Thread consumption formula
Thread needed (m) = total seam length (m) x thread ratio for that stitch type. The ratio is the metres of thread one metre of seam uses, needle and looper or bobbin threads together. The calculator uses these ratios: lockstitch 2.5, 3-thread overlock 4, 4-thread overlock 5 and 5-thread safety stitch 6. So 10 m of seam needs 25 m of thread in lockstitch, 40 m in 3-thread overlock, 50 m in 4-thread overlock and 60 m in safety stitch. These ratios are StitchMagic calculator defaults, not a thread maker’s chart. Stitch density, fabric thickness and thread size change the real figure, so if your thread supplier publishes a ratio for your stitch type and stitches per inch, use theirs: multiply your seam length by it the same way.
How to calculate sewing thread consumption for a garment
Measure every seam on one finished sample with a tape and group the lengths by stitch type, because each type has its own ratio. Multiply each group by its ratio, add the results, then add 5–10% for thread ends, rethreading and machine setup, as the calculator notes under its result. Example: a garment with 4 m of lockstitch seams and 2 m of 3-thread overlock needs 4 x 2.5 = 10 m plus 2 x 4 = 8 m, so 18 m of thread; with 10% added, 19.8 m. Run the calculator once per stitch type and add the answers. For a batch, multiply the per-garment figure by the number of pieces.
Sewing thread consumption calculator: 1 cone thread to meter
A cone’s length is printed on its label in metres (or yards; 1 yard = 0.9144 m), and it differs by brand, thread size and cone size, so read the label rather than converting from weight. Garments per cone = metres on the cone ÷ thread per garment. For example, an illustrative 5,000 m cone divided by the calculator’s default 27.5 m (25 m plus 10%) covers 181 garments. On an overlock machine each needle and looper draws from its own cone, so divide the overlock figure between the cones on the machine.
Thread count formula is a different thing
Thread count measures fabric, not sewing thread: it is the number of warp and weft threads in one square inch of woven cloth, used mostly for bed linen. It does not tell you how much sewing thread a garment needs. For sewing thread, use the consumption formula above: seam length x the stitch type’s ratio.