A box cushion is five honest pieces: two plates, a boxing strip, a zipper run, and, when it dresses up, piping around both plates. Enter the foam; every cut and the total yardage come back.
Your Foam
Fine-tune: fabric width and cord
Fabric to Buy
1 ½yd
24 × 24 × 4″ foam · piped · 54″ fabric
- Plates, cut 225 × 25″ → 25″ of fabric
- Boxing strip100″ at 5″ tall → 2 strips, 10″
- Pipingcord 200″ · bias 5 ⅝ yd from a 16″ square
- Zipper to buy28″ (#5), set in a back plaque
- Fabric total1 ½ yd (51″ of length)
The Formulas
plates: 2 × (width + 1) × (depth + 1) · boxing strip: perimeter + 4, at height + 1piping: 2 × perimeter + 8 · bias strip width = π × cord + ¾ · bias square = √(length × width)Every cut carries ½ in seams all around. The boxing strip wraps the foam’s sides; its back stretch hides the zipper in a two-piece plaque, which is where the extra 4 inches go.
Worked Example
A 24 × 24 × 4 in chair cushion on 54 in fabric, piped with 5/32 cord:
- Plates: two at 25 × 25: both fit across the width → 25 in of fabric.
- Boxing: perimeter 96 + 4 = 100 in at 5 in tall → two crosswise strips → 10 in.
- Piping: 2 × 96 + 8 = 200 in of 1¼ in bias → a 16 in square, cut continuously.
- Total: 25 + 10 + 16 = 51 in → 1½ yd. Skip the piping and it’s a tidy 1 yd.
How to Measure Your Inputs
- Measure the foam itself, compressing nothing: width side to side, depth front to back, height at the edge.
- Wrapped foam (dacron or batting around the slab) adds ½ in to every dimension: measure after wrapping, or add it here.
- Patterned fabric: center a motif on each plate and expect one extra repeat per plate beyond this yardage: the calculator prices plain cloth.
Assumptions and Defaults
- Half-inch seams throughout, the upholstery standard; the cover sews ½ in smaller than the foam on purpose, for a tight fit.
- The zipper runs the back side plus 2 in around each corner: measure the back edge and add 4 for the zipper to buy.
- Piping bias cuts from one continuous-marked square, the same math as bias quilt binding.
Cushion Questions, Answered
How much fabric does a box cushion need?
A box cushion 24 in square and 4 in thick needs about 1½ yd of 54 in fabric with piping, or 1 yd without it. Larger cushions grow fastest in the top and bottom plates, so once the two plates no longer fit side by side, the yardage jumps by a full plate.
Should a cushion cover be smaller than the foam?
Yes, a cushion cover should be sewn snug, about ½ in smaller than the foam, so the foam fills the corners and the cover stays smooth. The seam allowances in this calculator build that in, so cut to its measurements and sew accurate ½ in seams.
How much piping cord does a cushion need?
A cushion needs enough piping cord to go around both plates plus slack for joining, which is 2 × the perimeter + 8 in. Order the cord by that length, and cut the bias strip from the single square the calculator sizes, using the continuous method.
Where does the zipper go in a box cushion?
The zipper in a box cushion goes in the back section of the boxing strip, set into a plaque: the boxing splits into two half-height strips that enclose the zipper, then rejoin the loop. Buy a zipper as long as the back edge plus 4 in, so it wraps both back corners and the cover opens wide enough to insert the foam.