Fiber strength is the force needed to break a bundle of cotton fibers, expressed in grams-force per tex (g/tex). The HVI measures it with a 1/8-inch (about 3.2 mm) gap between the clamp jaws. By USDA's descriptions, 25.5 to 28.4 g/tex is average, 28.5 to 30.4 is strong and 30.5 or more is very strong.
What g/tex means
Tex is a unit of linear density: the weight in grams of 1,000 meters of fiber. Strength in g/tex is the force, in grams, required to break a bundle of fibers one tex in size. USDA reports it to the nearest tenth, for example 30.2 g/tex.
How the HVI measures it
Before testing, each sample is conditioned to a standard humidity. The instrument clamps a bundle of fibers between a pair of jaws 1/8 inch apart and increases the force until the bundle breaks. In 1991 USDA began offering module and trailer averaging as a way to improve the accuracy of instrument strength readings. In 2011 to 2015 data, averaging raised strength reproducibility from 80.8% to 93.3%.
Strength categories
| Category | HVI 1/8-inch gauge strength (g/tex) |
|---|---|
| Weak | 23.4 and below |
| Intermediate | 23.5 to 25.4 |
| Average | 25.5 to 28.4 |
| Strong | 28.5 to 30.4 |
| Very strong | 30.5 and above |
Cotton Incorporated and Australian Classing Services publish the same categories rounded to whole numbers: 23 and below, 24 to 25, 26 to 28, 29 to 30, and 31 and above. In the 2024/25 U.S. crop, the average by USDA classing office ranged from 29.1 to 32.0 g/tex.
Why it matters in spinning
Fiber strength significantly affects spinning efficiency and yarn strength: strong fibers correlate with strong yarns. In the USDA loan schedule for Upland cotton, the base strength range is 26.0 to 28.9 g/tex.
What affects it and how it relates to other properties
- Variety: the main factor; the environment has a smaller effect. Most current Australian varieties fall in the strong category.
- Weathering: exposure to the weather in late summer and into the fall can cause significant declines in strength.
- Maturity: immature fibers, which read as low micronaire, are less desirable because they reduce strength, spinning efficiency and dyeing quality. More in cotton micronaire.
- Length: longer fibers also help spin stronger yarns, so strength and staple length are best read together.
Frequently asked questions
What fiber strength is considered good?
On USDA's scale, 28.5 to 30.4 g/tex is strong and 30.5 or more is very strong. What a mill needs depends on the yarn it will produce.
Are g/tex and cN/tex the same?
Not exactly. One gram-force equals 0.98 centinewtons, so 30 g/tex is about 29.4 cN/tex.
Is fiber strength the same as yarn strength?
No. The HVI tests a bundle of loose fibers; yarn strength is measured separately on the finished yarn. The two are related, but they are not the same number.
Sources
- USDA AMS: Cotton Classification, Understanding the Data
- Cotton Incorporated: U.S. Cotton Fiber Chart 2024/2025
- Alabama Cooperative Extension System: How to Think About Fiber Quality in Cotton
- Australian Classing Services: Fibre Strength
See how to read a full HVI report in this guide or browse Max and Safe's cotton qualities.