---
title: "Fibreglass (GRP) — density, melting point and properties"
description: "Fibreglass (GRP) has a density of 1,900 kg/m³ (1,500–2,100). Melting point, strength, conductivity, what it is used for, and whether it floats, rusts or is magnetic."
url: https://atoneplace.net/material/fiberglass/
site: "At One Place"
---

# Fibreglass (GRP)

**Density of fibreglass (GRP): 1,900 kg/m³ (1,500–2,100)**

1.90 g/cm³ · 118.6 lb/ft³

Glass fibre is about a third the stiffness of carbon at a fifth of the price, which is why boats are fibreglass and only racing yachts are carbon. The strength range here is mostly lay-up, not chemistry: chopped strand mat lands near 100 MPa and woven roving near 300. Its long-term enemy is water — polyester laminates absorb it slowly and blister, which is what osmosis in old boat hulls is, and epoxy laminates resist it far better.

## Properties

- **Density:** 1,900 kg/m³ (1,500–2,100) — 1.90 g/cm³
- **Tensile strength:** 100–300 MPa
- **Young's modulus:** 20 GPa
- **Thermal conductivity:** 0.35 W/(m·K)
- **Floats in water:** No
- **Magnetic:** No

## What it is used for

- Boat hulls and decks
- Corrugated roof sheeting and tanks
- Vehicle body panels and truck bodies
- Printed circuit board substrate (FR-4)

## How it compares

*Water is 1,000 kg/m³. Anything with a shorter bar than water floats in it.*

- air: 1.2 kg/m³
- expanded polystyrene foam: 20 kg/m³
- rigid polyurethane foam: 40 kg/m³
- Water — the float line: 1,000 kg/m³
- fibreglass (GRP): 1,900 kg/m³
- tungsten: 19,250 kg/m³
- gold: 19,300 kg/m³
- platinum: 21,450 kg/m³

*Water is 1,000 kg/m³. Anything below that floats.*

| Material | Density | Relative to water |
| --- | --- | --- |
| rigid polyurethane foam | 40 kg/m³ | 0.04× |
| expanded polystyrene foam | 20 kg/m³ | 0.02× |
| air | 1.2 kg/m³ | 0.00× |
| fibreglass (GRP) ← | 1,900 kg/m³ | 1.90× |
| platinum | 21,450 kg/m³ | 21.45× |
| gold | 19,300 kg/m³ | 19.30× |
| tungsten | 19,250 kg/m³ | 19.25× |

## Does it rust?

Fibreglass (GRP) contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

## Is it magnetic?

Fibreglass (GRP) is not a metal and has no ferromagnetic response.

## Common questions

### What is the density of fibreglass (GRP)?

1,900 kg/m³ (1,500–2,100), or 1.90 g/cm³.

### Does fibreglass (GRP) float?

No. At 1,900 kg/m³ it is denser than water.

### Is fibreglass (GRP) magnetic?

Fibreglass (GRP) is not a metal and has no ferromagnetic response.

## Related pages

- [Does fibreglass (GRP) float?](https://atoneplace.net/does-it-float/fiberglass/)
- [Unidirectional carbon fibre composite](https://atoneplace.net/material/carbon-fiber-ud/)
- [Woven carbon fibre composite](https://atoneplace.net/material/carbon-fiber-woven/)
- [Asphalt (hot-mix)](https://atoneplace.net/material/asphalt/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)