Carbon Fiber Yarn: Complete Guide to Tow Grades, Sizing and Applications

I’ve had customers ask me for “carbon fiber yarn” when they actually needed fabric, and others who ordered “carbon fiber tow” when what they really needed was prepreg. The terminology gets confusing fast. Here’s a clear breakdown of what carbon fiber yarn is, how the grades differ, and what to look for when buying.

Carbon fiber yarn — also called carbon fiber tow — is the raw material that everything else starts from. It’s a bundle of thousands of individual carbon filaments, each 5-7 micrometers in diameter, running parallel to each other. These tows get woven into fabric, wound into prepreg, or used directly in processes like filament winding and pultrusion.

I source carbon fiber yarn for customers every week. The two most common mistakes I see: buying the wrong tow size for the application, and ignoring the sizing chemistry. Let’s go through what matters.

Toy Sizes: From 1K to 50K and Beyond

A 1K carbon fiber yarn contains 1,000 filaments. A 50K contains 50,000. The “K” stands for thousand — it’s just filament count.

Tow Size	Filaments	Typical Tex (g/km)	Best For
1K	1,000	60-70	Aerospace interiors, medical devices, fine textiles
3K	3,000	200-210	General-purpose weaving, sporting goods, automotive trim
6K	6,000	400-420	Marine reinforcement, structural woven fabrics
12K	12,000	800-850	Industrial prepreg, pultrusion, wind energy, large parts
24K	24,000	1,600-1,650	Heavy-duty layups, infrastructure, cost-sensitive industrial
50K	50,000	3,200-3,500	Pultrusion profiles, construction, low-cost large structures

Tex values vary by manufacturer and precursor type (PAN vs pitch-based). These are typical for standard-modulus PAN-based fiber.

Standard Modulus vs Intermediate vs High Modulus

This is where a lot of buyers get tripped up. The tow size (3K, 12K, etc.) tells you the filament count, but it doesn’t tell you the mechanical grade. A 12K standard modulus yarn (230 GPa) is a completely different material from a 12K high modulus yarn (390 GPa+).

Grade	Tensile Modulus	Tensile Strength	Typical Applications
Standard Modulus (SM)	230 GPa	3,500 – 4,500 MPa	General-purpose: fabric weaving, sporting goods, automotive
Intermediate Modulus (IM)	290 – 310 GPa	5,000 – 6,000 MPa	Aerospace primary structure, high-performance parts
High Modulus (HM)	390 – 440 GPa	3,000 – 4,000 MPa	Stiffness-critical: satellite struts, precision instruments
Ultra-High Modulus (UHM)	600 – 800 GPa	2,500 – 3,500 MPa	Space applications, thermal management, specialty

Important trade-off: as modulus goes up, both elongation at break and tensile strength typically go down. High modulus fiber is stiffer but more brittle. For impact-critical applications, standard modulus is often the better choice despite being “lower grade” on paper.

Our Carbon Fiber Yarn

Carbon Fiber Yarn Available from 1K to 24K. Standard and intermediate modulus grades. Epoxy-compatible sizing. Used for weaving, braiding, filament winding, and pultrusion. View Details

Carbon Fiber Fabric Woven from our carbon fiber yarns. 3K, 6K, 12K. Plain, twill, satin weaves. 160-600 gsm. View Details

Sizing: The Invisible Variable That Makes or Breaks Your Part

Sizing is a thin chemical coating applied to the fiber surface during manufacturing. It protects the fiber during handling and — most importantly — determines how well the fiber bonds with your resin system.

The default is epoxy-compatible sizing. That’s what comes on most standard carbon fiber yarn. If you’re using epoxy resin, you’re fine. But if you’re using polyester, vinyl ester, or phenolic resin, the standard sizing won’t bond properly. The interface between fiber and resin will be weak, and your laminate will fail at lower stress than expected.

I’ve seen this happen twice with the same customer. They ordered standard carbon fiber yarn, used polyester resin (cheaper for their marine application), and the parts delaminated after six weeks. The fiber and resin were both good materials — they just weren’t compatible.

Common Applications by Tow Size

Weaving (3K, 6K, 12K). Most carbon fiber fabric is woven from 3K to 12K tows. 3K gives the finest surface finish — the fabric looks smooth and even. 12K gives a coarser texture but costs less per square meter. For cosmetic parts, 3K is standard. For structural parts where appearance doesn’t matter, 12K saves money.

Filament Winding (12K, 24K). Tanks, pressure vessels, drive shafts. Filament winding uses larger tows because the process lays them down in precise patterns at high speed. 12K and 24K are the standards.

Pultrusion (24K, 50K). Continuous profiles — beams, tubes, channels. Pultrusion uses the largest tows because the process pulls them through a resin bath and die at high speed. 50K industrial tow is the most cost-effective option here.

Prepreg Manufacturing (3K, 12K). Unidirectional prepreg tape is typically made from 12K or 24K tow. Woven prepreg fabric uses 3K or 6K. The tow size affects the prepreg’s drape and tack.

Frequently Asked Questions