Thread is sold with numbers on the spool that look like they should mean something obvious and do not. The most confusing part is that the system runs backwards: a higher number means a finer thread.
The Weight System
The common domestic marking is wt — 50wt, 40wt, 30wt. It derives from how many kilometres of thread make up a kilogram, so more length per unit weight means a finer thread.
So:
- 100wt — extremely fine. Used for invisible appliqué and fine machine work.
- 60wt — fine. Bobbin thread for embroidery, lightweight fabric, fine piecing.
- 50wt — the standard all-purpose weight. If a spool is unmarked, it is probably around here.
- 40wt — slightly heavier. The usual weight for machine embroidery, and for visible quilting.
- 30wt — noticeably heavier. Topstitching and decorative work where you want the stitch to show.
- 12wt — very heavy, almost like a fine perle cotton. Big visible stitching, often hand-look quilting.
You may also see Tex or denier on industrial cones. Both run the intuitive way — higher number, heavier thread — which is precisely why they are easier and less common.
Matching Thread to Needle
This is the part that actually affects your sewing. Thread and needle are a pair: the needle must make a hole large enough, and have an eye large enough, for the thread to pass without friction.
- 50wt all-purpose — 80/12 universal for most work.
- 40wt embroidery — 75/11 or 80/12 embroidery needle, which has a larger eye.
- 30wt topstitching — 90/14 or 100/16 topstitch needle. Do not try this through a standard 80/12; the thread will shred.
- 12wt — 100/16 topstitch, and expect to sew slowly.
Our guide to needle sizes and types covers the needle side of that pairing in more detail.
Fibre Matters Too
Polyester has slight stretch and good strength, which makes it the sensible default for garments and anything that will be worn, washed and moved in.
Cotton has almost no stretch and a matte finish. Traditional for quilting, where the lack of give is an advantage and the appearance is preferred. It is weaker than polyester and produces more lint.
Rayon is used for embroidery because it has a high sheen. It is not strong and is not for construction seams.
Nylon and specialist bonded threads are for upholstery, bags and heavy work.
A common piece of advice is to avoid sewing a strong thread on a weak fabric, so that a seam fails before the fabric tears. It is reasonable guidance for garments.
Top and Bobbin Do Not Have to Match
Using a heavy decorative thread on top and a fine 60wt in the bobbin is standard practice for topstitching and embroidery. It reduces bulk on the underside and uses less of the expensive thread.
Expect to adjust your upper tension when you do this — usually looser. Different weights top and bottom is one of the genuinely legitimate reasons to move the tension dial.
Old Thread
Thread degrades. Cotton in particular goes brittle with age and dry storage, and old thread snaps constantly, sheds lint into the machine and produces weak seams.
If a spool is inherited, dusty, or snaps when you pull a length firmly between your hands, do not use it for construction. Keep thread out of direct sunlight; a closed box or drawer is better than an open display rack, however nice those look.
Practical Advice
For most sewing, 50wt polyester all-purpose in a handful of neutrals covers nearly everything. Add 40wt if you embroider and 30wt if you want visible topstitching.
Cheap bargain-bin thread is a false economy — it is usually poorly spun, sheds heavily, and the lint ends up compacted under your bobbin case, which is its own set of problems. See cleaning a sewing machine.
