Cushion Fabric Formulas: Box, Knife-Edge & Round Calculation

Notation

Throughout this guide:

• W = finished cushion width
• L = finished cushion length
• T = finished cushion thickness (depth)
• s = seam allowance (typically 0.5”)
• D = diameter (for round cushions)
• fw = fabric bolt width (typically 54”)

Formula 1: Box Cushion

A box cushion has 6 pieces: 2 main panels + 4 boxing strips.

Panel dimensions (cut 2)

Panel cut width  = W + 2s
Panel cut length = L + 2s

Example: 24” × 24” cushion, ½” seam:

Panel cut = 24 + 1 = 25" wide × 25" long

Long boxing strip dimensions (cut 2)

Boxing strip width  = L + 2s   (runs along the length of the cushion)
Boxing strip height = T + 2s

Example: 4” thick:

Long boxing = 25" wide × 5" tall

Short boxing strip dimensions (cut 2)

Boxing strip width  = W + 2s
Boxing strip height = T + 2s   (same height as long boxing)

Example:

Short boxing = 25" wide × 5" tall

Layout and running length

Panels per row        = ⌊ fw ÷ panel width ⌋   (round down)
Panel rows needed     = ⌈ 2 ÷ panels per row ⌉  (round up; 2 panels per cushion)
Panel running length  = panel rows × panel cut length

Long boxing per row   = ⌊ fw ÷ long boxing width ⌋
Long boxing rows      = ⌈ 2 ÷ per row ⌉
Long running length   = long boxing rows × boxing strip height

Short boxing per row  = ⌊ fw ÷ short boxing width ⌋
Short boxing rows     = ⌈ 2 ÷ per row ⌉
Short running length  = short boxing rows × boxing strip height

Total running inches  = panel running + long running + short running
Total yards           = total inches ÷ 36

Full worked example: 24” × 24” × 4”, one cushion, 54” fabric, ½” seam

Panel cut = 25" × 25"
Panels across 54": ⌊ 54 ÷ 25 ⌋ = 2 → both panels fit in one row
Panel running = 1 row × 25" = 25"

Long boxing = 25" × 5"
Long across 54": ⌊ 54 ÷ 25 ⌋ = 2 → both long strips in one row
Long running = 1 row × 5" = 5"

Short boxing = 25" × 5"
Short across 54": ⌊ 54 ÷ 25 ⌋ = 2 → both short strips in one row
Short running = 1 row × 5" = 5"

Total = 25 + 5 + 5 = 35" = 0.97 yards → order 1.25 yards (add 10% + rounding)

Formula 2: Knife-Edge Cushion

Two panels only — no boxing strip.

Panel cut width  = W + 2s
Panel cut length = L + 2s

Panels across fabric  = ⌊ fw ÷ panel cut width ⌋
Panel rows needed     = ⌈ (2 × quantity) ÷ panels across ⌉
Total running inches  = panel rows × panel cut length
Total yards           = total inches ÷ 36

Example: 18” × 18” throw pillow, 2 pillows, ½” seam, 54” fabric:

Panel cut = 19" × 19"
Panels across: ⌊ 54 ÷ 19 ⌋ = 2
Total panels = 2 pillows × 2 = 4 panels
Panel rows = ⌈ 4 ÷ 2 ⌉ = 2
Total = 2 × 19" = 38" = 1.06 yards → order 1.25 yards

Formula 3: Round Cushion

Cut squares for each circle (cutting a circle starts with a square):

Cut square side = D + 2s

Without boxing (two circle panels only)

Circles per row       = ⌊ fw ÷ cut square side ⌋
Circle rows needed    = ⌈ (2 × quantity) ÷ per row ⌉
Running length        = circle rows × cut square side
Total yards           = running length ÷ 36

With boxing strip

Circumference        = π × D  ≈  3.1416 × D
Boxing strip width   = circumference + 12"  (for joining; per cushion)
Boxing strip height  = T + 2s

Boxing strips needed = ⌈ (strip width × quantity) ÷ fw ⌉
Boxing running       = boxing strips × boxing strip height

Total yards          = (circle running + boxing running) ÷ 36

Example: 18” diameter, 3” thick, boxing, 54” fabric, ½” seam:

Cut square = 19" × 19"
Circles: 2 per row → 1 row × 19" = 19"

Circumference = π × 18 ≈ 56.5"
Boxing width = 56.5 + 12 = 68.5"  →  ceil(68.5 ÷ 54) = 2 strips
Boxing height = 3 + 1 = 4"
Boxing running = 2 × 4" = 8"

Total = 19 + 8 = 27" = 0.75 yards → order 1 yard

Formula 4: Piping / Welting

Piping perimeter per cushion:
  Box cushion:    4 × (W + L) + 12"   (top seam + bottom seam = 2 × perimeter, plus joining)
  Knife-edge:     4 × (W + L) + 12"
  Round cushion:  2 × (π × D)  + 12"

Piping strip width = 1.5" (covers standard ⅜" piping cord)
Strips needed = ⌈ (total piping length × quantity) ÷ fw ⌉
Piping yards  = (strips × 1.5") ÷ 36

Example: 24” × 24” cushion with piping, 54” fabric:

Perimeter = 4 × (24 + 24) = 192" + 12" joining = 204" per cushion
Strips needed = ⌈ 204 ÷ 54 ⌉ = 4 strips
Piping fabric = (4 × 1.5") = 6" = 0.17 yards → order ¼ yard

For step-by-step piping construction and cutting instructions, see How to Add Piping to Cushions.

Quick Reference: Yardage by Cushion Size

Standard box cushion, 54” fabric, ½” seam, no piping:

Bold = most common sofa seat cushion size. For full sofa set yardage (seat + back cushions), see How Much Fabric for Sofa Cushions.

Use the Cushion Fabric Calculator for exact figures with your dimensions and fabric width.

See also: Cushion Types and Construction Guide and How to Calculate Fabric for Box Cushions.