Piping & Bias Strip Calculator: Strip Width, Cutting Length and Fabric for Welting
Work out how wide to cut bias strips for piping (welting) from the cord diameter and seam allowance, the strip length with joins, the continuous-bias square, fabric needed for individual bias or straight-grain strips, cord to buy and how many cushions the piping covers. Metric and inch units.
How it works
Strip width = π × cord diameter + 2 × seam allowance (round up to 0.5 cm or 1/8 in) · Cutting length = piping length × (1 + extra %) · Continuous-bias square side = √(cutting length × strip width) + strip width · Individual bias strips: fabric length = cutting length × strip width ÷ (fabric width × 0.75)
Piping (welting) is a strip of fabric folded around a cord and caught in a seam. The strip has to go all the way round the cord, which is the cord circumference (π × diameter), and then leave a seam allowance on each side, so the cut width is π × d + 2 × seam allowance. For 5 mm cord and 1.5 cm allowances that is 4.57 cm, which the calculator rounds up to 5 cm (1 7/8 in) because a little extra width makes the piping easier to sew and trim; a strip that is too narrow will not reach the seam line. The strip is cut on the bias, 45° to the selvage, so it bends around corners. For long runs the continuous-bias method is the most economical: a square whose area equals the cutting length times the strip width (plus one strip width for the angled ends) is seamed into a tube and cut in a spiral. If you prefer individual strips, cut a full-width piece at 45° into parallel strips; the angled ends and the short corner pieces are lost, so only about 75% of the piece becomes usable strip (this matches the published Sailrite yields of 19, 25 and 28 yards of bias piping per yard of 36, 46 and 54 in fabric). The fabric length is therefore cutting length × strip width ÷ (fabric width × 0.75), and the longest strips are the shorter of the piece length and the fabric width times √2. Straight-grain strips across the width waste least fabric but should only be used on straight edges. The cushion check divides the finished length by 4 × side + 5 cm overlap per piped edge. Results are planning estimates: heavy or slippery fabric, matching stripes and very thick cord need more.
Worked example
6 m (600 cm) of piping over 5 mm cord with 1.5 cm seam allowances, on 140 cm fabric. Round the cord: π × 0.5 = 1.57 cm. Strip width: 1.57 + 3 = 4.57 cm, rounded up to 5 cm (1 7/8 in). With 10% for joins, cut 660 cm (6.60 m) of strip. Continuous bias: √(660 × 5) + 5 = 62.4, so a 63 × 63 cm square. Individual bias strips: 660 × 5 ÷ (140 × 0.75) = 31.4, so a 32 cm piece of full-width fabric, cut into strips up to 45 cm long: about 15 strips with 14 joins. Buy 6.30 m of cord (5% extra). One piped edge on a 45 cm cushion takes 4 × 45 + 5 = 185 cm, so the run covers 3 cushions.
Sources: University of Kentucky Extension: cording is cut to the cord circumference plus seam allowance · Sailrite: how much fabric to make piping (yield per yard of fabric) · Bsewinn: bias binding square size (√area + strip width)
Frequently asked questions
How wide should I cut bias strips for piping?
Measure round the cord (or use π × diameter) and add two seam allowances. With 1.5 cm (5/8 in) allowances: 3 mm cord needs about 4 cm, 5 mm cord 5 cm, 6 mm (1/4 in) cord 5 cm and 10 mm (3/8 in) cord 6.5 cm. Round up rather than down; you can trim the excess after sewing.
How much fabric do I need to make piping?
Multiply the cutting length by the strip width to get the area, then take the square root and add one strip width for a continuous-bias square. 6.6 m of 5 cm strip needs a 63 cm square. Individual strips cut across the width need more because of the corner triangles and joins.
Do I need to cut piping on the bias?
For curves, corners and round cushions, yes: bias-cut fabric stretches and lies flat around a bend. For perfectly straight edges such as the sides of a box cushion, straight-grain strips work and use less fabric.
How much piping does a cushion need?
The perimeter of each piped edge plus about 5 cm to overlap the ends. A 45 cm square cushion takes 185 cm per edge, or 3.7 m if both front and back are piped.
Can I use ready-made bias binding for piping?
Single-fold binding that opens to about 5 cm (2 in) covers cord up to about 4 to 5 mm with small allowances. For thicker cord or a fabric match, cut your own strips.
How much cord should I buy?
The finished piping length plus about 5%. Pre-wash cotton cord, or buy extra, because it can shrink when the finished item is washed.
Related calculators
Fabric estimate. Add extra for pattern matching, directional prints, shrinkage and mistakes, and check against the pattern or a layout before cutting.