The backing is not the size of the quilt
A quilt backing has to be bigger than the top, and the reason is mechanical. On a longarm frame the quilt is loaded onto rollers and held under tension by clamps at the sides; the backing carries that load, and the quilting itself draws the layers in as it progresses. Four inches beyond the top on every side is the figure most longarm quilters specify, which means eight inches on the width and eight on the length.
That margin also absorbs drift. Even a carefully basted quilt shifts slightly as it is quilted, and a top that starts centred can finish an inch off. With four inches of spare backing you trim it away; with one inch you have a corner with no backing under it and no way to fix it.
Domestic machine quilting is more forgiving because you baste rather than clamp, and two to three inches per side is usually adequate. Ask your quilter before you cut, because their frame and their clamps set the number, not the pattern. If you are sending the quilt out, the requirement is a condition of the service, not a suggestion.
The other measurement that trips people up is the quilt itself. Measure the top across the middle in both directions, not along the edges. Pieced borders stretch on the bias and a top that measures 60 inches at the edge can measure 58 through the centre. The centre measurement is the honest one.
Vertical seams, horizontal seams, and why the winner changes
Once the backing is bigger than one width of fabric, you have to piece it, and there are two ways round. Join panels side by side and the seams run parallel to the quilt's length — a vertical layout. Stack panels one above the other and the seams run across the quilt — a horizontal layout. The two use different amounts of fabric, and which one wins depends entirely on the quilt's proportions.
The arithmetic is short. In the vertical layout you need enough panels to span the backing width, and each panel must be as long as the backing length. In the horizontal layout the roles swap. Because panels come in whole units, a quilt just over a panel width in one direction can cost you an entire extra panel in one orientation and nothing in the other.
Take the 60 by 80 inch quilt in the worked example. Vertically it needs two panels of 88 inches, 4.89 yards. Horizontally it needs three panels of 68 inches, 5.67 yards. Vertical wins by three quarters of a yard. Now take a 100 by 108 inch king: both orientations need three panels, but the horizontal panels are shorter, so horizontal wins by two thirds of a yard. There is no rule of thumb here; you have to compute both, which is what this calculator does.
Do not forget the seam allowances. Two 43-inch panels do not give 86 inches of backing — a half-inch seam on each edge takes an inch out, leaving 85. On a three-panel back you lose two inches. Quilters who ignore this are the ones who find their backing an inch short after they have already trimmed the selvedges off.
Worked example: a 60 × 80 inch quilt on 44-inch cotton
The top finishes 60 inches wide by 80 inches long. Your longarm quilter asks for 4 inches of overhang on every side. The backing fabric is 44 inches wide, you trim half an inch of selvedge from each side, and you join panels with a half-inch seam pressed open.
- Backing size. 60 + (2 × 4) = 68 in wide; 80 + (2 × 4) = 88 in long.
- Usable width per panel. 44 − (2 × 0.5) = 43 in.
- Vertical layout. One panel gives 43 in, not enough for 68. Two panels give (2 × 43) − (1 × 2 × 0.5) = 86 − 1 = 85 in, which covers 68 comfortably. Each panel must be 88 in long, so 2 × 88 = 176 in = 4.89 yd.
- Horizontal layout. To span 88 in you need three panels: two give 85 in, three give 129 − 2 = 127 in. Each panel must be 68 in long, so 3 × 68 = 204 in = 5.67 yd.
- Choose. Vertical, at 4.89 yards — buy 5 yards. You will cut two 88-inch panels and join them down the middle.
- Seam to piece. One join, 88 inches long.
- Wide-back comparison. A 108-inch wide backing covers the 68-inch width in one piece, so you need only the 88-inch length: 88 ÷ 36 = 2.44 yd, with no seam at all.
The wide-back option uses half the yardage but costs more per yard, so compare in money. At $9 for 44-inch cotton, five yards is $45. At $16 for 108-inch wide back, two and a half yards is $40 — and you save an hour of cutting, pressing and piecing.
Reading the result: yardage, seams and where they land
Round the yardage up to the next quarter yard when you buy, and up to the next half yard if your fabric is directional or you are nervous about the cutting counter's accuracy. Shops cut with scissors along a torn or folded line, and a yard is frequently a yard less half an inch.
Look at the seam length as a time estimate as much as a fabric one. A single 88-inch seam is one pass at the machine and one pressing; two 108-inch seams on a king back is a considerably longer job, and every extra seam is another chance for the back to end up skewed. If two layouts are within a fraction of a yard of each other, prefer the one with fewer seams.
Think about where the seam lands. On a bed quilt, a horizontal seam sits across the bed under the sleeper, and a vertical seam runs the length of the quilt. Many quilters route a two-panel vertical back so the seam falls down the centre; others deliberately offset it so it does not line up with the centre of the top. A three-panel back is usually arranged with a full width in the middle and equal partial widths at the sides, which looks deliberate rather than accidental.
Always press backing seams open rather than to one side. A pressed-open seam is half the thickness, which matters when a longarm needle has to cross it, and it distributes the bulk so the quilting stitches sit evenly. Trim the selvedges off before you sew: a selvedge caught in a backing seam shrinks at a different rate from the rest of the cloth and puckers the whole length of the seam at the first wash.
Backing yardage for standard quilt sizes
| Quilt size | Top (in) | Backing needed (in) | Best layout on 44 in | Yards, 44 in | Yards, 108 in |
|---|---|---|---|---|---|
| Crib | 36 × 52 | 44 × 60 | 2 panels, horizontal seam | 2.44 | 1.67 |
| Throw | 50 × 65 | 58 × 73 | 2 panels, horizontal seam | 3.22 | 2.03 |
| Twin | 70 × 90 | 78 × 98 | 2 panels, vertical seam | 5.44 | 2.72 |
| Full / double | 84 × 92 | 92 × 100 | 3 panels, horizontal seams | 7.67 | 2.78 |
| Queen | 90 × 108 | 98 × 116 | 3 panels, horizontal seams | 8.17 | 3.22 |
| King | 100 × 108 | 108 × 116 | 3 panels, horizontal seams | 9.00 | must be pieced |
Every figure comes from the formula on this page: panels are the smallest count where p × 43 − (p − 1) × 1 covers the dimension, and yardage is panels × the other dimension ÷ 36. The winning orientation is not fixed — it is horizontal for a crib and a throw, vertical for a twin, and horizontal again from full size upward. A king backing is 108 in wide before trimming, so a 108 in wide back cannot cover it in one piece either.
Mistakes that leave a backing short
- Buying backing the size of the top. The overhang is not optional on a longarm. Eight inches on each dimension is the standard requirement.
- Ignoring the seam allowances. Two 43-inch panels give 85 inches of finished backing, not 86. On three panels you lose two inches.
- Leaving the selvedges in the seam. Selvedge is woven more tightly and shrinks differently, so a seam containing it puckers permanently after the first wash.
- Assuming the bolt is the width on the label. Quilting cotton labelled 44 inches often arrives at 42 or 43, and after trimming you may have only 41 usable. Measure before you plan a tight layout.
- Measuring the top at the edges. Borders stretch. Measure through the middle in both directions and use that.
- Pressing backing seams to one side. Press them open: the seam is then half as thick where the needle has to cross it.
- Forgetting shrinkage on an unwashed backing. If the top is pre-washed and the backing is not, the back shrinks around the front at the first wash and the quilt goes wavy. Treat both the same way.
- Piecing a back from many small offcuts without adding length. Every extra seam consumes another two allowances and adds another chance for the back to skew.
Wide backing, pieced backs and what to do with the offcut
Wide-back quilting cotton at 108 inches exists precisely to remove this problem, and for anything larger than a throw it is usually competitive. It halves the yardage, eliminates the seam entirely on most sizes, and removes the risk of a skewed back. Its drawbacks are price per yard, a narrower range of prints, and the fact that some longarm quilters find very wide fabric harder to load squarely.
A pieced back — one made from leftover blocks, strips or offcuts — is a different calculation, because the constraint becomes the total area rather than the panel count. Work out the finished backing size here, then treat it as the target area for whatever piecing you plan, and add a generous margin: a pieced back always needs squaring up afterwards. If you are cutting rectangles for it, the fabric yardage calculator will tell you how they nest on the bolt.
Whatever is left over from the backing is worth planning for. A two-panel vertical back on the worked example leaves nothing across the width but does leave a strip if you buy five yards rather than 4.89. Binding is the natural use: the quilt binding calculator tells you how many strips and how much fabric that takes, and for a 60 by 80 quilt it is under two thirds of a yard.
Batting follows the same overhang rule as backing and should be cut to the same size, or a shade smaller than the backing but still larger than the top. If your batting comes on a roll narrower than your quilt, it can be butted and joined with a wide zigzag or a fusible tape rather than a seam, because batting has no grain and no seam allowance. Plan the top's dimensions first with the half square triangle calculator if your blocks are still at the cutting stage.