What this calculator does
A quilt back is almost always bigger than one width of fabric. Quilting cotton comes off the bolt about 42 inches wide, and even a lap quilt is wider than that once you add the extra your quilter needs. So the back gets pieced from two or three long panels sewn side by side, or from one length of extra-wide backing.
The tricky part is which way to run the seams. A 60 × 72 inch quilt with 4 inches added per side needs a 68 × 80 inch back. Two panels running top to bottom need 2 × 80 = 160 inches of fabric, about 4½ yards. Two panels running side to side need 2 × 68 = 136 inches, just under 3⅞ yards. Same quilt, same fabric, more than half a yard apart. The calculator tries both directions and shows you the cheaper one first.
How to use it
- Measure your finished top through the middle, not along the edges. Edges stretch.
- Enter the width and length in inches. Fractions such as
60 1/2work. - Set the extra per side. Ask your longarm quilter; 4 inches is the most common request.
- Leave the usable width at 42 inches unless you have measured your washed fabric.
- Buy the yardage in the result and cut the number of panels it shows, each at the length shown.
The math
backing width = top width + 2 × extra
backing length = top length + 2 × extra
panels = round up (backing side ÷ usable width)
yards = panels × other side ÷ 36, rounded up to 1/8
We do that sum twice: once with the panels running the length of the quilt (vertical seams) and once across it (horizontal seams). Whichever needs fewer yards wins. For wide backing, if either side of the back fits inside 106 inches of usable width, one piece does the job and you only buy the other side's length.
Worked example: a queen quilt
A 90 × 108 inch queen top with 4 inches per side needs a 98 × 116 inch back. Running seams top to bottom: 98 ÷ 42 = 2.33, so 3 panels, each 116 inches long. That is 348 inches, or 9 3/4 yd. Running seams side to side: 116 ÷ 42 = 2.76, also 3 panels, but each only 98 inches long. That is 294 inches, or 8 1/4 yd, so side-to-side seams save a yard and a half. With 108 inch backing the back fits in one piece 98 inches wide, so you buy the 116 inch length: 3 1/4 yd.
Backing for common quilt sizes
Computed with 4 inches per side and 42 inch usable cotton. Your pattern's own size beats these, but they are handy when you are standing in the shop.
| Quilt | Top size | 42″ cotton | 108″ wide |
|---|---|---|---|
| Baby | 36 × 52″ | 2 1/2 yd | 1 1/4 yd |
| Lap / throw | 50 × 65″ | 3 1/4 yd | 1 5/8 yd |
| Twin | 70 × 90″ | 5 1/2 yd | 2 1/4 yd |
| Full | 85 × 108″ | 7 3/4 yd | 3 1/4 yd |
| Queen | 90 × 108″ | 8 1/4 yd | 3 1/4 yd |
| King | 108 × 108″ | 9 3/4 yd | 6 1/2 yd |
A king with 4 inches per side needs a 116 inch back, wider than 108 inch fabric, so even wide backing has to be seamed. 120 inch wide backing covers it in one piece.
Tips before you cut
Trim the selvages first. They are woven tighter than the rest of the fabric and pucker when the quilt is washed. Cut them off before you join panels, then use a ½ inch seam pressed open so the back lies flat under the quilting.
Directional prints. If the print has an up and a down, check that the seam direction the calculator picks keeps the print the right way up. If not, use the other direction, shown in the result, and buy that amount.
Leftover backing is not wasted. A 20 inch strip from the side of a pieced back is often enough to cut the binding, so run the binding calculator before you go back to the shop.
Questions people ask
How much bigger than the quilt top should the backing be?
Most longarm quilters ask for 4 inches more on every side, so 8 inches added to the width and the length. If you quilt on a domestic machine, 2 to 3 inches per side is usually enough. Change the extra-per-side box to match what your quilter asks for.
Why does the calculator use 42 inches and not 44?
Quilting cotton is sold as 42 to 44 inches wide, but once you trim off the selvages and allow for shrinkage you can count on roughly 40 to 42 inches. Using 42 matches the published charts from most fabric companies. If your fabric is narrower after washing, type the width you actually measured.
Should the backing seam run vertically or horizontally?
Either is fine for strength. Pick the direction that uses less fabric, which is what the calculator does. Some longarmers prefer the seam to run parallel to the rollers (side to side) so the backing loads flat, so ask yours if you are unsure.
Is wide backing cheaper?
Often. 108-inch backing costs more per yard, but a queen quilt may need about 3 yards of it instead of more than 8 yards of standard cotton, and there is no seam to sew. Compare the two prices per project, not per yard.
Do I need to prewash backing fabric?
It is a personal choice. If you prewash, buy about 5 percent more because cotton can shrink in both directions, and measure the washed width before you cut.