decides it. A 50 × 70 backing off 42" fabric has an 86" diagonal and a 40.7" nesting height, which fits, so it takes 2½ yards against the 2¾ yards the better straight orientation needs. The 68 × 88 backing above has a 53.8" nesting height, so it does not fit at all — and this calculator says so rather than printing a number that cannot be cut.

One thing to know before choosing it: the seam runs off the straight grain, so it stretches while you handle it. Starch, pin more than feels necessary, and do not press along it.

Binding: cross-grain or bias

Binding length is the quilt's perimeter plus enough to turn four mitred corners and close the loop. Ten to fifteen inches covers it, and the field defaults to ten. A 60 × 80 quilt is 280" around, so 290" of binding — seven 2½" strips off 42" fabric, half a yard, with 4" to spare.

Bias binding is cut at 45° to the grain, from a square rather than across the width. A square cut into bias strips yields roughly its own area divided by the strip width, so the square you need is the square root of (binding length × strip width): 290" of 2½" binding wants a 27" square, or ¾ of a yard — half again as much fabric as cross-grain. That is the trade, and it buys two things cross-grain cannot: bias goes round a curved or scalloped edge without puckering, and it wears longer on the fold, because no single thread in the fabric runs the whole length of the edge to break along.

If you are making flange or faux-piped binding, run the calculator twice — once for each strip — and make one of them ¼" wider than the other to account for the seam that joins them.

Borders: each one is bigger than the last

The mistake that costs fabric here is treating a second border as though it went round the original quilt. It does not: the first border already made the quilt bigger. A 2½" border on a 60 × 80 top makes it 65 × 85, so a 5" border outside that goes round 65 × 85 and finishes at 75 × 95.

That compounds. The two borders above take 7 strips then 8 strips — 5/8 of a yard and then 1¼ yards, 1⅞ yards in total if they are the same fabric. Two borders of the *same* width would not be equal either; the outer one is longer and costs more.

Sashing, and what cornerstones are actually for

A 4 × 5 grid of 12" blocks with 2" sashing between them finishes at 54 × 68: four blocks plus three gaps across, five blocks plus four gaps down. The blocks are cut at 12½" and the sashing strips at 2½", each ½" over finished for the seams on both sides.

The interesting part is the horizontal sashing. Without cornerstones, each run between rows is as long as the whole row — 54½" cut — which is longer than your fabric is wide, so it has to be joined from two strips. Turn cornerstones on and that run breaks into block-length segments with a small square at each crossing, and suddenly every piece fits inside one width. That is most of why cornerstones make a sashed quilt easier to sew straight, quite apart from how they look: nothing has to be pieced and then kept square over four and a half feet.

Shape cut sizes, and the rule about rounding

For squares the seam allowance is simple — add ½", ¼" for each side. Triangles are where the familiar odd numbers come from. A half-square triangle is two triangles cut from one square, and the square is cut 7/8" over the finished size, because as well as the ¼" on the two straight sides there is a seam across the diagonal. A quarter-square triangle is four triangles from one square, cut 1¼" over, because it is the long edge and not the short one that sits on the straight grain.

For anything that is not a right angle — a 45° or 60° diamond, a hexagon, an equilateral triangle — the allowance does not simply add to the side. Offsetting an edge outward by ¼" pushes the corner out along its bisector, and how far depends on how sharp the corner is. A sharp 45° point grows much more than a blunt 120° one. The arithmetic works out at ¼" times the cotangents of the two half-angles at each end of the side, which sounds abstract and is the reason the published rules look arbitrary: a 60° triangle wants ¾" added to its height and a hexagon wants ½" across the flats. Both fall out of the same identity, and so does the plain ½" on a square.

Every cut size here is rounded up to the next 1/8" ruler mark, never to the nearest one. This is deliberate and it is the opposite of what a converter would do. A piece cut a sixteenth small is scrap — you cannot add fabric back — while a piece cut a sixteenth large gets trimmed square with the rest of the block. When the exact geometry lands between two marks, the calculator shows both: the mark to cut on, and the exact figure underneath.

What this does not do

It assumes one fabric per job and no directional print. A print with an obvious up will change how you can lay pieces out, and the calculator's habit of turning a piece 90° to save fabric is not available to you if the pattern has to run one way.

It also does not lay out a cutting diagram. For pieces that are not rectangles the yardage assumes each one comes out of its own rectangle, which over-estimates — nesting diamonds and triangles along a strip fits noticeably more. If you want the packing worked out properly for rectangular pieces, the cut layout calculator does that, with a diagram. For converting the fractions on a pattern into decimals or millimetres, the fraction converter is the tool.

And it does not know about shrinkage. Quilting cotton loses 2–3% in the first wash. If you prewash, prewash everything and measure afterwards; if you do not, expect the finished quilt to come up slightly smaller the first time it is washed.