---
title: "Gray cast iron — density, melting point and properties"
description: "Gray cast iron has a density of 7,150 kg/m³ (7,050–7,300). Melting point, strength, conductivity, what it is used for, and whether it floats, rusts or is magnetic."
url: https://atoneplace.net/material/cast-iron/
site: "At One Place"
---

# Gray cast iron

**Density of gray cast iron: 7,150 kg/m³ (7,050–7,300)**

7.15 g/cm³ · 446.4 lb/ft³

The 2-4% carbon that makes it castable comes out as graphite flakes, and those flakes are simultaneously its great strength and its fatal weakness: they damp vibration superbly, which is why machine tool beds are cast iron, and they act as internal cracks, which is why it snaps rather than bends. It is roughly 10% less dense than steel because the graphite is light. Melting 350 °C below steel is what made it the first mass-produced metal casting, and is also why a cast iron pan can be left on a fire that would ruin an aluminum one.

## Properties

- **Density:** 7,150 kg/m³ (7,050–7,300) — 7.15 g/cm³
- **Melting range:** 1150–1200 °C
- **Tensile strength:** 150–400 MPa
- **Young's modulus:** 100 GPa
- **Thermal conductivity:** 46 W/(m·K)
- **Thermal expansion:** 10.5 µm/(m·K)
- **Floats in water:** No
- **Magnetic:** Yes

## What it is used for

- Engine blocks and cylinder liners
- Machine tool beds and lathe ways
- Drain covers, pipe and radiators
- Frying pans and Dutch ovens

## 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
- gray cast iron: 7,150 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× |
| gray cast iron ← | 7,150 kg/m³ | 7.15× |
| platinum | 21,450 kg/m³ | 21.45× |
| gold | 19,300 kg/m³ | 19.30× |
| tungsten | 19,250 kg/m³ | 19.25× |

## Does it rust?

Yes, and readily. Rust is iron oxide, and it is flaky rather than adherent — it does not seal the surface, so oxygen and water keep reaching fresh metal underneath and the process continues until the section is gone. That non-protective quality is what makes rust different from the oxide layers on other metals.

## Is it magnetic?

Iron, nickel and cobalt are the three common ferromagnetic elements, and anything based mostly on them is strongly attracted to a magnet.

## Common questions

### What is the density of gray cast iron?

7,150 kg/m³ (7,050–7,300), or 7.15 g/cm³.

### Does gray cast iron float?

No. At 7,150 kg/m³ it is denser than water.

### Is gray cast iron magnetic?

Iron, nickel and cobalt are the three common ferromagnetic elements, and anything based mostly on them is strongly attracted to a magnet.

## Related pages

- [Does gray cast iron float?](https://atoneplace.net/does-it-float/cast-iron/)
- [Mild steel](https://atoneplace.net/material/mild-steel/)
- [Stainless steel 304](https://atoneplace.net/material/stainless-304/)
- [Brass](https://atoneplace.net/material/brass/)
- [High-carbon steel](https://atoneplace.net/material/carbon-steel/)
- [Stainless steel 316](https://atoneplace.net/material/stainless-316/)
- [Aluminum 6061](https://atoneplace.net/material/aluminum-6061/)
- [Bronze](https://atoneplace.net/material/bronze/)
- [Aluminum 7075](https://atoneplace.net/material/aluminum-7075/)

## Sources

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