A cubic foot is a standard unit of volume, representing a cube that measures twelve inches on each side. Determining the weight of rock that fills this space is not a straightforward calculation, as the resulting mass is highly variable. The final weight of one cubic foot of rock depends entirely on the rock’s intrinsic geological properties and its physical condition when measured.
Defining the Average Weight Range
For a solid, non-porous block of common rock, the weight of a single cubic foot typically falls within a range of 150 to 175 pounds. This range provides a practical, initial estimate for large pieces of quarried stone or boulders. Construction and engineering industries often use an average figure of approximately 165 pounds per cubic foot for general calculations involving solid rock material.
These figures represent the theoretical mass of a solid block with minimal internal air pockets. The actual density of the rock type is the primary factor driving this inherent weight. However, the practical weight of rock used in landscaping or construction often involves materials that are not solid blocks, introducing variables that can drastically alter the final measurement.
How Geological Type Impacts Weight
The inherent weight of a solid rock is determined by its density, a physical property directly related to its mineral composition and specific gravity. Specific gravity is a ratio comparing the density of the rock to the density of water. Rocks rich in heavier elements, such as iron and magnesium, possess a naturally higher density, leading to greater mass per cubic foot.
Igneous rocks, like basalt and granite, generally represent the denser end of the spectrum because they are rich in these heavier, darker minerals. Basalt, a common volcanic rock, can have a density that yields a weight exceeding 185 pounds per cubic foot. In contrast, granite, a well-known intrusive igneous rock, typically weighs around 165 to 175 pounds per cubic foot, due to its quartz and feldspar content.
Sedimentary rocks, such as limestone and sandstone, often exhibit lower densities because they are composed of lighter minerals and tend to be more porous. For example, a cubic foot of dense limestone may weigh up to 175 pounds, but a lighter, more porous sandstone may only weigh about 155 pounds. This difference highlights how the geological history and mineral makeup of the rock dictate its foundational mass.
Crushed Stone, Void Space, and Moisture
When rock is crushed into aggregate for use as gravel or base material, the weight per cubic foot significantly decreases compared to a solid block. This reduction is caused by “void space,” which is the volume occupied by air pockets between the irregularly shaped rock fragments. A cubic foot of crushed granite, for instance, may weigh only about 100 to 110 pounds, a substantial drop from the 175 pounds of a solid block.
The degree of void space is influenced by the size and uniformity of the crushed pieces. A mixture of different sizes packs together more efficiently, reducing the air pockets and increasing the bulk density. Conversely, uniformly sized gravel will have more empty space and a lower weight per volume. This mass measurement for loose, crushed material is often referred to as “bulk density” in the construction industry.
The presence of moisture is another factor that alters the weight of both crushed aggregate and porous rock. Water is absorbed into the pore spaces within the rock and held within the void spaces of the aggregate. Since water weighs approximately 62.4 pounds per cubic foot, absorbed water directly adds to the overall mass. A cubic foot of saturated aggregate can weigh 10 to 20 percent more than the same volume of dry material, which is a consideration for transport and structural load calculations.