Spool labels look simple until two brands describe the same cone in different units. One says 40 wt. Another says Tex 25. A third prints denier. They are often the same thread. The heavy sizes used for bags—Tex 45, Tex 70, Tex 80—make the mismatch more expensive, because the wrong one either shreds in the needle or fails the seam.
What the numbers mean
Thread size is mass per length, not a caliper reading. Fiber and construction still change how a given size feels in the hand. Cotton and bonded nylon of the same Tex are not interchangeable.
Weight (wt) is the U.S. home-sewing label. It is inverse. The number is roughly how many kilometers of thread weigh one kilogram. A 40 wt thread needs about 40 km to make 1 kg. A 12 wt thread needs only about 12 km for the same kilogram, so it is much thicker. Higher weight number, finer thread.
Tex is the system industrial and bagmaking suppliers use. Tex is the weight in grams of 1,000 meters. Tex 25 means 1,000 meters weighs 25 grams. Tex 70 means 1,000 meters weighs 70 grams. Higher Tex, thicker and stronger thread.
Denier is the same idea on a longer sample: grams per 9,000 meters. Denier is about Tex times 9.
The conversion
Tex ≈ 1000 / wt
wt ≈ 1000 / Tex
denier ≈ Tex × 9
A standard 40 wt all-purpose thread is about Tex 25 and 225 denier. That is garment and quilt-piecing size.
Tex 40 is not 40 wt. Tex 40 converts to about 25 wt. It is already heavier than everyday sewing thread.
Tex 45 is about 22 wt and 405 denier. Tex 70 is about 14 wt and 630 denier. Tex 80 is about 12.5 wt and 720 denier. Those three are the bagmaking range, not the garment range.
Why bagmakers step up
A bag seam is loaded differently from a shirt seam. Straps, handles, boxed corners, and base panels take point loads and abrasion. Canvas, waxed canvas, vinyl, faux leather, and webbing are also thicker than shirting. A Tex 25 seam can look fine on day one and creep or snap at a stress point later.
Bonded nylon or bonded polyester is the usual fiber here. The light bonding reduces fray as the thread punches through dense layers and keeps the twist from opening in a thick seam. Spun all-purpose polyester at 40 wt does not do that job.
Tex 45 is the medium bag size. It sews most cotton canvas, lighter vinyl, and structured totes, and many domestic or heavy-duty home machines can run it with a 100/16 or 110/18 needle. It is strong enough for construction seams on everyday bags without the bulk of a true heavy topstitch thread.
Tex 70 (often sold as size 69) is the heavy bag standard. Makers use it for strap attachment, stress seams, heavy canvas, and vinyl, and for topstitching that has to be visible and durable. Needle size is typically 110/18 to 125/20. Most basic domestic machines struggle with it; a compound-feed or industrial straight stitch handles it cleanly. Using Tex 70 on a light lining makes the seam gather and look stiff—match the thread to the layer.
Tex 80 sits just above that, in the same heavy-duty class as Tex 70–90. It is used when the fabric stack is thick (multiple layers of canvas plus webbing or interfacing) or when the stitch line is structural, as on duffels, tool rolls, and outdoor packs. Denier is about 720. Needle size starts around 110/18–120/19 and goes up with the stack. It is overkill for a lightweight fashion tote and underkill for marine covers, which often jump to Tex 90–135.
Common sizes
| Size | Approx. equivalent | Typical use | Needle starting point |
|---|---|---|---|
| 60–50 wt | Tex 16–20 | Bobbin, fine piecing, linings | 70/10–75/11 |
| 40 wt | Tex 25, ~225 denier | Garments, quilt piecing, light bag linings | 80/12–90/14 |
| Tex 40 | ~25 wt | Light-medium construction, light bags | 90/14–100/16 |
| Tex 45 | ~22 wt, ~405 denier | Medium bags, canvas, everyday totes | 100/16–110/18 |
| Tex 70 | ~14 wt, ~630 denier | Heavy bag seams, straps, vinyl, topstitching | 110/18–125/20 |
| Tex 80 | ~12.5 wt, ~720 denier | Thick stacks, structural bag seams, outdoor packs | 110/18–120/19 |
At the machine
Match the needle eye to the thread. A Tex 45 thread in a 80/12 needle shreds. A Tex 70 thread in a machine that cannot form the loop cleanly birdnests or skips. Recheck tension every time the Tex changes; the setting that balanced Tex 25 will be wrong for Tex 70.
Keep the labels readable in storage. Weight and Tex only help if you can still tell a Tex 45 cone from a Tex 70 cone six months later. Covered storage beats open racks for thread you will not sew this season.
Short version: 40 wt is Tex 25. Tex 45, 70, and 80 are the bag sizes—about 22 wt, 14 wt, and 12.5 wt. Higher Tex is thicker. Use the heavier ones where the seam carries load, not where you want the stitch to disappear.