Cushion Fabric Calculator
Box cushion yardage without the headache. Enter length, width and thickness — the calculator sizes the top and bottom panels, works out the boxing strip, adds seam allowance and tells you exactly what to buy.
Calculate your box cushion fabric
All math runs in your browser. Results update as you change inputs.
How the cushion calculator works
A box cushion is three parts: a top panel, a bottom panel, and a boxing strip that wraps around the edge. Each part is sized up by seam allowance, then laid out across the fabric width:
Worked example — 18 × 18 × 4 in seat cushion
With 1/2-inch seams and 54-inch fabric:
Buying 4 of these cushions: 116 in ≈ 3¼ yd, and with a 10% buffer about 3⅝ yd. Piping for each cushion: 2 × (18+18) + 6 = 78 in ≈ 2¼ yd of premade welt.
Quick reference — common cushion sizes
| Cushion (in) | 54 in fabric, no pattern | Buy (rounded up) |
|---|---|---|
| 16 × 16 × 3 | ≈ 0.75 yd | ¾ yd |
| 18 × 18 × 4 | ≈ 0.87 yd | 1 yd |
| 20 × 20 × 4 | ≈ 1.0 yd | 1 yd |
| 24 × 24 × 4 | ≈ 1.3 yd | 1½ yd |
| 36 × 18 × 4 (bench) | ≈ 1.6 yd | 1¾ yd |
| 72 × 18 × 4 (window seat) | ≈ 3.1 yd | 3¼ yd |
Striped or one-way patterns: add 10–15% and plan the stripe direction before cutting — the boxing strip should run so stripes wrap the cushion continuously.
Tips that save fabric on cushions
Join the boxing at the back. Place strip joins at the rear zipper edge where they are invisible, not at the front corners.
Railroad the boxing when you can. If fabric is railroaded and your perimeter fits within the bolt length, one continuous strip avoids joins entirely.
Zippers add nothing to yardage — the zipper sits in the bottom panel seam or the boxing join. Just buy a zipper 2–3 inches shorter than the back edge.
Outdoor cushions: use the same math, but add 10% buffer — marine and Sunbrella-type fabrics are often 46/60 inch widths, and every seam is a water path, so fewer joins is better.
Frequently asked questions
How much fabric do I need for a box cushion?
For a 18 x 18 x 4 inch box cushion in 54-inch fabric you need about 0.9 yard: two 19-inch squares for top and bottom, plus a boxing strip of about 74 inches joined from two strips. A 24 x 24 x 4 inch cushion needs about 1.3 yards.
How do I calculate the boxing strip length?
The boxing strip runs around the whole cushion, so its length is the perimeter: 2 × (length + width). Add about 2 inches for joining seams, then cut it as strips across the fabric width and join them end to end.
How much extra fabric for piping or welt cord?
Piping needs the same perimeter as the boxing strip: 2 × (length + width) plus 4–6 inches for corners and joins. Premade piping is sold by the yard — buy perimeter plus half a yard. Cutting your own bias piping from the same fabric roughly doubles total yardage.
Should I add seam allowance to cushion fabric?
Yes. Add 1/2 inch on every edge that gets sewn: the top and bottom panels grow by 1 inch in each direction, and the boxing strip height grows by 1 inch total. The calculator does this automatically from the seam allowance field.
How much fabric for outdoor bench cushions?
A typical bench cushion 36 x 18 x 4 inches needs about 1.6 yards of 54-inch fabric. Use the calculator with your exact measurements, and buy 10% extra for pattern matching or striped fabric.