To cut fabric on the bias, place the grainline of each pattern piece at 45° to the selvage, on a single layer of fabric lying flat on a table, and cut with a rotary cutter around pattern weights rather than pins. Find the true bias by folding the straight crosswise edge up to meet the selvage: the fold is the bias. Allow about 50 percent more fabric than for the same pieces on the straight grain, cut mirror-image pairs by flipping the pattern piece, and let the cut pieces hang for a day before you sew, because bias stretches under its own weight.

What does "on the bias" mean?

A woven fabric has two grains: the lengthwise grainline, parallel to the selvage, and the crosswise grain, from selvage to selvage. Neither stretches much. The bias is any diagonal between them, and the true bias is the 45° diagonal, where the threads are free to shift against each other and the fabric stretches the most, drapes the most and clings to curves.

Cut on the bias, a plain woven becomes something else. A slip dress skims the body and swings; a cowl neckline falls into soft folds; a binding strip bends smoothly around a neckline. The same fabric cut on the straight grain would hang stiff and flat.

Why cut fabric on the bias?

  • Drape and cling for slip dresses, cowl necklines, 1930s-style skirts and fluid trousers.
  • Stretch without elastic: a bias-cut dress pulls on over the head with no zip.
  • Curved edges: bias binding and piping must be bias cut to follow necklines and armholes.
  • Design: stripes and plaids turn into chevrons and diamonds when cut on the bias, on a yoke, a pocket or a whole skirt.
  • Softer hems: a bias-cut hem falls in ripples rather than a straight line.

The costs are more fabric, more care in cutting and sewing, and pieces that behave differently from one another if the grain is not exact.

What do you need?

  • A table larger than the pattern piece, or a clean floor. The fabric must lie flat with nothing hanging over an edge.
  • A 45° guide: a quilting ruler with a 45° line, a set square, or the folded-corner method below.
  • Pattern weights and a rotary cutter with a cutting mat. Pins and shears lift and shift bias fabric.
  • Chalk or a water-soluble pen, and a long ruler for drawing lines.
  • Lightweight fusible stay tape or a strip of selvage for necklines.

How do you cut fabric on the bias, step by step?

  1. Straighten the crosswise edge. Pull a crosswise thread and cut along the gap, or tear a cotton, so the cut edge is exactly on the crossgrain. On a slippery fabric, press it first and use spray starch.
  2. Find the true bias. Fold the straightened edge up until it lies along the selvage. The diagonal fold is the true bias. Press it lightly or mark it with chalk every 20 cm (8 in), then open the fabric out again. A 45° line on a ruler gives the same result on a smaller piece.
  3. Lay out a single layer, right side up, flat, with the whole area you will cut on the table. Smooth it without pulling; do not let any part hang off the edge, or it stretches before you have cut anything.
  4. Draw the bias grainline on the pattern if the piece was drafted for the straight grain: a line at 45° to the original grainline, using the ruler's 45° mark. Pieces designed for the bias already carry it.
  5. Place the pattern piece with its grainline exactly on a marked bias line. Measure from the grainline to the selvage at both ends; the two numbers should differ by the same amount as the length between them (a 45° line moves 10 cm across for every 10 cm it travels). A degree or two off is enough to make one side of a dress hang differently from the other.
  6. Weight and cut with long, smooth rotary strokes, moving around the piece rather than turning the fabric. Cut every notch as a small snip, not a triangle.
  7. Cut the pair. Flip the pattern piece over to cut the mirror image, so the left and right pieces have the bias running in opposite directions and the garment hangs evenly. Mark the wrong side of each piece with a chalk cross straight away; bias pieces are easy to confuse.
  8. Transfer markings and stabilize at once: thread-trace the center front and center back, and press fusible stay tape, cut on the straight grain, along necklines and armholes before they can stretch. Staystitching alone is less reliable on bias.
  9. Hang the pieces on a hanger or a line, clipped at the top, for 24 hours before sewing. They lengthen and narrow slightly; the hem can drop 2 to 5 cm (3/4 to 2 in) on a dress length.
Tip: Widen the seam allowances to 2 cm (3/4 in) on bias-cut garment seams. Bias pieces change shape as they hang, and the extra width lets you fit the dress on the body and sew the seam where it should be.

How much extra fabric does a bias layout take?

Pattern pieces set diagonally waste the triangles at the top and bottom of the layout, and a single layer means every piece is placed individually. Count on 50 percent more than the straight-grain amount, and up to double for a long dress or a narrow 110 cm (45 in) fabric. Lay out the real pieces on paper the width of your fabric before buying, or make a scaled sketch. The Sewly slip dress pattern carries a 45° grainline on the front and back and gives the fabric quantity for 110, 140 and 150 cm (45, 55 and 60 in) wide fabric with a layout drawn for the bias, which is a good reference for a dress of that length.

Bias strips for binding are more economical: a 50 cm (20 in) square of fabric gives about 9 m (10 yd) of 2.5 cm (1 in) strips cut corner to corner. The bias binding calculator converts the length you need into fabric.

How do you sew bias-cut pieces without stretching them?

Handle them as little as possible and do not pull them under the foot. Sew with a 2 to 2.5 mm stitch, holding the fabric gently taut in front of and behind the needle so the seam has a little stretch sewn into it; a seam sewn slack on bias puckers when the fabric relaxes. Use a fine needle and polyester thread. French seams suit light bias fabrics. For hems, hang the garment, level it from the floor, and sew a narrow hem; a deep hem on bias ripples. The bias-cut slip dress project walks through a whole garment, and sewing slippery fabric covers the satin and charmeuse that bias dresses are so often made of.

Common mistakes when cutting on the bias

  • Folding the fabric and cutting two layers. The layers slide; one piece comes out longer than the other.
  • Pinning. Each pin lifts and distorts bias fabric. Use weights.
  • A grainline a few degrees off. Check it with measurements at both ends, not by eye.
  • Letting fabric hang off the table while cutting, which stretches that section before the piece exists.
  • Cutting both of a pair the same way up instead of flipping, so the two sides hang differently.
  • Sewing immediately. Hang first, or the hem drops after the first wear.
  • Deep hems, wide facings and heavy interfacing, which fight the bias instead of moving with it.

When should you cut on the bias instead of on grain?

Cut on the bias when drape is the point: a slip dress, a cowl, a flared skirt in a light fabric, a scarf, a binding, a piping strip, or a yoke in stripes for a chevron. Stay on the straight grain for anything structured or close-fitting through the waist, such as a pencil skirt or trousers; for stiff fabrics, which gain nothing from the bias; and for a first project, because the handling takes patience. The cutting layout and grainline guide covers the straight-grain layout that most patterns assume, and a circle skirt gives you bias behavior on part of the hem without a full bias layout.