Quilt Backing Calculator: Yardage, Seams and Batting Size

How to work out quilt backing

The backing has to be larger than the quilt top so it can't slip short while you quilt. Most quilting cotton is narrower than a quilt, so the backing is pieced from two or three lengths of fabric sewn side by side.

Add the extra to every side: backing width is quilt width plus twice the extra, and the same for length.
Work out how many widths of fabric cover the backing, allowing for each seam.
Multiply the number of widths by the length each one runs, then convert to yards and round up to the next 1/8 yard.
Try it both ways, with the lengths running up and down or side to side, and buy for the one that uses less.

Backing width = quilt width + 2 × extra Backing length = quilt length + 2 × extra Widths needed = (backing width − seam) ÷ (fabric width − seam), rounded up Fabric to buy = widths needed × backing length ÷ 36 in per yard

Example: a 60 × 70 in throw

With 4 in of extra on each side the backing has to be 68 × 78 in. One 42 in width isn't enough in either direction, so it takes two.

Layout	Pieces	Fabric	To buy
Vertical seam	2 × 78 in	156 in	4 3/8 yd
Horizontal seam	2 × 68 in	136 in	3 7/8 yd
108 in wide back	1 × 68 in	68 in	2 yd

Turning the seam saves half a yard here. With wide backing there is no seam at all.

Batting sizes

Batting needs the same extra as the backing. These are the standard package sizes, with the largest quilt top each one covers at 4 in of extra per side.

Package	Batting size	Quilt top up to
Crib	45 × 60 in	37 × 52 in
Twin	72 × 90 in	64 × 82 in
Full	81 × 96 in	73 × 88 in
Queen	90 × 108 in	82 × 100 in
King	120 × 120 in	112 × 112 in

Package sizes vary a little between brands, so check the label. Batting is also sold by the yard for quilts that don't match a package.

Piecing the backing

Trim the selvages first. They shrink differently from the rest of the fabric and can pucker the seam.
Sew with a 1/2 in seam and press it open. A wider seam is stronger, and pressing it open keeps the backing flat.
Avoid a seam down the middle if you like. Cut one width in half lengthwise and sew a half to each side of a full width.
Mark the top edge of directional prints so the backing doesn't end up upside down on the frame.

Questions

How much backing do I need for a throw quilt?

A 60 × 70 inch throw with 4 inches of extra on every side needs a 68 × 78 inch backing. From 42-inch quilting cotton that is two widths sewn with one horizontal seam, so 3 7/8 yards. From 108-inch wide backing it is 2 yards with no seam.

How much bigger should the backing be than the quilt top?

For quilting on a home sewing machine, 2 to 3 inches on each side is usually enough. Longarm quilters usually ask for 4 inches on each side, and some ask for 6. Check with your longarm quilter before you buy.

Should the backing seam be vertical or horizontal?

Either works, so pick the layout that uses less fabric. The calculator compares both. Many longarm quilters prefer seams that run across the quilt, parallel to the frame's rollers, so ask yours if someone else is quilting it.

What size batting do I need for my quilt?

Batting needs the same extra as the backing, usually 3 to 4 inches on each side. Standard packages are crib 45 × 60, twin 72 × 90, full 81 × 96, queen 90 × 108 and king 120 × 120 inches. A queen-size quilt top often needs king batting once the extra is added.

Can I use wide backing fabric instead?

Yes. Wide backing comes about 108 inches wide, with roughly 104 inches usable. With 4 inches of extra on each side it covers quilts up to about 96 inches in one direction without any seams.

Should I prewash backing fabric?

If you prewash, buy a little extra, because cotton can shrink a few percent in the first wash. Enter the shrinkage under Advanced and the calculator allows for it in both the length you buy and the usable width.