If we know the weight of a rock, what is its volume?

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If we know the weight of a rock, what is its volume?

If we know the weight of a rock, what is its volume?


The volume of a rock can always be estimated if it is approximately rectangular in shape and you are able to measure its maximum length, width, and height. Just multiply your three measurements together, and the result is the volume of a rectangular solid that could enclose your rock. The actual volume of the rock would be less than this, of course.

Another way to find the rock's volume—assuming it is not a huge rock—is to measure the volume of water it displaces when submerged in a container of water filled to the brim.

How to find the density of a rock

What if it is a larger rock? You can calculate the volume of the rock using its weight and the density (the amount of mass per unit of volume) of the material in the rock. Density is expressed in units of mass per unit volume, such as grams per cubic centimeter (g/cm3) or pound mass per cubic foot (lbm/ft3). If you can identify the type of rock, you can estimate its density from what is known about rock material. Here are some typical rocks:

  • Density of basalt: 3 g/cm3 (187 lbm/ft3)
  • Density of granite: 2.7 g/cm3 (169 lbm/ft3)
  • Density of sandstone: 2.3 g/cm3 (144 lbm/ft3)

To calculate the volume of a rock, divide its mass by its density. Here, we would use the mass as indicated by its weight on Earth. For example, a 13,600 g (30 lb) piece of sandstone would have a volume of 13,600 / 2.3 or about 5,910 cm3 (30 / 144 or about 0.2 ft3). Such a piece of sandstone would measure about 15 cm by 20 cm by 20 cm (about 6 in by 8 in by 8 in).

Try some of PlanetSEED's many density and volume activities and experiments.