Thread weight is a number for thickness, and on the common scale a higher number means a finer thread. 50 wt is thinner than 30 wt, which surprises everyone at first.
Where the number comes from. The wt scale is based on how many kilometres of thread make up a kilogram. More length per kilogram means a finer thread, so the number rises as the thread gets thinner.
The weights you will actually meet.
50 wt — the general purpose weight for garment construction and piecing. If a thread carries no number, it is probably around here.
40 wt — machine embroidery, and slightly more visible topstitching.
30 wt — decorative stitching and quilting where you want the thread to show.
12 to 20 wt — heavy topstitch and jeans thread. These need a topstitch needle and usually a looser tension.
60 wt and finer — bobbin thread for embroidery, appliqué and fine work, where bulk is the enemy.
Tex and denier are other systems you will see on cones and industrial thread, and both work the other way round: a higher number means a thicker thread. Tex 30 is heavier than Tex 18.
What to do with the information. Heavier thread needs a larger needle eye, a longer stitch and usually less top tension. Finer thread needs a finer needle, or it will not be gripped properly and will loop.
And weights are not standardised across manufacturers, so treat them as a guide rather than a specification.
Questions people actually ask
What weight is ordinary sewing thread?
Around 50 wt for most general purpose polyester, though few domestic spools state it. It is the weight everything else is compared against.
Can I use 40 wt for construction?
Yes. It is slightly heavier and slightly more visible, and it is perfectly strong for seams.
Last reviewed 2026-08-11