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Sewing a rectangle into a tube rings of “square peg in a round hole” hopelessness. And yet, it’s one of the most common techniques when creating bags, boxes, and baskets. If you want a flat, stable base, an inset base panel is the go-to option. To make this geometric magic happen doesn’t take a hammer, but it does require careful measuring and marking upfront, and sewing each side independently during construction. Follow our easy steps to achieve an expert finish.

And remember, especially for all the math nerds out there: a square is a special kind of rectangle, one where all the sides have the same length. Every square is a rectangle because it is a quadrilateral with four right angles. However not every rectangle is a square, to be a square its sides must have the same length. So these steps work equally well for all rectangles, including the square ones.

Photo top of page: left top to bottom: The Quintessential Plaid Tote,  Extra-Large Wash & Dry Clean Laundry BinsNesting Baskets in Loomstate & Yarn Dyed Cotton; right: Rocky Mountain Satchel
Photo above: left top to bottom: Bundle of Box Style Floor Cushions, Cool & Cozy Pet Bed, French Mattress Edge Floor Cushions; right: Extra Cushy Square Pouf

For our sample photography, we made a miniature box since it’s always easier to capture close-up images of smaller items. And, we used a contrasting color of thread to make sure you could clearly see all the seams. For your project, you’d cut the pieces as directed by your project’s instructions and would use matching thread.

Photo above: Stylin’ Damask & Canvas Duffle

We are also using the most traditional “tube” – one that is formed by sewing together two flat panels along both sides, resulting in a tube with a seam at the exact center of each side. Your project might have a single seam, or it could be assembled from multiple panels prior to creating the final tube. In any event, once you have a tube, you need to isolate what will become each side of the eventual “box” then find the center point of each side.

Preparing the tube

  1. Following the directions for your project, cut and assemble the main panels into a tube. We placed our panels right sides together, pinning along each side.
  2. Using a ½” seam allowance, stitch each seam.
  3. Roll the tube so the two seam allowances are directly one top of one another. Press open both seam allowances. Each seam allowance creates the center point of the shorter side of our eventual box.
  4. With the tube still flattened in this position, move the iron to the outer edges and press the outer folds.
  5. These crease lines are the exact center points of the longer sides of our eventual box. Highlight these crease lines with your fabric pen or pencil.

Preparing the base panel

  1. Cut out the base panel per your project instructions. As above with the tube, you must find the exact center point of each side.
  2. To do this fold the panel in half and press the fold to set a crease. You can use an iron or just your fingers. You need only a light crease.
  3. Unfold and highlight the crease line, top and bottom, with your fabric pen or pencil. This marks the center points of two sides.
  4. Fold in the opposite direction to create center points for the remaining two sides of the base panel.
  5. With the four center points marked, use your ruler to draw a ½” box in each corner. This marks will you will start and stop each side seam.

    NOTE: This is based on ½” seam allowances for your project. If your project has a narrower or wider seam allowance, adjust the box to match your seam allowance.
  6. Use your ruler to double check the center and corner marks along each side of your base panel.

Inserting the base into the tube

NOTE: Our preference is to start the base panel insertion along one of the sides, which is traditionally a shorter side when working with a standard rectangle. As we move through the steps below, you’ll also see we are recommending stitching both shorter sides first, then the two longer sides. Again, this is a recommendation, and is probably the best route if you are brand new to the technique. That said, the steps can be done in any order – the technique is the same.

  1. Turn the tube wrong side out. Place the first side of the base panel right sides together with the appropriate side of the tube, matching up all your centering marks. Pin in place.
  2. Using the drawn box as your guide, start your seam ½” in. Lock your stitch.
  3. Using a ½” seam allowance, stitch to the opposite marked corner. Lock your stitch.
  4. Remove the project from under the needle. This completes your first side.
  5. Align the base panel to the tube for the next side to be stitched. As mentioned above, we are doing the opposite shorter side next.
  6. As with the first side, use a ½” seam allowance to stitch from marked corner to marked corner.
  7. Remove the project from under the needle, and pin the remaining sides of the base panel to the tube.
  8. In order to create the flattest base possible, you need to clip into each corner of the tube. Snip into the corner at a diagonal at a depth of about ⅜”. You are clipping right up to but not through your stitching line.
  9. This frees up the seam allowance so it can bend or “ease” around the corner, allowing you to more easily stitch the final two sides  independently. The image below shows you the view from the “tube side” with each of the corners snipped and eased.
  10. Stitch the remaining two sides in the same fashion. Work with the base panel facing up.
  11. Stitch one side at a time, starting and stopping at the marked corners.
  12. Below is what the finished piece should look like from the panel side…
  13. … and the tube side. If your fabric is apt to ravel, you can place a dot of seam sealant at each corner.
  14. All your center point markings should be nicely aligned, so the base will sit straight and true.
  15. Here’s your nice, flat base looking down into your new box.
  16. And, turned right side out from the exterior.
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Wendy Lichti
Wendy Lichti
1 year ago

What if you want to line the box. How much smaller do you make the lining? Is there a formula for that?

Liz Johnson
Liz Johnson
1 year ago
Reply to  Wendy Lichti

Hi Wendy – there isn’t one true formula since there are a lot of variables in terms of fabric type, seam allowance, box shape, etc. etc. In general, there are three main options: 1) make it the same size as the outer box and rely on any upper binding to hold it in place – perhaps also tacking down the bottom corners – hiding those tiny tacks in the exterior seam line, 2) make it about 1/2″ smaller all around – that’s a pretty good starting point, but again, there are a lot of variables in play, 3) layer the… Read more »

2 years ago

Thank you for this. You’ve made the process much easier.

Liz Johnson
Liz Johnson
2 years ago
Reply to  Laverne

Hi Laverne – We happy to hear you found the info helpful!

4 years ago

I’ve been looking for a

I’ve been looking for a better way to do bag bottoms than the boxed corners, and you’ve done it!  Thank you!  It’s been difficult to find tutorials other than the ‘three ways to box corners for bottoms’ and I’m SO HAPPY I found this. I see bags everywhere with your type of bottom and it’s taken me a few years to find out how to do it. Boxed corners seem to be for beginners and totes, and you’ve now taken my sewing up several levels. THANK YOU!!

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