The Continuous Bias Tape Method, Step by Step

Bias tape is cut at 45 degrees to the fabric's grain, which is what gives it the stretch and drape to wrap smoothly around a curved neckline, armhole, or hem without puckering. Cutting it strip by strip and joining the short ends together works, but it's slow and wastes fabric at every join. The continuous-bias method solves both problems by turning one square of fabric into a single long spiral of tape in just a few cuts and one seam.

Why bias, and why continuous

Fabric cut on the straight grain resists curving smoothly — it wants to pucker or fight the fabric it's being sewn to. Cut at a true 45-degree angle to the grain, the same fabric gains just enough give to ease around a curve and lie flat. The continuous method exploits a neat geometric trick: cut a square of fabric on the diagonal, rejoin the two triangles offset by one strip-width to make a parallelogram, sew the long edges into a tube, and slice around that tube in one uninterrupted spiral. What comes off is one long bias strip, with only a single seam in the whole length instead of one seam per short strip.

Step by step

1. Cut a true square. Square up your fabric so all four sides are equal and the corners are true right angles — any distortion here throws off every following cut.
2. Cut the square in half diagonally. One straight cut corner to corner gives you two identical triangles.
3. Rejoin the triangles, offset. Flip one triangle and align its diagonal (bias) edge against the other's straight edge, offset by one strip-width, right sides together, and stitch a standard seam. Press the seam open. The result is a parallelogram with two bias edges running along its long sides.
4. Mark the strip lines. On the wrong side, draw parallel lines at your chosen strip width, running parallel to the bias edges, from one edge of the parallelogram to the other.
5. Form the tube. Fold the parallelogram right sides together so the marked lines align, but offset by exactly one strip-width — this offset is what turns straight lines into a continuous spiral once cut. Pin carefully at each line intersection, matching the marks precisely, and stitch a seam along that edge. Press the seam open.
6. Cut the spiral. Starting at the loose end, cut along the marked line continuously. Because of the offset seam, the cutting line spirals around the tube rather than closing into a set of separate rings, producing one long continuous strip.
7. Press. Run the finished strip through a bias tape maker, or fold and press it by hand into single- or double-fold tape, ready to apply.

Sizing the square you'll need

The length of tape a square can produce is approximately its area divided by the strip width, minus a bit for the seam and the trimmed ends. A 36in square cut into 2in-wide strips can, in principle, yield around 583in of continuous tape once a reasonable waste allowance is subtracted — and working backward, the side of the square needed for a target length is the square root of (length × strip width), again with a waste allowance built in. The bias tape length calculator runs both directions of that math, so you can size the fabric square you need for a project, or check how much tape a square you already own will realistically yield.

Choosing your strip width

For double-fold bias tape — the kind that's pressed with both raw edges folded to the center, then folded again — cut roughly four times the finished width you want, since two folds and a bit of take-up consume most of that extra fabric. Single-fold tape needs roughly twice the finished width plus a small seam allowance. If you're using a bias tape maker, check its packaging; they're sold and labeled by the finished output width.

A few practical notes

Squaring up the starting fabric carefully matters more than almost any other step — a slightly off-square start compounds into wavy, inconsistent strips by the time you reach the far end of the spiral. Press as you go rather than all at the end; bias-cut fabric holds a crease better warm, and it's much easier to correct a stretched section early than after the whole length is cut. Finally, budget a real waste allowance rather than cutting the square to the bare minimum — the diagonal seam and the trimmed ends always take a little more than the raw area math suggests.