What Is the Density of FRP in kg m3 and How Does It Compare with Steel?

What Is the Density of FRP in kg m3?

If you are checking the density of frp in kg m3 for a drawing, purchase order, or freight estimate, do not start with one fixed number. Most solid FRP laminates are around 1360 to 2002 kg/m3, based on ACMA’s 2016 guideline for architectural FRP products. For more values across related materials, you can also browse the Properties section.

A Practical Density Range

In normal buying work, glass fiber reinforced plastic, often called GFRP or fiberglass reinforced plastic, is usually handled as 1500 to 2000 kg/m3. This range works for many flat panels, molded covers, grating parts, and pultruded profiles.

ACI 440.1R-15 also reports FRP bar density at 1.25 to 2.1 g/cm3, equal to 1250 to 2100 kg/m3. That is a helpful check because FRP bars and FRP laminates are made in different ways, but their density ranges still overlap.

Why One Number Is Not Enough

FRP is a composite, not a pure metal, so the same name can cover products with different weights. A resin-rich hand lay-up panel may be lighter than a glass-rich pultruded beam.

A filled fire-retardant panel may weigh more than a clean structural laminate. A sandwich panel with a foam core can be much lighter per cubic meter when the full panel volume is counted, and that is the number that matters when the crate is lifted on site.

Quick Unit Conversion

The conversion is simple: 1 g/cm3 equals 1000 kg/m3. So a specific gravity of 1.70 is about 1700 kg/m3.

This small rule is useful when a supplier sends a datasheet in g/cm3 and your bill of materials uses kg/m3. It also prevents a common quoting mistake, where a buyer compares a solid laminate value with a core-filled panel value and then finds the shipment weight does not match.

Why Does FRP Density Change from Product to Product?

Density changes because FRP takes its final weight from every part of the laminate. Fiber, resin, filler, pigment, core, surface veil, and void level all affect the number. The datasheet value should match the exact grade, thickness, and process you plan to buy.

Fiber Type Sets the Starting Point

Glass fiber is heavier than many polymer resins. ACMA’s 2016 FRP guideline lists common glass fiber specific gravity at about 2.45 to 2.62.

Carbon fiber is often listed at about 1.70 to 2.2, while aramid fiber public references commonly place para-aramid near 1.44 to 1.45 g/cm3. This is one reason carbon or aramid composites can be lighter than a similar glass fiber laminate, although cost and mechanical behavior may be quite different.

Resin and Filler Add Real Weight

Thermoset resins in FRP often sit around 1.10 to 1.40 specific gravity, according to ACMA’s constituent table. Fillers can be much heavier, with typical mineral fillers near 2.58 to 2.71.

If a factory adds calcium carbonate, alumina trihydrate, or another filler for fire behavior, surface finish, or cost control, density can rise quickly. The product may still fit the job, but crate weight, bracket load, and freight cost will not be the same.

Voids and Cores Pull Density Down

Air voids lower measured density, but they are not a good way to reduce weight. Too many voids can reduce strength and water resistance.

Foam, honeycomb, balsa, or syntactic core is different because it is designed into the part. A planned core gives stiffness at lower weight, and ACMA lists structural and non-structural core materials at much lower specific gravity than solid laminate constituents. That is why a thick sandwich cover can feel oddly light the first time a metalworker picks it up.

How Can You Calculate FRP Weight Before Ordering?

You do not need a complex model for an early weight estimate. For purchasing, packing, and early design checks, volume times density is usually enough. Keep the full laminate schedule for final engineering review.

The Basic Mass Formula

Use this formula: mass in kg equals volume in m3 multiplied by density in kg/m3. For a flat sheet, volume equals length times width times thickness.

A 1 m3 block of FRP at 1700 kg/m3 weighs 1700 kg. This is the cleanest way to compare FRP, aluminum, and steel by equal volume. If the shape has ribs, cutouts, or molded lips, use CAD volume when you can.

A Panel Example for Buyers

Take a panel measuring 2.4 m by 1.2 m by 4 mm. The volume is 2.4 x 1.2 x 0.004, or 0.01152 m3.

At 1700 kg/m3, the panel weighs about 19.6 kg before packaging, fasteners, labels, and edge trimming. At 2000 kg/m3, the same panel weighs about 23.0 kg. That 3.4 kg gap per panel becomes important when a container carries hundreds of pieces.

A Pipe and Tank Note

For FRP pipe or tank shells, do not use outside diameter as if the full cylinder is solid. Calculate the shell volume, or ask for net laminate volume from the drawing.

A small wall thickness error can change the estimated weight by several percent. On export orders, this can affect pallet design, crane selection, and inland trucking. It is plain paperwork, but it avoids bad surprises at unloading.

How Does FRP Density Compare with Steel and Aluminum?

Buyers usually ask about FRP density because they want to reduce weight. A lighter part can cut installation labor, reduce support loads, and lower maintenance linked to corrosion. Equal-volume density is only the first check, though. Strength, stiffness, temperature, fire rules, and service fluid also need to be reviewed. See also: Application.

Equal Volume Weight Difference

NIST Technical Note 1771 lists steel density at 7856.2 kg/m3, while a common bulk aluminum reference from MIT and CRC data gives aluminum around 2700 kg/m3. If an FRP laminate is 1700 kg/m3, it is about 22 percent of steel weight and about 63 percent of aluminum weight for the same volume.

That weight gap is easy to see in handling. A steel panel that weighs 78.6 kg by volume could be near 17.0 kg if replaced by an FRP part of equal volume and 1700 kg/m3 density.

Specific Strength Matters More

Do not select FRP by density alone. FRP can carry load well in the fiber direction, but it behaves differently across laminate thickness and around drilled holes.

Steel is isotropic for many practical designs, while FRP is directional. If you need a long grating span, a ladder rail, or a tank wall under pressure, compare density together with tensile strength, flexural modulus, fiber direction, safety factor, and the relevant code.

Shipping and Handling Effects

Lower density helps on site and in the warehouse. Workers can move smaller FRP covers by hand where steel may need a hoist.

A lighter guard or enclosure may also need fewer heavy brackets. Freight bills can improve, but bulky shapes may reach volume weight limits before actual weight limits. For export buyers, ask the supplier for gross weight, net weight, and packing dimensions because density explains the part weight, while packaging decides the invoice weight.

How Should You Specify FRP Density on a Datasheet?

A useful FRP purchase specification should be clear enough that two suppliers cannot quote two different products under the same name. Density is one line item, but it works better when it is tied to process, fiber content, test method, and tolerance.

Ask for Test Method and Tolerance

ASTM D792-20 covers density and specific gravity of plastics by displacement. For composites, ASTM D3171-22 covers constituent content, and ASTM D2584-25 covers ignition loss of cured reinforced resins.

You do not need to print a long standard list on every order, but it is worth asking which method the supplier uses. A density value without a test method is only a rough claim, and it is hard to compare across factories.

Match Density with Fiber Content

When two FRP profiles have the same shape but different density, fiber content may explain the difference. For example, a simple rule-of-mixtures estimate using 40 percent E-glass fiber at 2580 kg/m3 and 60 percent resin at 1200 kg/m3 gives about 1752 kg/m3 before voids and fillers.

If the glass share goes up, the laminate gets heavier. If a low-density core is added, the overall panel gets lighter. This is why density should be checked together with the laminate structure, not only the outside size.

Check Accessories and Surface Layers

Gel coat, grit surface, metal inserts, rubber seals, hinges, and stainless hardware are easy to miss. They are not always included in the FRP laminate density.

A molded enclosure might have a laminate density near 1650 kg/m3, but the finished assembly can weigh more because of locks and mounting plates. For a clean commercial order, request both laminate density and finished part weight because these two numbers answer different questions.

FAQ

Q1: What Is the Most Common Density of FRP in kg m3? A: Many solid GFRP laminates fall around 1500 to 2000 kg/m3. ACMA’s public guideline gives a broader typical laminate range of 1360 to 2002 kg/m3.

Q2: Is FRP Lighter Than Steel? A: Yes. A typical FRP laminate near 1700 kg/m3 is about 22 percent of steel weight by equal volume, using NIST’s steel density value near 7856 kg/m3.

Q3: Is Carbon FRP Always Lighter Than Glass FRP? A: Often, but not always. Carbon fiber can be lighter than glass fiber, but resin content, filler, fabric style, voids, and core design can change the final composite density.

Q4: How Do You Convert Specific Gravity to kg/m3? A: Multiply specific gravity by 1000. A specific gravity of 1.75 is about 1750 kg/m3.

Q5: What Density Should Be Used for an FRP Weight Estimate? A: Use the supplier’s tested datasheet value when available. For early estimates, 1700 kg/m3 is a practical middle value for many GFRP laminates, but confirm it before final ordering.