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One diagonal seam instead of two vertical panels can save up to a yard and a half of backing fabric on baby, crib, and small throw quilts.
| Quilt | Diagonal (yds) | Two Panels (yds) |
|---|---|---|
| Wall | 1 | 2-1/8 |
| Baby / Crib | 1-7/8 | 3-3/8 |
| Large Crib | 2-3/8 | 3-7/8 |
| Small Throw | 3 | 3-7/8 |
| Throw | 4 | 4-1/4 |
Assumes 42″ of usable fabric width, a 1/2″ diagonal seam, and 4″ of overage per side (8″ total). Yardage rounded up to the nearest 1/8 yard. For quilts wider than about 53″ (a 61″ backing), two vertical panels or 108″ wideback fabric is the better buy — see the backing calculator.
The diagonally pieced method and its yardage formulas were devised by John Flynn (flynnquilt.com).
Cut your calculated length of fabric as a single piece. Cut it corner to corner on the diagonal — one long cut from the top-left corner to the bottom-right corner — so you have two long right triangles. Slide one triangle along the diagonal edge (don't rotate or flip it) until the combined piece is as wide as your backing, then sew the two diagonal edges together with a 1/2″ seam. Press the seam open and trim the assembled piece square to your final backing dimensions.
Because the pieces slide rather than rotate, directional prints keep pointing the same way on both sides of the seam. The trade-off is a visible diagonal seam line on the quilt back — busy all-over prints hide it almost completely, while stripes and regular geometrics will show a mismatch along the seam.
A busy all-over print hides a diagonal seam almost completely — and saves you pattern-matching.
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Instead of sewing two full-length vertical panels together, you cut a single length of fabric corner-to-corner on the diagonal, slide one triangle along the cut, and sew the two pieces back together with a 1/2" seam. The offset makes the fabric wider than it started, so backings a little wider than the fabric need far less length. The method and its yardage formulas were devised by quilter John Flynn.
A diagonal backing uses less fabric than a traditional two-panel backing whenever the backing width (quilt width plus overage) is 1.5 times the fabric width or less. With standard 42" of usable width, that means backings up to about 61" wide — baby, crib, and small throw quilts. The savings are biggest on narrow backings: a 44"-wide backing needs 1-7/8 yards diagonally versus 3-3/8 yards as two panels.
Cut your backing length as one piece. Cut it corner to corner on the diagonal so you have two long triangles. Slide one triangle along the diagonal edge until the piece is as wide as your backing needs to be, then sew the diagonal edges together with a 1/2" seam. Press the seam open, then trim the assembled piece square to your backing dimensions.
Yes — the two pieces are slid along the cut, not rotated, so the print stays pointing the same direction on both sides of the seam. The seam itself runs diagonally across the quilt back. Regular stripes or strong geometric prints will show a visible mismatch along the diagonal seam, so busy all-over prints hide it best.
Most do, and some prefer them: because the seam is not parallel to the frame's rollers, the seam allowance bulk never stacks up in one spot as the backing rolls. Ask your longarmer first — a few charge extra for any pieced backing — and always press the diagonal seam open to reduce bulk.