Do you want to know how to add any size box corners to a bag pattern? Let me show you how I like to do it.
I hope you’ll have fun sewing along with my video tutorial. Or if you prefer, just scroll on down and you’ll find step by step instructions with plenty of photos.
First, I decide how big I want my box corner to be. The bigger it is, the roomier the bag ends up.
On this bag I’ve made my box corner about 6 cm across at that seam, so that makes the width of the bottom of my bag 6cm wide.
To get a roomier shape at the bottom of the bag, I need to add extra fabric to cover that shape. So, to get that shape without changing the original bag size, I add half the width I want – so 3 cm for mine – to both the height and the width on both sides of the bag pattern.
Then I mark out a square with that half-width measurement on both sides in the bottom corners – so mine is 3 cm by 3 cm.
I find marking them out accurately really helps them line up properly later, especially with a bigger box corner.
If I were to just cut the square out of the original pattern without adding that extra fabric first, the finished bag would end up much smaller than planned.
Next I cut those squares off.
Then I sew the side seam of the bag as normal, just a regular straight stitch down the side. I stop at the cut-out corner and anchor the stitches with a few extra forward and back stitches. I don’t sew around the cut-out square at all. I just move the needle straight past it, then continue sewing again just after the cut-out, anchoring those stitches again as I start then doing the same at the other corner before coming up again on the next side seam.
This quilting cotton is prone to fraying, so I add a quick overcast stitch over the edge of the fabric as I go. I find it makes the bag more durable, and it also keeps everything much neater when I sew the seams in stages rather than trying to do it all at once at the end. If you don’t have an overlocker or an overcast foot, then a zigzag stitch inside the seam allowance works just as well.
Once I’ve trimmed away those loose threads, I take a corner of the bag and pinch either side of the missing square, so that I’m pulling the fabric apart. That brings the side seam and bottom seam into the middle so it creates a new straight line running across the bottom edge with those 2 seams meeting in the middle.
I like to push the seams so they face in opposite directions. That helps spread the bulk out so the corner lies flatter once it’s finished. Then I stitch a straight seam right across there. Then you should find that the finished seam comes out at the bag bottom width that you wanted – so about 6 cm across for mine.
Then I just finish off that seam with another quick overcast stitch, zigzagging over the edge of the fabric for a stronger corner.
Then repeat it on the other side exactly the same. The only thing to pay attention to on the second corner is when I’m push the seams in opposite directions on my box corner, I like to make sure that I’m folding the bottom seam in the same direction as the other side. That way it sits neater and flatter. Then it’s done!
Easier than you thought, right? See how nicely it holds its shape even with just 1 layer of quilting cotton. If you want it even firmer, you might want to consider adding interfacing, or quilting your bag lining and outer layers together with some wadding (aka batting) or foam in the middle.
And here it is with the bag fully sewn up – matching the box corner adjustments exactly the same for the lining as I did for that outer fabric layer. It makes a lovely spacious bag bottom, doesn’t it? What do you think?
If you have a go at making one, I’d love to know how you got on. Or, if you have any other helpful tips for making a pocket like this, then please do share them.
Happy Sewing!
