Composite Density Calculator

A composite does not have one fixed density like a metal, because it is two materials, a fiber and a resin, and its weight depends on how much of each is in the mix. This works out the density from the fiber, the resin and the ratio, by the rule of mixtures, so you can weigh a composite part or compare a layup before you make it.

A composite is a blend, so its density is too

Carbon fiber, fiberglass and the like are a stiff fiber held in a resin, and each has its own density: carbon fiber around 1.8, glass near 2.6, epoxy about 1.15. The finished composite lands somewhere between them, pulled toward whichever there is more of. That is the rule of mixtures: the density is the fiber's share times its density plus the resin's share times its density. Pack in more fiber and the part gets denser and stiffer; more resin and it gets lighter and weaker.

By volume or by weight

There are two ways to say how much fiber is in a composite, and they give different numbers. By volume is the share of the space the fiber fills, which is what strength and stiffness figures are based on. By weight is the share of the total mass, which is how you often measure out cloth and resin on the bench. Because fiber is usually denser than resin, a given weight fraction is a smaller volume fraction. The calculator takes either and converts, so you are never comparing the wrong two.

What a real layup achieves

How much fiber you can pack in depends on the method. A plain hand layup, brushing resin into cloth, lands around 40 to 50% fiber by volume. Vacuum bagging squeezes out excess resin and pushes it higher; resin infusion and pre-preg in a press higher still, toward 60% and beyond, which is where the best strength-to-weight lives. More fiber is denser but also stiffer and stronger, so the ratio is a real design choice, not just a weight number.

Then weigh the part

Once you have the density, the weight of a composite part is that density times its volume, the same as any material. Work the density out here, then use the Metal Weight tool, pick Custom and enter this number, and give it the shape and size to get the weight. Bear in mind real parts carry some voids and resin-rich spots, so the true density runs a little under the ideal, and a made part weighs a touch more than the fiber count alone suggests.

More MakerKiln tools

• metal weightMetal Weight CalculatorWeight of a bar, tube, sheet or plate; use the Custom density for a composite.→
• mix ratioEpoxy Mix Ratio CalculatorResin and hardener amounts for a batch, by volume or weight.→
• epoxy coverageEpoxy Coverage CalculatorResin for a flood coat or deep pour, from the area and depth.→

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