Join bias tape by sewing the ends at a 45-degree angle

When you run out of bias tape mid-project or need to extend a single strip, you join two pieces by sewing their ends together at a diagonal seam. This diagonal join—called a bias seam—is stronger than a straight seam and lies flatter when pressed open, so it won't create a thick ridge under your fabric.

The method works because bias tape is cut on the bias (at 45 degrees to the grain), so when you sew two bias-cut ends together at that same angle, the grain lines up and the seam sits invisibly along the tape's fold. A straight seam across the ends would bunch and pucker when you fold the tape around fabric.

Key Takeaways

  • Lay the two tape ends perpendicular to each other in an L-shape, right sides facing, and mark a diagonal line from corner to corner.
  • Sew along that diagonal line with a short stitch length (1.5 to 2.0 mm), then trim the seam allowance to 1/4 inch and press it open.
  • Test the join by folding the tape in half lengthwise—it should fold smoothly without puckering or bunching at the seam.
  • If the join puckers when folded, the angle was off; unpick it and try again, making sure the tape ends are truly perpendicular before marking.

Prepare the tape ends and mark the seam line

Lay the two pieces of bias tape on a flat surface with their right sides (the printed or finished side) facing up. Position them in an L-shape so one tape runs horizontally and the other runs vertically, with the ends overlapping about 1/2 inch. The corner where they meet should form a 90-degree angle.

Flip the top tape over so the right sides are now facing each other. The two tape ends should still overlap, and you should see the wrong side of the top tape. Using a ruler and a fine-tip fabric pencil or chalk, draw a diagonal line from the upper left corner of the overlap to the lower right corner. This line should run at a 45-degree angle—the same angle as the bias grain of the tape itself.

Double-check that your line runs corner to corner across the overlap. If it angles off to one side, the seam will pull the tape out of shape when you fold it.

Sew the seam and trim the excess

Thread your machine with thread that matches the tape color. Set the stitch length to 1.5 to 2.0 mm—shorter than your normal seam, because a short stitch holds the bias grain more securely and prevents the seam from stretching later.

Sew along the diagonal line you marked, starting at one corner and ending at the other. Backstitch at both ends to lock the thread. Remove the tape from the machine and trim away the two triangular flaps of excess tape on either side of the seam, leaving a 1/4-inch seam allowance. Use small scissors or a rotary cutter and ruler to keep the trim line straight and parallel to your seam.

Press the seam open with a hot iron, laying the tape flat on the ironing board. The seam should lie flat and invisible when you look at the tape from above.

Test the join by folding the tape

Fold the joined tape in half lengthwise, the way you would fold it to wrap around fabric. Run your finger along the seam as you fold. The tape should fold smoothly and the seam should sit flat without puckering, bunching, or creating a ridge.

If the seam puckers or the tape twists as you fold it, the diagonal angle was not quite 45 degrees. Unpick the seam by snipping a few stitches and pulling the thread free, then lay the tape out again and re-mark the line. Make sure the tape ends are truly perpendicular (forming a perfect 90-degree angle) before you draw the new line.

Common mistakes and how to avoid them

The most frequent error is sewing a straight seam across the tape ends instead of a diagonal one. A straight seam works for woven fabric, but bias tape is cut on the grain bias, so a straight seam cuts across the grain and causes the tape to bunch when folded. Always mark and sew at 45 degrees.

Another common mistake is overlapping the tape ends too much or too little. An overlap of 1/2 inch gives you enough fabric to sew without the seam running off the edge, and enough excess to trim cleanly. If you overlap less than 1/4 inch, your seam may run off the tape entirely.

Pressing the seam open is also straightforward to skip, but it is essential. An unpressed seam will create a thick lump under your fabric when you fold the tape around it. Take 10 seconds to press it flat before you use the tape.

Joining pre-made bias tape versus homemade strips

Pre-made bias tape from a store is already folded and pressed, so after you sew the diagonal seam and trim it, you can fold the joined tape in half and press it again to re-fold the new seam. The process is the same as for homemade strips, but the tape is narrower and more delicate, so use a shorter stitch length (1.5 mm rather than 2.0 mm) to avoid stretching it.

If you are joining strips you cut and pressed yourself, the seam handling is identical. Just make sure both strips are pressed open flat before you lay them out in the L-shape, so you can see the grain and mark the diagonal accurately.

What to do if the seam still puckers after pressing

If you have pressed the seam open and it still puckers when you fold the tape, the angle of your seam was off. This usually happens when the tape ends were not quite perpendicular, or when you drew the diagonal line at a slight angle instead of true corner-to-corner.

Unpick the seam completely and lay the tape out again. Use a ruler to check that the corner angle is exactly 90 degrees before you mark the diagonal. If you are having trouble seeing the angle clearly, place the tape on a gridded cutting mat—the grid lines will help you align the ends perfectly. Mark the line again, sew it, and test the fold.

Frequently Asked Questions

Can I sew bias tape together on a serger instead of a regular machine?

Yes, a serger works well for joining bias tape and creates a very find seam. Use a 3-thread or 4-thread overlock stitch, set the stitch length to 2.0 to 2.5 mm, and sew along the same 45-degree diagonal line. Trim the excess tape after serging. The serger seam is slightly thicker than a regular seam, so test the fold to make sure the tape still lies flat.

What if I sew the seam and it is crooked or off-angle?

Unpick the seam by snipping a few stitches and pulling the thread free. Lay the tape out again, re-mark the diagonal line, and sew it again. There is no penalty for redoing it—the tape is inexpensive and the extra time is worth a seam that folds smoothly.

Do I have to press the seam open, or can I press it to one side?

Press it open. Pressing to one side creates a thick ridge that shows through when you fold the tape around fabric. Open seams lie flat and are nearly invisible in the finished project.

How much overlap do I need when I lay the tape ends together?

Overlap them about 1/2 inch. This gives you enough fabric to sew without the seam running off the edge, and enough excess to trim to a clean 1/4-inch seam allowance. Less than 1/4 inch of overlap is risky because your seam line may miss the tape entirely.

Can I join bias tape while it is still folded, or do I have to unfold it first?

Unfold it completely. You need to see both layers flat so you can mark the diagonal line accurately and sew through both layers evenly. If you sew folded tape, the seam will be uneven and the tape will twist when you fold it again.