Density is measured as mass per unit volume (mass divided by volume). Soil particle density depends on the chemical composition and structure of the minerals in the soil. Most mineral particles in soils have a particle density ranging from 2.60 to 2.75 g/cm³. However, the density can be as high as 3.0 g/cm³ for very dense mineral particles, and as low as 0.9 g/cm³ for organic particles.

Field density is defined as the weight of unit volume of soil present on site.

Density of soil, γ_t = Total weight of soil (W) / Total volume of soil (V)

In soils, mass of air is considered to be negligible, and therefore the saturated density is maximum, dry density is minimum and wet density is in between the two. Dry density of the soil is calculated by using the following equation:

Dry density of soil, γ_d = Wet density of soil (γ_t) / (1 + Water content of soil (w)).

Density or unit weight of soils may be determined by using the following methods:

1. Core cutter method
2. Sand replacement test
3. Rubber balloon test
4. Water displacement method
5. Gamma ray method

The particle density of a soil measures the mass in a given volume of particles. Particle density focuses on just the soil particles themselves and not the volume they occupy in the soil. Bulk density includes both the volume of the solid (mineral and organic) portion of the soil and the spaces where air and water are found.

If we have information on both the particle density and the bulk density of the soil, we can calculate the pore space (or porosity) that is occupied by air and water. This is useful because it helps us to understand other important soil properties such as how much water can be stored in the soil, how fast water and heat will move through the soil, how easily roots can move through the soil, and the potential for flooding or drought in an area.