Thread Yardage Estimator

Pick the project, size, fabric, layers, stitch, thread weight, seams, allowance and stitches per inch, and the tool estimates top (needle) thread in yards, bobbin or looper thread, the number of 500-yard spools and a cost at $6.50 a spool. The defaults (size M garment, medium fabric, two layers, 8 seams, 1.5-inch allowance, straight stitch at 14 spi, 40wt) come to 44.2 yd of top thread plus 44.2 yd of bobbin thread, 88.3 yd in all: one spool. The big number is the top thread only, so buy for the total with bobbin.

A size L jacket in heavy fabric with a serged stitch needs 322.2 yd of needle thread and 1288.9 yd of looper thread. A serger cannot pool thread across cones, so the tool counts one needle cone and two looper cones of 644.4 yd each (2 spools apiece): 5 spools, $32.50.

seam length (cm) = project base × size factor × layers + allowance × 2.54

all thread (yd) = seam length × seams × ratio × density × fabric × weight ÷ 91.44

Project bases run from 80 cm (hemming) to 500 cm (quilt); the size factor runs from 0.76 (XS) to 1.48 (3XL). The ratio is Coats' thread consumption ratio, in cm of thread per cm of seam at 7 stitches per cm (18 spi): 2.5 for straight stitch (301 lockstitch), 7.0 for zigzag (304) and 14.0 for serge (504 overlock). Coats splits that thread 50/50 between needle and bobbin for 301 and 304, and 20% needle to 80% loopers for 504; the tool shows the needle share as top thread and the rest as bobbin or looper thread.

Coats advises 10% to 15% extra for wastage, not added here.

Everything else is this tool's own rough allowance with no published basis. The project bases are fixed guesses at total seam length. Layers multiply the whole seam length, and the seam allowance (a width, how far the stitching sits from the edge) is added as extra length, so treat both as scaling knobs rather than measurements. Stitching denser than 18 spi multiplies the thread by spi ÷ 18, while 8 to 18 spi, including the default 14, changes nothing. The fabric factor runs from 0.8 light to 1.6 very heavy, and thread weight scales the total from ×1.5 for 30wt to ×0.4 for 80wt, although a finer thread does not really use less length for the same seam. Blind hem and decorative stitches have no Coats ratio, so they use a per-stitch estimate with bobbin thread at 12% of top thread.