This document has been declared "Stabilized" and will no longer be subjected to periodic reviews for currency. Users are responsible for verifying references and continued suitability or technical requirements. New technology may exist. This Aerospace Information Report (AIR) describes a mathematical model that can be used to analyze particle count data. Particle counts that fit the model can be graphically displayed, converted from one counting size-frequency range to another, and extrapolated to estimate counts beyond the measured range. Derivation, applications and calculations are described. Cumulative particle counts larger than stated size-range are fitted against a modified log-normal distribution function by plotting on special log-log graph paper. Many real particle distributions approximate a straight line, showing good fit for this log-normal model. The resultant plot provides a basis for further analysis, including size-range conversion and extrapolation.