How this calculator works
Backing and batting are bought by length from a roll or bolt, and a quilt wider than the roll needs two or more widths joined. Add the overage to each side of the quilt first, because a longarm needs fabric beyond the quilt edge. Then divide the quilt width by the usable fabric width and round up for the number of widths. That many widths, each as long as the quilt plus overage, gives the length to buy. If the fabric has no direction, the pieces can be rotated, and the calculator compares both layouts and reports the shorter one. A directional print must keep the quilt length running along the fabric.
Formula
quilt plus overage = quilt size + 2 x overage. widths = (quilt width + 2 x overage) / fabric width, rounded up. length to buy = widths x (quilt length + 2 x overage). The rotated layout swaps the width and length in those formulas.
Worked example
A 150 by 200 cm quilt with 10 cm overage is 170 by 220 cm. On 110 cm fabric, 170 / 110 = 1.55 widths, so 2 widths of 220 cm, or 4.4 m. Rotated, 220 / 110 = 2 widths of 170 cm, or 3.4 m, so the rotated layout is shorter for a non-directional fabric.
Advertisement
Frequently asked questions
Why add overage to both sides?
A longarm frame and many home frames clamp the backing outside the quilt edge. Ten centimetres on each side is a common allowance; a few quilting styles need more. The extra fabric is trimmed away after quilting.
Can batting be joined?
Yes, with a seam or a fusible tape, but a join can show as a ridge. Where the batting roll is wide enough, one piece is better. The calculator reports the widths needed for the chosen batting width.
What does directional mean here?
A print with a clear up and down, such as trees or text, or a woven stripe, cannot be rotated without the pattern running sideways. If your fabric is like that, keep the quilt length along the fabric length.
Should I pre-wash backing fabric?
Pre-wash if the quilt will be washed and you want to avoid shrinkage after quilting. Pre-washing also removes sizing and can change the usable width, so measure again after washing.