Quick Answer
A quilt backing calculator tells you how much fabric to buy by adding a 4-inch margin to every side of your quilt top, then dividing the backing width by your fabric's usable width (about 42 inches) to find how many widths to sew together. Backing yardage = (number of widths × backing length) ÷ 36, rounded up to the next quarter yard.
Quilt Backing Formula & Yardage Chart
To calculate fabric for quilt backing, the backing-for-quilts calculator adds a margin to every side of your quilt top, then works out how many widths of fabric to piece together and the total yardage to buy. The backing has to be bigger than the top so a longarm or basting frame can clamp it — 4 inches on every side is the standard rule.
Backing = quilt top + (2 × extra per side);widths = ⌈ backing width ÷ usable fabric width ⌉ (≈ 42 in cotton);yardage = (widths × backing length) ÷ 36, rounded up to the next ¼ yard.| Quilt size | Quilt top (in) | Backing needed +4″/side (in) | 42″ cotton | 108″ wide backing |
|---|---|---|---|---|
| Baby | 30 × 40 | 38 × 48 | 1½ yd | 1¼ yd |
| Crib | 36 × 52 | 44 × 60 | 2½ yd | 1¼ yd |
| Lap / Throw | 55 × 70 | 63 × 78 | 3½ yd | 1¾ yd |
| Twin | 70 × 90 | 78 × 98 | 5½ yd | 2¼ yd |
| Full / Double | 80 × 90 | 88 × 98 | 7½ yd | 2½ yd |
| Queen | 90 × 108 | 98 × 116 | 8¼ yd | 3¼ yd |
| King | 108 × 108 | 116 × 116 | 9¾ yd | 6½ yd |
How to Use the Quilt Backing Calculator
-
Pick your quilt size
Open the size dropdown and tap your quilt — Baby, Twin, Queen, King, and more. It fills in the width and length for you. Not listed? Just type your own two numbers.
-
Type your quilt’s width and length
Measure the finished quilt top, not the bed, and enter the two numbers. Tap Inches or Centimetres so it matches your tape measure.
-
Set the extra on each side
Leave it on 4 inches for most quilting, or type a bigger number if your longarm quilter asks for more.
-
Read your answer on the right
The calculator shows the yards to buy, how many fabric widths to cut, and a small picture of where the seams go. Open More options to switch to wide backing or change the seam direction.
Worked Example: Calculating Quilt Backing Yardage
Here is how the yardage calculator for quilt backing works out a standard 70 × 90-inch twin quilt on regular 42-inch cotton, so you can check it by hand.
-
Add the margin to every side
The twin top is 70 × 90 in. Add 4 in on all four sides, so the backing must be 78 × 98 in.
-
Count the fabric widths
Divide the backing width by the fabric width: 78 ÷ 42 = 1.86, rounded up to 2 widths.
-
Buy the length
Two widths, each 98 in long, = 196 in. Divide by 36: 196 ÷ 36 = 5.44, rounded up to 5½ yards.
-
Cut and sew
Cut two pieces 98 in long, trim the selvages, and sew them down the middle with one vertical seam to make the back.
