How to sew a chair seat cushion

Step by step

1. Step 1

   Materials and tools

   • Fabric: sturdy woven fabric (upholstery fabric, canvas, home-décor fabric), approx. 300 g/m² (9 oz/yd²).
   • Filling: firm upholstery foam (any height you like, 3/4–2 in recommended).
   • Notions: hook-and-loop tape (3/4–1 1/4 in wide), sewing thread matching the fabric.
   • Tools: sewing machine, tape measure, ruler, fabric scissors, pins or clips, tailor's chalk or fabric marker, paper for templates. For a round cushion a piece of sturdy string and a pin (for a makeshift compass); for other shapes a French curve helps.

2. Step 2

   Templates and measuring

   Before you cut anything, prepare two paper templates: “d” for the foam, exactly the size of the chair seat, and “D” for the fabric cover. How you get them depends on the shape of the seat.

   Round seat: measure the diameter

   Measure the diameter of the seat “d” and draw both templates with a string compass. This is what I show in the video.

   How to draw a large circle: make a string compass. Stick a pin into the centre of the paper, tie on a piece of string cut exactly to the circle's radius (d/2 or D/2), tie a pencil to the other end and draw the circle with the string pulled taut. Works for any size: nothing to print or tape together.

   Formula for a round cushion

   D = d + t + (2 × s) − 3/8

   d

   diameter of the chair seat

   t

   thickness of the foam

   s

   seam allowance (3/8 in throughout this tutorial)

   −3/8

   3/8 in is subtracted so the finished cover fits snugly around the foam, whose sharp edges we'll trim off

   Example: the chair seat is 18 in, the foam is 3/4 in thick, sewing with a 3/8 in seam allowance.
   D = 18 + 3/4 + (2 × 3/8) − 3/8 = 19 1/8 in

   Any other shape: trace it

   A rounded square, a D shape like my armchair, anything. You can't measure a diameter here, so trace the outline of the seat:

   1. Lay a sheet of paper on the seat and hold it down with one hand, or weigh it down, so it does not move at all. Anything large will do; in 2020 I used a forgotten roll of Christmas wrapping paper.
   2. All the way round, lightly fold the paper over the edge of the seat. The crease shows you the exact outline. Then trace it with a pencil or marker.
   3. Cut out the outline. This is template “d”: use it to trace and cut the foam.
   4. Work out the offset “o” with the formula below.
   5. Lay template “d” on another sheet and trace it. Then, all the way round, measure the distance “o” out from the outline with a ruler, at a right angle, and make a mark. Make a mark every 1 to 2 in, closer together on tight curves. Join the marks with a smooth freehand line, or with a French curve. Cut it out and you have template “D” for the fabric.

   Formula for a traced shape

   o = (t + 2 × s − 3/8) / 2

   o

   offset of the fabric outline from the seat outline, the same all the way round

   t

   thickness of the foam

   s

   seam allowance (3/8 in throughout this tutorial)

   −3/8

   3/8 in is subtracted so the finished cover fits snugly around the foam, whose sharp edges we'll trim off

   Example: the foam is 3/4 in thick, sewing with a 3/8 in seam allowance.
   o = (3/4 + 2 × 3/8 − 3/8) / 2 = 9/16 in

   It is the same formula as for the circle, just for one side: circle “D” is circle “d” offset by “o” all round, D = d + 2 × o.

   

   

3. Step 3

   Cutting the fabric

   Cut a total of 3 pieces from the fabric:

   1. Piece A (top): one whole piece from template “D”.
   2. Pieces B and C (bottom), round cushion:
      • Draw a line through the centre of template “D”.
      • Draw a parallel line 1 1/4 in from the centre line.
      • Fold the template along this offset line.
      • Cut this “oversized half-circle” from the fabric 2×.
   3. Pieces B and C (bottom), any other shape:
      • Split template “D” with a straight line into a front and a back half, across the seat at its widest point. That is where the foam opening goes, and the wider it is, the easier the foam slips through.
      • Draw a parallel line 1 1/4 in away on each side of the split line.
      • For piece B, fold the template along the back parallel line and trace the front half, now 1 1/4 in bigger, onto the fabric.
      • For piece C, fold it along the front parallel line and trace the back half, also 1 1/4 in bigger.

   Here pieces B and C differ, but both reach 1 1/4 in past the split line, just like on the circle. On a circle both halves are the same, which is why one parallel line and the same piece twice are enough there.

   The 1 1/4 in past the split line serves a purpose on each piece: on piece B it carries the hook-and-loop tape, on piece C it flips over into a fold that hides the whole closure (see step 5).

   Cutting list

   fabric	1× whole piece from template “D” (piece A), pieces B and C from template “D”, each 1 1/4 in past the split line
   foam	1× from template “d”
   tape	hook-and-loop tape: 1× length of the split line on template “d” − 2 1/2 in, for a round cushion d − 2 1/2 in (see step 4)

4. Step 4

   Finishing the edges and attaching the hook-and-loop tape

   1. Finishing: zigzag or serge (overlock) the entire edge of all three pieces (A, B, C).
   2. Cutting the tape: cut the hook-and-loop tape to the length of the split line on template “d” minus 2 1/2 in. For a round cushion that is the diameter “d” minus 2 1/2 in (15 1/2 in in our example). The ~1 1/4 in margin at each end leaves room to firmly secure the opening (step 5), while the opening stays large enough to slip the folded foam through.
   3. Attaching: both halves of the tape (hook and loop) are sewn onto the right side of the fabric, along the straight edge of each piece.
   4. Centre the tape on both pieces (equal gaps at each end) and pin it to the straight edge.
   5. Stitch the tape down around its entire perimeter (all four sides, not just one line), otherwise it will peel off over time.

5. Step 5

   Assembling the bottom

   1. Lay piece A on the table as a guide.
   2. Place piece B on it, right side up.
   3. Place piece C on piece B right side down, straight edges aligned, and close the hook-and-loop tape.
   4. Flip piece C over along the tape line to the other side. The joined pieces now form the same shape as piece A; use it to align them. The flip creates a 1 1/4 in fold on piece C with the tape hidden inside.
   5. Pinning the fold: pin or clip the resulting fold on piece C along its whole length.
   6. Separating the pieces: open the hook-and-loop tape and separate pieces B and C again. Only piece C gets stitched.
   7. Topstitching the fold (2×): stitch the fold on piece C with a straight stitch, parallel to the straight edge, twice:
      • the first line along the edge of the folded fabric, 1/16 in from that edge: this locks the fold down;
      • the second line, for reinforcement, at the same 1/16 in offset along the folded crease, over its whole length.
   8. Putting it back together: place the finished piece C back on piece B so the hook-and-loop tape closes again. Pieces B and C now form the bottom of the cushion, matching the top piece A.
   9. Securing the ends of the tape: where the tape ends, sew a short, dense zigzag perpendicular to the straight edge, through all layers. This locks pieces B and C together so the foam opening won’t tear.

   

6. Step 6

   Sewing the cover

   1. Lay piece A on the table right side up.
   2. Place the assembled bottom piece (B+C), with the hook-and-loop tape closed, onto piece A right side down (right sides together).
   3. Align the edges and pin all the way around.
   4. Sew with a straight stitch at a distance of “s” from the edge (your chosen seam allowance, 3/8 in in our example) all the way around.

7. Step 7

   Finishing and inserting the foam

   1. Turning: open the hook-and-loop closure and turn the whole cover right side out through it. Smooth out the seams.
   2. Cutting the foam: cut the foam using template “d”.
   3. Shaping: with scissors, trim the top and bottom edges of the foam all the way around (about 1/4 in at a 45° angle). The cushion will have a nicer, softer shape.
   4. Inserting: fold the foam, slide it into the cover and smooth everything into place.