Quilt backing calculator

How much backing fabric do I need?

Enter the size of the quilt top. You get the backing size, how many widths of fabric to sew together, which way the seams should run to use less fabric, and how much to buy.

Your backing

4 1/2 ydbacking fabric to buy (4.1 m)
2widths of fabric, 1 seam
68″ × 80″backing size: top plus 4″ each side

Pieces of backing fabric with the seams; dashed outline: the quilt top, centred.

Cut 2 lengths of 80″ from quilting cotton, 42–44″ wide and sew them together with ¹/₂″ seams, seams running top to bottom (lengthwise). Trim off the selvages first.
Sewn together they measure 79″ across the seams; the backing needs 68″, so 11″ is spare. That leftover strip, 11″ × 80″, is often enough for some binding strips.
To avoid a seam down the middle, cut one length in half lengthwise and sew a half to each side of the other. It uses the same fabric.
The other way round (seams running side to side (crosswise)): 3 lengths of 68″, 5 3/4 yd (5.2 m), 44″ more fabric. Choose it for a directional print.
On 108″ wide backing (104″ usable): one piece of 68″, 2 yd (1.8 m).
Batting: at least 68″ × 80″, the same as the backing.

Binding for this quilt

How the backing is worked out

The backing has to be bigger than the quilt top. Quilting draws the layers in a little, and a longarm machine needs fabric to clamp onto, so most longarm quilters ask for 4″ extra on every side: 8″ added to the width and to the length. If you quilt it yourself you may manage with less, but the calculator assumes whatever you enter on all four sides.

Standard quilting cotton is 42–44″ wide, and about 40″ is usable once the selvages are trimmed, so most backings are pieced from two or three lengths. Each join is sewn with a 1/2″ seam pressed open, which takes 1″ off the width.

The calculator tries both ways: lengths running top to bottom (seams lengthwise) and lengths turned to run across the quilt (seams crosswise), and shows the one that needs less fabric. Turn it the other way if your print has a direction. Wide backing, 108″ or 118″ on the bolt, often covers a whole bed quilt in one piece.

Assumptions: the backing is centred under the top with the same extra on every side; fabric is bought by length and rounded up to the next 1/8 yard or 10 cm; shrinkage from prewashing is not included.

Worked example

A 60 × 72″ quilt top with 4″ extra needs a 68 × 80″ backing. Two 40″ widths sewn together make 79″, so cut two 80″ lengths: 160″, or 4 1/2 yards. Turned crosswise it would take three 68″ lengths, 204″, so lengthwise wins.

Sources

Missouri Star Quilt Co., How to figure yardage for quilt backing: most longarm quilters ask for 4–6″ extra all the way round; 42″ cotton has about 40″ usable; piece backings with 1/2″ seams pressed open; 108″ and 118″ wide backs have about 104″ and 114″ usable.
Quilted Joy, How much backing fabric do I need?: a longarm business asking for backing 4″ beyond the top on every side.
Nebraska Quilt Company, How to calculate longarm backing fabric: some quilters add 5″ per side, and seams parallel to the rollers are often preferred for loading.

Backing questions

Which way should backing seams run for longarm quilting?

Quilters disagree, and sources even use horizontal and vertical differently. Many longarm quilters prefer seams that run parallel to the machine's rollers because the backing rolls on more evenly. Ask your quilter which they want, then use the calculator's other orientation if needed.

Is 4″ extra on each side really necessary?

For a longarm quilter, usually yes: the frame clamps the edges and the quilting draws the quilt in. Some ask for more, up to 6″. When you quilt at home you can use less, but too little leaves you short if the layers shift.

Is wide backing worth it?

For bed quilts it often is. A 108″ wide back can cover a queen quilt in one piece with no seams to sew or press, and the calculator shows how much you would need next to the pieced option so you can compare prices.

Why avoid a seam down the middle?

Some quilters find a centre seam wears faster where the quilt is folded. With two widths, cut one in half lengthwise and sew a half to each side of the other: the seams move off-centre and the fabric needed stays the same.

What about the batting?

Cut the batting about the same size as the backing, so it reaches the edge of the frame or the edge of your basting. Check the shrinkage figure on the packet if you plan to wash the finished quilt.