Kevlar vs Carbon Fiber vs Fiberglass: Strength and Weight Compared
12 November 2026
Kevlar vs carbon fiber vs fiberglass is a stiffness, impact and cost decision, not a strength one, because at the fiber level all three have tensile strengths in the same 3 to 3.5 GPa range. What separates them is modulus and density: carbon at about 230 GPa and 1.76 g/cc, Kevlar(R) 49 aramid at about 112 GPa and 1.44 g/cc, E-glass at about 72 GPa and 2.55 g/cc. Carbon buys stiffness, aramid buys impact and abrasion, and E-glass buys everything you can repair with a grinder and a can of resin.
Kevlar vs carbon fiber vs fiberglass: the fiber numbers
| Property | E-glass | Carbon, standard modulus | Kevlar 49 aramid |
|---|---|---|---|
| Density | 2.55 g/cc (0.092 lb/in³) | 1.76 g/cc (0.064 lb/in³) | 1.44 g/cc (0.052 lb/in³) |
| Tensile strength, fiber | About 3.4 GPa (500 ksi) | About 3.5 GPa (510 ksi) | About 3.0 GPa (435 ksi) |
| Tensile modulus, fiber | About 72 GPa (10.5 Msi) | About 230 GPa (33 Msi) | About 112 GPa (16 Msi) |
| Elongation at break | About 4.8% | About 1.5% | About 2.4% |
| Compression in a laminate | Good | Good, well over half of tensile | Poor, roughly a fifth of tensile |
| Impact and abrasion | Fair | Poor, brittle failure | Best of the three |
| UV and moisture | Stable | Stable | Yellows in UV, absorbs moisture |
Those are fiber values. A laminate is fiber plus resin, and the resin is doing nothing structural, so laminate numbers scale with fiber volume fraction: about 30 to 40% on a hand layup, 50 to 55% under a vacuum bag, 60% in prepreg. A hand-laid carbon panel at 35% fiber is not three times stiffer than a bagged glass panel at 55%. Check the TDS for the fabric and process you are actually running before you put a number on a drawing.
Which is strongest?
In pure tension, all three are close enough that the question is the wrong one. The difference that matters on a boat is stiffness per weight, and there carbon is about three times E-glass and twice aramid. A stringer capped with carbon deflects a third as much as the same stringer in glass at the same laminate weight, or matches the glass deflection at a third of the weight.
In compression the picture flips. Aramid fiber buckles internally, and a Kevlar laminate in compression carries roughly 20% of what it does in tension. A mast, a spar, a deck beam or anything that sees bending has a compression face, and that face cannot be aramid. Carbon and glass both hold most of their tensile strength in compression.
In impact, aramid wins outright. It absorbs energy by stretching and fibrillating instead of cracking, which is why it goes in canoe hulls, kayak keel strips and the bottom of a race boat that hits things. Carbon is the worst of the three here: it is brittle, it fails without warning, and a carbon hull that takes a rock delaminates in a way you cannot see from outside.
Kevlar weight vs carbon fiber vs glass, same thickness
The fabrics people compare are all around 0.010 in dry: 6 oz E-glass plain weave, 5.7 oz 3K carbon plain weave, and 5 oz Kevlar 49 style 281. Same thickness per ply, so a four-ply skin is about 0.040 in cured in any of them. The weight difference is the density difference: the aramid ply is about 17% lighter than the glass ply, the carbon ply about 5% lighter, and the glass ply drinks the most resin because it is heaviest per yard. On a 30 ft hull that adds up. On a 10 ft skiff it does not, and the glass boat is the one you can fix with a grinder in a driveway.
Working with each
Cutting. Glass and carbon are abrasive and eat a smooth blade; aramid is worse, it grabs and slides. Serrated shears grip the fabric so it cuts instead of running ahead of the blade.
Sanding. Glass and carbon sand clean; aramid fuzzes into a beard that never goes away, so a Kevlar laminate gets a glass ply over the top as a sanding layer.
Wet-out. Carbon is black and hides a dry spot until it is cured; glass goes clear so you can see it. Roll carbon harder than you think you need to and check it under a raking light.
Galvanic and moisture. Carbon in contact with aluminum, a mast step, a chainplate, a rudder post, corrodes the aluminum. Put a ply of glass between them. Aramid absorbs moisture, so dry it and seal it under resin or paint. Use a fitted P100 respirator with organic vapor cartridges when cutting or sanding any of these, nitrile gloves, and eye protection.
Where each one wins in a boat shop
E-glass: hull skins, transoms, tabbing, anything that has to be repaired later, anything where the budget matters. Ninety percent of marine laminating is E-glass because it does the job at a third the material cost and every yard is a repair that can be ground back and relaminated.
Carbon: stiffeners, spars, stringer caps, rudder stocks, tiller extensions, anything long and thin that has to hold shape under load. The 11oz Unidirectional Carbon Fiber Tape (12K) is the part for that: 11 oz of 12K fiber all running one direction, in 1 in to 12 in widths, laid along the load path on top of a glass laminate. Two plies of 2 in UD carbon on a stringer cap stiffens it more than four plies of 1708 at a fraction of the weight, and it stays in the shear plane of the glass underneath.
Chopped carbon is the other carbon that belongs in a repair shop. The 1/4" Chopped Carbon Fiber stirs into epoxy, polyester or vinyl ester for castings, deep fills, and forged carbon parts where cloth will not fit; the 1/2" Chopped Carbon Fiber is the same strand at twice the length, which knits across a joint better and gives a stiffer casting at the cost of a lumpier mix.
Aramid and carbon-aramid hybrids: impact zones. Keel strips, canoe and kayak hulls, chine guards, race boat bottoms. Buy it for what hits the water, not for stiffness, and put glass over it so you can sand.
Before committing to carbon, get a Carbon Fiber Square 12 in x 12 in - 5.7oz 3K 2x2 Twill in your hands: weave scale, drape and how much a cloth distorts when it is cut decide whether a part comes out clean, and none of that shows on a screen. Then buy the Wiss Fiberglass & Carbon Cutting Scissors or the 12" Serrated Fiberglass & Carbon Scissors and stop using the kitchen shears; a dull blade pushes carbon weave out of shape instead of cutting it and you wet out a distorted ply.
Frequently asked questions
Is Kevlar stronger than carbon fiber?
In tension, no; carbon fiber is about 3.5 GPa and Kevlar 49 about 3.0 GPa, close enough not to matter. In stiffness carbon is twice as stiff at about 230 GPa versus 112. In impact Kevlar wins, and in compression Kevlar loses badly, carrying roughly a fifth of its tensile strength. Use carbon for stiffness and spars; use Kevlar for keel strips and hulls that hit things.
Is carbon fiber stronger than fiberglass?
Stiffer, by about three times: roughly 230 GPa fiber modulus against 72 GPa for E-glass. Tensile strength is nearly the same. For a hull skin that mostly sees impact and needs to be repaired someday, fiberglass is the better material. For a stringer cap, a spar or a tiller, carbon UD tape at the same weight deflects about a third as much. Put a glass ply between carbon and any aluminum.
How much lighter is Kevlar than carbon fiber?
At the fiber level Kevlar is 1.44 g/cc and carbon 1.76 g/cc, so a Kevlar ply is about 18% lighter than a carbon ply of the same thickness and 43% lighter than E-glass at 2.55 g/cc. In a real laminate the resin closes the gap, because you need the same resin per ply. Buy Kevlar for impact, not for the weight saving; carbon at the same weight is much stiffer.
Can you mix carbon fiber and fiberglass in the same laminate?
Yes, and most carbon on a boat should be laid that way. Lay the glass skin first, then run carbon UD tape along the load path, two plies of 2 in 11 oz tape for a stringer cap, and cap it with a ply of 6 oz glass for sanding and UV. Use the same resin throughout; do not put polyester over cured epoxy. The glass also isolates the carbon from aluminum hardware.
What do you cut carbon fiber and Kevlar with?
Serrated shears. Carbon and glass dull a plain blade in a few yards, and Kevlar slides ahead of it and frays. The teeth grip the fabric so the blade goes through instead of pushing the weave out of shape. A 10 in or 12 in drop-forged pair holds an edge through a full cut list and can be resharpened. Keep one pair for dry fabric only; resin on the blade ruins the cut.
What You Need For This Job
- 11oz Unidirectional Carbon Fiber Tape (12K): one-direction carbon for stiffeners, spars and stringer caps, 1 in to 12 in
- 1/4" Chopped Carbon Fiber: stiff reinforced resin for castings, fills and forged carbon
- 1/2" Chopped Carbon Fiber: longer strand for structural castings and joint repairs
- Carbon Fiber Square 12 in x 12 in - 5.7oz 3K 2x2 Twill: one square foot of twill to handle and cut before you buy a roll
- Wiss Fiberglass & Carbon Cutting Scissors: drop-forged shears that hold an edge in glass and carbon
- 12" Serrated Fiberglass & Carbon Scissors: serrated 5.5 in blade for fabric that slides ahead of a smooth edge
