Sewing Thread Types and Weights Explained

Why two spools marked 40 can be different sizes

A spool labelled 40 might be a medium thread or a fairly fine one, depending on the system behind the number. Coats, one of the largest thread makers, says plainly that a ticket number is a reference number and not a measurement, and that Ticket 40 cotton is not the same thickness as Ticket 40 corespun.² Until you know which system a number belongs to, it tells you very little.

Machine thread passes through the tension discs and the needle eye thousands of times, so it needs to be smooth as well as strong, and thin enough to sit in the groove on the front of the needle. Choose the fibre first and the thickness second; the needle size follows from the thickness.

The main thread types

All-purpose polyester is what most beginners should buy. It stretches a little and is strong for its size, and it will not rot or mildew, so it works on wovens and knits. South Dakota State's extension sheet lists spun polyester and long-staple polyester as durable choices with some give, with long-staple being smoother and stronger.⁵

Cotton thread is soft, with a matte finish that presses well, but it is weaker and less stretchy than synthetics. South Dakota State recommends it only for natural-fibre fabrics that do not stretch, with size 50 for general sewing and 60 for finer work.⁵ Quilters like it because it ages at the same rate as cotton fabric. YLI notes that longer cotton fibres (the staple) make stronger, less linty thread, and that mercerizing (a chemical treatment) adds strength and sheen.⁴

Cotton-wrapped polyester, sometimes called corespun or cotton-covered polyester, has a polyester filament core wrapped in cotton. The core gives strength and the cotton cover protects it from iron heat.⁵ It is a good all-rounder if you sew a lot of cotton and want extra strength.

• Topstitching or heavy-duty thread: a thicker thread meant to be seen, as on jeans. Use it with a larger needle so it does not shred.⁵
• Upholstery thread: very thick and strong, usually bonded nylon or polyester, for furniture, canvas and outdoor gear.
• Button and carpet thread: extra-strong thread for hand sewing buttons and heavy fabric.⁵
• Silk: fine, strong and elastic, mostly for silk fabric, tailoring and hand basting that leaves no marks.⁵
• Invisible (monofilament) thread: a single clear or smoke strand. YLI makes a nylon version at 0.004 in and a heavier polyester one at 0.005 in, and notes the polyester version takes heat better.⁴
• Embroidery thread: built for sheen rather than seam strength. YLI describes trilobal polyester, with a triangular cross-section that reflects light, as an embroidery and decorative thread.⁴

Do not use embroidery thread to sew a seam. It is designed to look good on the surface rather than carry load, and it is usually finer than all-purpose thread.

Weight, Tex and why the numbers conflict

Thread is sized by comparing length to mass, and different systems do it in opposite directions. YLI's explanation is a clear starting point.¹

Converting takes one division. Weight equals 1,000 divided by Tex, so Tex 25 is 40 wt and Tex 40 is 25 wt.¹ Coats defines Tex as grams per 1,000 metres and gives a rule for synthetic ticket numbers: divide 1,000 by the Tex, then multiply by 3. That turns Tex 40 into Ticket 75 and Tex 100 into Ticket 30.²

The numbers clash for three reasons. The systems run in opposite directions, so a higher weight number is a thinner thread while a higher Tex is a thicker one. Many weight labels also describe the single strand, and the number of strands twisted together (the ply) changes the final thickness: a thread labelled 50/3 is three strands of 50 and is about half as thick again as a 50/2.⁴ And ticket numbers vary by fibre, so the same ticket on cotton and on corespun means different sizes.² If the spool or the maker's website gives a Tex number, trust it over the others, because Tex is a direct physical measurement with no ply arithmetic hidden in it.

Matching thread to fabric and needle

The thread has to pass freely through the needle eye and lie in the long groove on the front of the needle. Too thick a thread for the needle shreds and breaks; too thin a thread for the fabric gives weak seams. American & Efird, a major industrial thread maker, publishes minimum needle sizes for each thread size.³ The rows below are drawn from its chart, with the weight number worked out from the Tex.

YLI gives similar pairings for home machines: a 70/10 needle for 60 wt and finer, 80/12 for 50 wt, 90/14 for 40 wt, and 100/16 for 30 wt or heavier.⁴ Our needle guide explains needle points and sizes in more detail, and the needle-changing lesson shows how to swap one.

For fabric, South Dakota State groups the choice by weight: extra-fine polyester or cotton-wrapped polyester for sheers and tricot, and long-staple or general-purpose polyester (or cotton-wrapped) for everything from broadcloth up to canvas.⁵ For hand sewing, the same matching applies; see matching thread for hand sewing.

Testing thread quality

Cheap thread causes more trouble than almost any other supply. It sheds lint into the bobbin area and snaps under tension, and because its thickness varies along the length, the stitch tension wanders. YLI links lint and breakage to short cotton fibres and low-grade spinning.⁴ You can check a spool in the shop or at home:

• Pull off 18 in (45 cm) and hold it up to a light. Good thread looks even; poor thread has fuzzy halos and thick and thin spots.
• Run it between finger and thumb. It should feel smooth, not hairy.
• Snap a short length between your hands. It should resist and break cleanly. Compare with a spool you trust.
• Untwist the end. Two or three even plies are a good sign; loose, uneven plies are not.
• After sewing a few seams, look at the bobbin case. A small heap of lint means the thread is shedding.

Old thread from a relative's sewing box is worth the same test. Cotton thread weakens with age and light; if it snaps with a gentle tug, use it for basting only.

Worked example: denim hem vs a cotton shirt

Jeans hem. The denim is about 12 oz per square yard (roughly 400 g per square metre), and at the side seams the folded hem can stack six layers or more. You want a visible gold topstitch like the original. A Tex 60 topstitching thread is a good match for the look. Converted, that is about 17 wt (1,000 divided by 60). A&E pairs Tex 60 with a 110/18 needle as a minimum.³ Many home machines struggle with Tex 60 in both top and bobbin, so put the heavy thread on top only and use all-purpose polyester in the bobbin, an arrangement South Dakota State mentions for heavy decorative thread.⁵ Lengthen the stitch to about 3.5 mm (about 7 stitches per inch) so the thick thread sits neatly. If your machine skips with Tex 60, step down to a 30 wt thread (about Tex 33) with a 100/16 needle, which YLI pairs together.⁴ The jeans hemming lesson and topstitching lesson cover the steps.

Cotton shirt. A poplin or broadcloth shirt is light and closely woven. Here the thread should disappear. Choose an all-purpose polyester or cotton-wrapped polyester around Tex 24 to 30 (roughly 42 to 33 wt) in a colour one shade darker than the fabric, since thread looks lighter once sewn. A&E pairs Tex 24 shirting thread with a 70/10 or 75/11 needle and Tex 30 with an 80/12.³ A 2.5 mm stitch length suits the hem. Use the same thread top and bottom.

Compare the two: the jeans thread carries two to two and a half times the Tex of the shirt thread, and the needle jumps three or four sizes. Swap them and you get shredded thread on the jeans and heavy, ropy stitching on the shirt.

Common mistakes

• Comparing weight numbers across brands without checking the ply. A 50/3 and a 50/2 are not the same thickness.⁴
• Using a needle too small for the thread, which shreds the thread near the eye.
• Sewing seams with embroidery thread because the colour is right.
• Using 100% cotton thread on stretchy knit fabric; seams pop when the fabric stretches.⁵
• Putting heavy topstitching thread in the bobbin of a machine that cannot handle it, instead of only on top.
• Buying the cheapest multipack and spending the saved money on rethreading time.

When you are unsure, sew a 6 in (15 cm) test seam on a scrap folded to the same thickness as your project, and pull it gently crosswise. If the thread breaks before the fabric strains, go up one thread size and one needle size.