How it works
A backing has to be bigger than the quilt top so it can be loaded on a frame and so nothing ends up short after quilting. Most longarm quilters ask for 4" extra on every side. If you quilt on a domestic machine, 2–3" usually does it.
Backing width = quilt width + 2 × extra
Backing length = quilt length + 2 × extra
Panels = round up( (backing span − ½") ÷ (usable width − ½") )
Yardage = panels × panel length ÷ 36, rounded up to ⅛ yd
Vertical seams: panels run the length of the quilt and are sewn side by side. You need enough panels to cover the width, and each one is cut as long as the backing.
Horizontal seams: panels run across the quilt. You need enough panels to cover the length, and each one is cut as wide as the backing.
Each seam uses ½" (a ¼" allowance on each panel), so the calculator subtracts that before checking the panels are wide enough. It compares both layouts and highlights the one that needs less fabric. If they tie, it picks the one with fewer seams.
Worked example
A 60" × 80" throw with 4" extra per side needs a 68" × 88" backing. With 40" of usable width, vertical seams need 2 panels of 88" (176" = 4.89 yd, so buy 5 yd). Horizontal seams need 3 panels of 68" (204" = 5.67 yd, so buy 5 3/4 yd). Vertical wins.
Common mistakes
- Using 44" as the usable width. Once selvages are trimmed and the fabric is pre-washed, 40–42" is more realistic.
- Forgetting the seam allowance. Two 40" panels sew up to 79½", not 80".
- Skipping the extra around the edges. Longarm clamps need it, and quilting draws the top in.
Batting is cut the same way, a few inches bigger than the top on every side. The batting calculator finds the smallest package that fits. If you are still adding borders, size them first with the border calculator, because the backing is measured on the finished top.
Last checked October 2026 against standard quilting and knitting math. Found a mistake? Tell us and we’ll fix it.