Since linear density is a ratio, it is calculated by dividing the object's mass by its length. If the mass and length are not known, they can usually be measured directly with a scale and measuring tape or other equivalent measuring devices.
Using linear density and other mass densities is helpful when calculating moments of inertia. Transforming the variable being integrated from distance to mass simplifies the calculation immensely. Making a second substitution after the integral has been calculated gives the desired equation for moment of inertia.
Any property that is spatially distributed can be divided by volume, area or length, giving a density value. For example, energy density and number density are both commonly used in the sciences.