Two methods of estimating particle or droplet size distributions from chord measurements an presented and evaluated. The first is a probability apportioning method (PAM). This assumes that particles are cut randomly and calculates a diameter probability distribution from each chord size detected. The distribution is then accumulated over all chord measurements. PAM has proved to be a fairly robust method when the particle diameters are known. The second is a finite element method (FEM) that has been found to be more generally applicable where there is a wide range of unknown particle diameters. Both methods have been rigorously tested on sets of chord data developed from either the ideal probability functions, a distinct element simulation or true data from a chord measuring instrument (Par-Tec).