Carbon Fiber Grades: T300, T700, T800 and M40J
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Carbon fiber is sold by grade, and the grade names most spec sheets use come from Toray, the biggest maker. Other makers have equivalents. The grade tells you two things about the fiber: how strong it is and how stiff it is.
The common grades
| Grade | Tensile strength (MPa) | Modulus (GPa) | Elongation (%) |
|---|---|---|---|
| T300 | 3,530 | 230 | 1.5 |
| T700S | 4,900 | 230 | 2.1 |
| T800S | 5,880 | 294 | 2.0 |
| T1100G | 7,000 | 324 | 2.2 |
| M40J | 4,400 | 377 | 1.2 |
| M55J | 4,020 | 540 | 0.8 |
These are values for the bare fiber from Toray's data sheets. A laminate is roughly half fiber and half resin, and the plies run in different directions, so a finished part is far less strong and stiff than these numbers. They're still the right way to compare grades.
Standard modulus: T300 and T700
T300 has been the reference fiber since the 1970s and is still common in 1K and 3K woven fabrics. T700 has the same stiffness but about 40 percent more strength and more stretch before it breaks. It's usually sold as 12K or larger tow and is a very common choice for unidirectional prepreg and filament winding in industrial and sporting parts. When a spec just says "carbon fiber", it usually means one of these two.
Intermediate modulus: T800 and up
T800 is about 28 percent stiffer than T700 and stronger again. It's widely used in aircraft structure and in high-end bike frames and rackets, where the extra stiffness means fewer plies for the same stiffness. It costs noticeably more, so it earns its place in parts designed around stiffness, not in a cosmetic cover.
High modulus: M40J and M55J
The M grades trade strength and stretch for stiffness. M40J is over 60 percent stiffer than T700, and M55J more than twice as stiff, but they break at around 1 percent strain or less and are more brittle under impact. They're used where stiffness or thermal stability rules: satellite structures, precision fixtures, long thin tubes and some high-end sporting goods.
Which one does your part need?
- Cosmetic parts and covers: whichever weave looks right, usually a standard modulus fiber.
- Most structural parts: T700 class. Good strength, good toughness, sensible cost.
- Stiffness-driven parts where every gram counts: T800 class.
- Parts that must not bend or grow with temperature: high modulus, designed carefully around its brittleness.
Grade is only one choice. The resin, the weave and the layup usually matter more to the finished part. See what tow size means, and our carbon fiber page if you want a second opinion on a spec.