How To Make A Bench Seat Cushion With Box Corners

Custom bench cushions can be expensive but you can sew a bench seat cushion in just a few easy steps! Here I'll show you the steps for your own DIY bench seat cushion complete with box corners for a really polished look.

When it comes to putting the finishing touches on your decor, there is no better way to add texture, color, and pattern than with textiles and upholstery but when it came to adding a bench seat cushion to the DIY built in cabinets and window bench I built I needed a custom foam bench cushion without the expensive price tag of custom work.

So I set out to sew a DIY bench cushion and it was so much easier than I thought it would be. Let me show you how I did it so you can make your own custom bench cushion too!

In This Post:

What you'll need to make your own bench seat cushion:

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Upholstery Foam Cushion: I used 3 inch foam to give my cushion a substantial look. Be sure to buy a piece of foam that is at least the length and depth of your bench. You can trim it down, so don't worry if it's too big when you buy it.
Fabric of your choice: I used upholstery fabric with a bit of texture on my bench cushion. You will want to use a home decor fabric since it's a bit thicker and more durable.
A sewing machine: this is my favorite, not too expensive and not too complicated machine.
Needle and thread
Measuring tape and ruler
Scissors
Electric knife or hack saw for cutting foam

How to Sew a Custom bench cushion with box corners

Step 1: Cut your upholstery foam

You'll cut your upholstery foam depending on the dimensions of your bench. It's best to take your bench measurements and subtract about an inch on each side to give a little wiggle room on your finished bench cushion.

Lay your upholstery foam out flat and mark the length first with a marker.
Cut your foam using an electric knife. - Yes, an electric knife like you'd use to slice the holiday turkey. I know that may sound weird but trust me, it's the easiest way to cut through the foam and maintain a nice clean edge.
- If you don't have an electric knife, you can use a hack saw. It's a little more difficult, but it does the job.
Once you have your foam cut to the right length, measure and mark the depth of your cushion.
- Using a straight edge, draw a line where you'll cut.
Cut along the line using an electric knife or hack saw.

how to cut upholstery foam for diy bench seat cushion

Step 2: Dry fit the foam cushion on the bench

Once you've cut your foam, dry fit it on your bench to make sure it fits properly.

Cover Your Bench Seat Cushion With Fabric

Step 3: Measure your fabric

To find the length of your fabric: Take the length of your cushion + 1x the thickness of your foam. Example: my cushion length is 90.5 inches and my foam is 3 inches thick. So 90.5 + 3= 93.5 inches long.
To find the depth of your fabric: Take the depth of your foam + the thickness of your foam + 1 inch for the seam x 2. This allows the fabric to wrap around and gives you a seam in the back. Example: my cushion depth is 24.75 and my foam is 3 inches thick. So 24.75 + 3 + 1= 28.75 inches x 2= 57.5 inches.

Step 4: Cut the fabric for your DIY bench cushion

Make sure you're working from a perfectly straight edge to measure your fabric correctly. You may need to cut your fabric to create a perfectly straight edge to start from.

Once you have your straight edge, measure for the depth of your fabric and cut accordingly.
Fold your fabric over in half length wise so that the good side of the fabric is facing in, then starting at one edge make a straight edge at one end. You may want to use binder clips or pins to hold your fabric in place, especially when working with a long piece.
Measure for the length of your fabric from that end to the other end and cut.

Step 5: Sew down the edge of each end using a ½ inch seam

Step 6: Mark the folded edge

With a fabric marker, mark the folded edge of your cushion cover the entire length of your fabric.

How To Sew Box Corners

Step 7: Prepare the seams for your box corners

With the sewn fabric still good sides together, pinch the top layer of the fabric and pull it over until the sewn seam and the marked edge line up exactly.
- When doing this you'll create what looks like a new corner with your seam running diagonally from it.
Place pins along your seam and through all layers of fabric to hold the seam to the marked edge on the opposite side.

Step 8: Measure for the box corners

Slide a ruler down from the corner you just created until the inches indicated on your ruler (from one edge of your fabric to the other,) equal the thickness of your foam.
- Example: my foam is 3 inches thick. You can see in the image below that I slid my ruler down from the corner until there were three inches taken up on my ruler from one edge of the fabric to the other.
With your fabric marker, draw a line along the edge of your ruler at this point.

measure for box corners on cushion cover

Step 9: Sew along the line

Sew along the line you made using a straight stitch.
Cut the excess fabric off, leaving about ¼ an inch of excess fabric.

Step 10: Repeat steps 7-9 at the other end of your cushion cover

Step 11: Turn your cushion cover right side out

At this point you'll have what looks like a big envelope with one long edge open.

Step 12: Put the upholstery foam in your cushion cover

Starting at one end, hold the corner of one end of your foam and place as tightly against the inside box corner of your cushion cover as possible.
Slide your foam into your cover and when you get to the other end, hold the corner of the foam and push/pull it as tightly into the other box corner as possible.
- This will take a bit of adjusting as you go. Be sure that the box corners are lined up with the corners of the foam and that the fabric on both sides of the foam are as even as possible.

Stitching the back seam of your DIY bench cushion

Once you have your foam inside your cushion cover and it's adjusted the way you want it to fit, place it on your bench with the unfinished edge facing out.

Step 13: Create a fold at one end

At one end of your cushion cover fold the short edge in so it lays flat against the foam - like you're wrapping a gift.

Step 14: Stitch the top flap down

Fold the top flap down so everything lays flat and pin to hold in place.
With a needle and thread stitch from the top of the diagonal seam to the bottom to hold the top flap down flat.
Repeat step 13-14 at the other end of your cushion.

Step 15: Stitch the bottom flap

Grab the bottom flap and fold the unfinished edge of fabric in about ½ an inch.

how to make a bench cushion with box corners

Fold the bottom flap up and pin in place folding in the unfinished edge as you go.
Continue the entire length of your cushion cover.

Starting at one end with a needle and thread, stitch the entire length of your seam using a simple loop pattern.
Once you've completed the seam turn your bench cushion over so that the stitched seam faces the back.

Finishing the look of your bench seat

Add your favorite decorative pillows to complete the look!

Creating these built-ins with a bench seat has been a great way to have plenty of seating in this small sitting area.

I'd say this sitting room is coming together quite nicely!