Number vs. volume distributions

Interpreting results of a particle size measurement requires an understanding
of which technique was used and the basis of the calculations.
Each technique generates a different result since each measures different
physical properties of the sample. Once the physical property is measured a
calculation of some type generates a representation of a particle size distribution.
Some techniques report only a central point and spread of the distribution,
others provide greater detail across the upper and lower particle size detected.
The particle size distribution can be calculated based on several models: most
often as a number or volume/mass distribution.

NUMBER VS. VOLUME DISTRIBUTION
The easiest way to understand a number distribution is to consider measuring
particles using a microscope. The observer assigns a size value to each particle
inspected. This approach builds a number distribution—each particle has equal
weighting once the final distribution is calculated. As an example, consider the
nine particles shown in the image above. Three particles are 1μm, three are 2μm, and
three are 3μm in size (diameter). Building a number distribution for these particles
will generate the result shown in the first graph below, where each particle size accounts for one third of the total. If this same result were converted to a volume distribution, the result would appear as shown in the second graph below where 75% of the total volume comes from the 3μm particles, and less than 3% comes from the 1μm particles. When presented as a volume distribution it becomes more obvious that the majority of the total particle mass or volume comes from the 3μm particles.
Nothing changes between the left and right graph except for the basis of the
distribution calculation.