Volumetric water content, θ, is defined mathematically as:
where  is the volume of water and  is equal to the total volume of the wet material, i.e. of the sum of the volume of solid host material (e.g., soil particles, vegetation tissue) , of water , and of air .
Gravimetric water content is expressed by mass (weight) as follows:
where  is the mass of water and  is the mass of the solids.
For materials that change in volume with water content, such as coal, the gravimetric water content, u, is expressed in terms of the mass of water per unit mass of the moist specimen (before drying): 
However, woodworking, geotechnics and soil science require the gravimetric moisture content to be expressed with respect to the sample's dry weight:
And in food science, both  and  are used and called respectively moisture content wet basis (MC) and moisture content dry basis (MC).
Values are often expressed as a percentage, i.e. u×100%.
To convert gravimetric water content to volumetric water content, multiply the gravimetric water content by the bulk specific gravity  of the material: