How to Sew Neat 3D Boxed Corners – 5 Tutorials (+ Videos)

If you’re new to making bags and zipper pouches, you’ll need to learn how to make 3D shapes by “boxing” the corners. Here are 2 ways to sew neat boxed corners, 3 ways to finish the raw edges if your bag is unlined, and how to calculate the corner sizes. This guide includes step-by-step video snippets showing the whole process, making it even easier to follow along.

Method 1: boxed corners on finished bags
Method 2: cut-out corners
How to finish the raw edges of boxed corners
How to calculate boxed corner sizes

PS. If you need a project to practice boxed corners on, try a free unlined tote bag sewing pattern.

Method 1: Afterthought corners for finished bags

Description:

This method involves sewing the two pieces of fabric together regularly, pivoting at the corners, and then adding in boxed corners.

This is a great method for adding boxed corners to existing bags, as well as deciding on the depth after you’ve sewn the bag. The only disadvantage is that you’ll have to pay more attention to aligning the seams to achieve a perfect result.

Step-by-step instructions:

Step 1: Place the two pieces of fabric right sides together, and pin them along the raw edges. I would suggest starting with the edges of your two layers of fabric already finished using your preferred method to make the rest of the construction easier. You can use a serger (overlocker), a zigzag stitch, pinking shears, some bias tape, or any other method you prefer.
Step 2: Sew at your designated seam allowance until you reach the corner. When you reach the corner, stop with your needle down, and pivot the fabric 90 degrees. Continue sewing, and repeat at the other corner(s).
Step 3: Trim the excess fabric at the corners. First, cut at a 45-degree angle at the corner, and then trim triangular shapes out of the sides of the corner. Don’t worry about cutting into the stitching, as it won’t matter in this case.
Step 4: Press the seam allowances open so that you can clearly see the stitching lines in the next step.
Step 5: Pick the bag up, and match the side seams up so that they are perfectly aligned. You can test this by sticking a pin into the seam on one side, and checking to see if it comes out of the seam on the other side.
Step 6: Keep matching the side seams until the bottom of the bag forms a square at the base. It will eventually form a square, no matter the size or shape of the piece you’re sewing. Check that the bottom seam and the side seam are directly on top of each other by sticking a pin through the seam and seeing if it comes out on the seam on the other side.
Step 7: Looking at the square you formed, measure up from the corner, along the bottom seam, and mark. This measurement will determine the depth of your boxed corners. To learn how to determine this measurement, jump to this point in the article. Mark a straight line that is perpendicular to the bottom seam at the level of the marking you just made. Double-check that the bottom seam lands exactly in the center of your desired measurement. For example, if you want a 4” (10 cm) line, the side seam should be exactly at the 2” (5 cm) mark.
Step 8: Pin and sew at the line you marked, backstitching at the beginning and the end.
Step 9: Trim the corner off, leaving about ⅜” to ⅝” (1 to 1.5 cm) of fabric.
Step 10: Finish the raw edge in the method of your choosing. I outlined three suitable seam finishes here. Turn your project right side out.

Method 2: Cut-out corners

Description

This method involves cutting square-shaped notches out of the bottom corners of the fabric to form the boxed corners.

This is a good option if you’re making a bag, case, or any other project from scratch. It allows you to align the seams more easily than the first method, giving you a neater result without much effort. You will, however, need to determine the depth of your project ahead of time so that you can cut out the right-sized squares at the beginning.

Step-by-step instructions

Step 1: Take the two pieces of fabric you want to use for your finished piece, and determine which side will be the side with the boxed corners. Now, you will mark a square out of each corner based on how deep you want the bottom of the piece to be. To calculate the correct dimensions for your desired finish, jump to this point in the article.
Step 2: Cut the squares out of the corners following the markings you have just made.
Step 3: Pin the two layers of fabric with right sides facing, making sure the square notches are perfectly matched up.
Step 4: Sew along the sides and the bottom using your designated seam allowance. I used a ⅜” (1 cm) seam allowance for my sample.
Step 5: Finish the bottom and side seams using your preferred seam finishing technique. Here you can use a serger (overlocker), a zigzag stitch, pinking shears, some bias tape, or any other method you prefer. I outlined three suitable seam finishes here.
Step 6: Pick up one of the corners, and open the fabric. Align the side seam with the bottom seam, which will force the square cut-out into a straight line. Pin along this newly formed straight edge, making sure the side and bottom seams are perfectly on top of each other. Make sure the seam allowances are going opposite ways to reduce the bulk.
Step 7: Sew along the pinned edge using your designated seam allowance.
Step 8: Finish the freshly sewn edge using your preferred seam finishing technique. Turn it right side out.

How to finish the raw edges of boxed corners:

Now that you’re familiar with how to sew boxed corners, you can start thinking about how to finish the raw edges. This is only necessary if your sewing project is unlined. I’ll walk you through 3 types of seam finishes that work well with boxed corners. Both of the sewing methods I mentioned above are suitable for these seam finishes, so it doesn’t matter which one you choose to follow.

Method 1: Serged (Overlocked) / zigzagged edges

Description

Serging the raw edges is a simple and fast way to neaten your seam allowances. If you don’t have access to a serger (overlocker), you can also use a simple zigzag stitch or a more elaborate mock-overlock stitch on your domestic sewing machine.

Step-by-step instructions

Using a serger (overlocker): Simply run your raw edge through the serger, leaving a long tail at the start and at the end. Using a tapestry needle, weave the ends of the threads into the serger stitch and trim the excess. This will ensure that your serging doesn’t become loose and unravel with use.
Using a zigzag / mock-overlock stitch: Choose a medium zigzag (I used a width of 5 and a length of 3 ) or a mock-overlock stitch on your domestic sewing machine. Sew along the raw edge of the fabric.

Method 2: Bound edges

Description

Binding the raw edges of the boxed corners is my personal favorite technique. It gives a really clean and professional look to the inside of your project, and the additional binding material gives the bottom of the bag some extra structure and shape. I like to use pre-made or homemade bias binding, but you can also use twill tape or even a wide ribbon to enclose the edges. Store-bought make-up bags are often bound using twill tape. You can choose a fun pop of color or pattern to add a special touch to the inside of your project.

Step-by-step instructions

Step 1: Trim the excess seam allowance to about 8 mm (a little over a ¼”). Unfold the bias binding, and pin the right side of the binding to the boxed corner seam. The crease of the binding should be on top of the original seam of the boxed corner.
Step 2: Sew a straight line down the crease of the binding.
Step 3: Fold the binding up and over, so that the folded edge barely covers the stitching line that is visible on the other side.
Optional: You can tuck the short edges of the binding in to give it an extra neat finish, but this adds a bit of bulk which may be difficult to sew. The binding won’t fray as the binding is cut on the bias, so this isn’t absolutely necessary to do.
Step 4: Topstitch the binding down on the other side, making sure this new line of stitching is to the right of the original seam you sewed to form the boxed corner. This will ensure that you don’t accidentally change the depth of the boxed corner.
Step 5: Turn your project right side out.

Method 3: French Seams

Description

French seams are formed by sewing the two pieces of fabric wrong sides together, trimming the seam allowance, turning the piece inside out, and sewing the same seam with the right sides facing eachother. It completely encloses the raw edges, giving you a durable and clean-looking result on the inside and outside of your project.

Using this technique creates some bulk at the seams, so it’s usually best suited for light to medium-weight fabrics. However, since some extra weight and thickness can help the boxed corners keep their shape, you can also experiment with French seams on heavier-weight fabrics. Just keep in mind that it can be more challenging to sew through multiple layers of thick fabric. You will also need to be extra careful about the seam allowances to maintain your desired depth.

This technique is easier to execute if you follow the second method of sewing the boxed corners (the cut-out corners method) but it can also be done if you followed the first method. Here, I’ll demonstrate how to do it using the second method.

Step-by-step instructions

Step 1: Before you sew the corner seams, stop and turn the piece right side out. (After Step 4 of Method 2).
Step 2: Align the side seam with the bottom seam, and pin along the raw edge.
Step 3: Sew using half your original seam allowance. For example, I was aiming for a ½” (1.2 cm) seam allowance, so I used a ¼” (6 mm) seam allowance for this step.
Step 4: Trim the seam allowance down.
Step 5: Turn the piece inside out, so that you’re once again looking at the inside of your project. Pin along the seam you just sewed, making sure the side seam is aligned with the bottom seam.
Step 6: Sew a straight line, once again using half your original seam allowance – ¼” (6 mm) in my case.
Step 7: Turn your project right side out.

How to calculate boxed corner sizes:

If you’re using method 1 (afterthought corners):

In Step 7, you need to measure up from the corner and mark a line to determine the the depth. It’s really simple to calculate what the distance should be between the corner and the line to obtain your desired depth: it’s just half the final depth you would like! For example, if you want your bag to have a depth of 4” (10 cm), you need to mark 2” (5 cm) up from the corner. When you draw a perpendicular line to the bottom seam that touches the marking you just made, you’ll see that it’s exactly 4” (10 cm) long.

Keep in mind that the further up you go from the corner, the longer your stitching line becomes. This will mean that the depth of your bag will increase, and the length will decrease. Take your desired depth into account when you’re measuring and cutting out your fabric for you project.

If you’re using method 2 (cut-out corners):

With this method, you’ll need to know the desired depth before you start sewing as it will inform the dimensions of the square cut-outs you’re going to make at the corners. Similarly to the first method, the length of one side of the square cut-out will be half the depth of your finished box corner. So, if you want a 4” (10 cm) deep bag, you need to cut 2” by 2” (5 cm by 5 cm) squares out of the bottom corners.

Unlike with the first method where we directly marked a stitching line, with this second method seam allowance is really important. You need to make sure that you use the same seam allowance on the boxed corner seam as you did when you were sewing the side seams and the bottom seam. This way, you’ll ensure that your measurements work as you predicted them to work.

What to read next:

This article was written by Nisan Aktürk and edited by Sara Maker.

Nisan Aktürk (author)
Nisan started her sewing journey in December 2019 and already has a fully handmade wardrobe. She’s made 50+ trousers, 20+ buttoned shirts, and a wide array of coats, jackets, t-shirts, and jeans. She’s currently studying for her Sociology Master’s degree and is writing a thesis about sewing. So she spends a lot of her time either sewing or thinking/writing about sewing! Read more…