How to: sew a bento bag

Every year I like to make something quick and fun for friends. Last year, it was bento bags—little furoshiki tie bags that can carry a bento box, a sandwich, or a tupperware or pyrex container with your lunch. They also make good produce bags. And a knitting friend uses hers to carry her yarn around her wrist while she knits.

A while back, at a crafty retreat with The Makerie (highly recommended, check them out!) , I received my lunch in a sweet little linen sack that tied at the top.

I enjoy taking my lunch to work in this bag. It is easy to pack and easy to throw in the laundry.

It looked like it was made of one piece of fabric—a long parallelogram—wound around into the shape of a bag with ties.

I found several patterns online that were made of triangles and a square with two layers of fabric.

But that wasn’t quite right. These were 45° and 90° angles and thicker. Not as slim and elegant.

I found this pattern that looked more like what I wanted, with 30°/60° angles and one seam:
https://ramonaclothing.wordpress.com/2013/08/02/tutorial-for-making-an-azuma-bento-bag/

But the dimensions were still a mystery to me. After some experimentation, I whipped up a few bags.

Then, with my son Kadin’s help, we worked out the geometry so the bag could reliably be made in any size.

THE PATTERN

You start with the length you want the bottom of your bag to be. This we will call n. Your bag will end up being n inches wide and a little less* than n inches tall/deep at the join before the ties.
*86% as tall/deep as wide

To make your bag, you cut a rectangle with these dimensions: 

(3n + n/2) by (squareroot(3) / 2)(n)

Luckily, squareroot(3) / 2 is just a constant, 0.86602540378.

That makes our dimensions:

3n + (n/2) by 0.86602540378(n)

Here is a table, feel free to add your own rows for other dimensions. For example, if you want a bag 10in wide, cut a rectangle 35in long by 8.5in wide:

Bag width (n)	Rectangle length (3n + n/2)	Rectangle width (0.866)(n)
8	28	~6.9
10	35	~8.7
12	42	~10.4
36	126	~31.2

If all those numbers confuse you, here is an example rectangle that makes about a 10in wide bag that works great for a lunch box:

Fabric is pretty forgiving, so feel free to round up and allow for seam allowance, etc.

From your rectangle, you will want to cut a parallelogram with 60° angles like this:

Measure in n/2 from one side to make your slanted ends, each slanting in the same direction.

For the 35in x 8.5in example rectangle above, this translates to:

The long points that have acute angles will be your ties, the oblique angles will be your joins.

PREP WORK

Before you do anything else, finish the slanted ends of the parallelogram shown in red:

Iron the slanted ends (short ends in red) folded over twice, then top stitch to hem/finish the edge.

Once the slanted ends are finished, you will want to prepare your tie by prepping the other side of the tie with some more ironing. From the other side of the acute angle (your tie), fold over twice and iron to about 2/3rds of the way down towards the oblique angle. The areas to prepare with ironing are shown in orange-red:

Here is how to iron the point so that you do not end up with any raw edges:

Trim it flush with the raw edge:

Fold the point down and press (this is the key step!):

Iron the edge over:

Iron the edge over again:

You have a parallelogram with two finished edges and two edges that are folded over and pressed, about ⅔ of the way down.

THE FUN PART!

The acute angles will be the ties. Each tie should have two sides of matching length. Starting from the acute angle, measure n inches down the long side (you can use the finished, slanted edge as your ruler, it is n!). This spot, n down from the acute corner, is where your join will be.

Fold up from the oblique angle to this place n inches down from the acute angle. Fold the rectangle with right sides together and sew from the fold to the point. Like this:

Sew down the entire edge. By the time you get to the join (the corner of the oblique angle), you will transition from sewing two layers together to sewing on only one layer of fabric, but sewing on top of (top stitching) your prepared and ironed double-folded edge. This will finish this edge of your tie. You can zip right through the thick point at the end with all the raw edges folded and ironed inside.

Half done!

Find the other acute corner. Measure n inches down, fold the corner from the oblique angle up to this point, and sew down the entire side from fold to point. Once you get to the join, transition to top stitching along your double-folded edge to finish the last tie.

Voilá! You have a bento bag with all the edges finished!

FINISHING TOUCHES

I like to miter the corners. You can do it opened up or leave it closed. I’ve been doing them closed. Just run the corner of your bento bag through the sewing machine at a 45° angle:

Opened up corners—where you press the seam open and flatten the corner with the seam in the middle on top—would make the bag a little more symmetrical, but the asymmetrical feel is also nice.

And finally, you can reinforce the join with some fancy hand stitching, or do it by machine.

 

But if you’ve sewn through the join and transitioned to top stitching, your join will be pretty strong.

ENJOY!

Map of a bento bag’s six equilateral triangles: