Sewing thread is usually under 2% of a garment's cost, yet it decides seam strength, seam extension and how the piece looks after washing. Broken stitches, skipped stitches, puckering, needle-hole leakage and stitching that turns pale in the wash almost always trace back to the thread. Colour and unit price are not enough — work backwards from fabric, seam type, care level and end market.

This guide runs through six parts: what thread actually does in a finished garment, five fibre types compared, how to read thread size and twist, choosing by garment category, colour, fastness and shrinkage, and acceptance criteria — ending with the information to send your supplier before sampling.

1. What Sewing Thread Actually Does

Thread does three jobs: it makes the seam strong — tear and burst resistance often depends on the thread rather than the fabric; it follows seam extension — knits and stretch fabrics need a thread that stretches, or the first movement snaps it; and it protects the after-wash look — shrinkage, colour fastness and whiteness stability decide whether stitching puckers, pales or bleeds after a few washes.

💡 Tip: fix the fabric and care level first, then the thread size, and only then the colour. Invert that order and sampling loops — especially on knits and stretch fabrics, where one size up can produce a wavy seam.

2. Five Fibre Types Compared

  • Core-spun polyester: a continuous filament core wrapped in staple fibre — high strength, elastic, resistant to washing and abrasion, the workhorse for knits and stretch fabrics; heat resistance is moderate, so control the pressing temperature.
  • Spun polyester: general purpose, low cost, good colour fastness, suited to woven basics and overlocking; low elasticity, so it tends to snap on stretch seams.
  • Nylon: the best strength and elasticity, resistant to abrasion and fatigue — denim, leather, outdoor gear; light and bleach resistance are weaker, and prolonged sun exposure makes it brittle.
  • Cotton: natural handle, high temperature resistance, good for natural fibres and pressing operations; lower strength, prone to shrinkage and mildew, unsuitable for damp storage or high-load seams.
  • Silk and viscose embroidery thread: high lustre for decorative topstitching and couture; moderate strength and wash resistance, so keep it away from load-bearing and high-abrasion areas.

3. Reading Thread Size and Twist

Numbering such as 40/2, 20/2 or 60/3 reads as count / number of plies: the lower the number, the coarser and stronger the thread, and the denominator is how many plies are twisted together. Coarse thread needs a larger needle, fine thread a smaller one — and the size must be fixed together with fabric weight, seam type and stitch length, never on its own.

  • Needle matching: size 9–11 for fine thread, 14 for general garment work, 16–18 for denim and heavy fabric. Coarse thread in a fine needle guarantees skipped stitches and breakage.
  • Twist direction and level: Z-twist and S-twist run opposite ways, and multi-thread chain and overlock machines must run a matching twist or you get loops and kinking.
  • Thickness and handle: coarser thread gives firmer, stronger stitches but puckers light fabric; finer thread feels softer and suits next-to-skin categories and fine decorative stitching.
  • Measuring beats reading labels: compare actual diameter, twist and tensile strength against the machine settings, and ask about diameter variation within one batch.

4. Choosing by Garment Category

  • Denim and washed garments: nylon or high-tenacity polyester, with reinforced topstitching able to survive stone and enzyme washing — and shrinkage and fading that track the fabric.
  • Knits and stretch: core-spun polyester, matched across overlock and coverstitch; run an extension-and-recovery test and accept it only if the seam stretches without snapping.
  • Tailoring and coats: spun polyester as the base, with mercerised high-tenacity thread for topstitching; pressing temperatures are high, so test heat resistance and colour change first.
  • Down and waterproof garments: low-absorption polyester or nylon, never cotton, which wicks water along the fibre; use anti-down needles and finer stitch density to limit needle holes.
  • Next-to-skin and childrenswear: low-twist soft or core-spun thread to cut friction and prickle; for children's categories, confirm chemical safety reports for both thread and dye.
  • Home textiles, bags and leather goods: coarse nylon or polyester with a larger needle and longer stitch; leather and coated materials need abrasion- and fatigue-resistant thread.

5. Colour, Fastness and Shrinkage

  • Judge colour in a standard light box and grade it with a grey scale; approving colour from a phone photo is a leading source of later complaints.
  • Set fastness by use: washing, water, rubbing, light, chlorine and perspiration — export orders commonly require grade 4 or better.
  • Pale and white shades test thread stability hardest: an optical brightener applied wrongly turns yellow, and shade variation shows up far more than on dark colours.
  • Shrinkage must track the fabric: if the thread shrinks more than the fabric the seam puckers; if less, the seam pulls and waves. Run a wash test together before deciding.

6. Acceptance Criteria for Ordering

  • Accept by net weight per cone rather than metres alone: diameter and tension vary, so the same metreage covers very different numbers of garments.
  • Keep a reference cone: hold back one unopened cone per batch so a breakage dispute can be re-tested against the same lot.
  • Lubrication and smoothness: too much finish makes the thread slip at the needle and gives loose stitching; too little causes frequent breakage. State the finish requirement in writing.
  • Chemistry: export children's categories need reports for free formaldehyde, azo dyes and extractable heavy metals; OEKO-TEX Standard 100 thread saves a lot of repeat testing.
  • Packing and storage: thread hates damp and sunlight — protect cones from compression and humidity, keep them away from windows; damp thread loses tensile strength and can mildew.
💡 Tip: send these seven items before asking for a price, and the quotes become comparable — garment category and fibre content, fabric weight, seam type, care level, needle size, colour reference and fastness grade, and lead time. Leave one out and every supplier quotes its default, so changing thread later means re-sampling.

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Setting thread specifications for a new line? Send us the garment category, fibre content and weight, seam type, care level and the colour reference you need — we match thread with neck labels, care labels and hang tags as one set, samples in 3–7 days.

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