This paper presents an analytical solution to quantify the uncertainty of volumetric fraction (Vf) estimates in a heterogeneous material using 2-D probes. The analytical solution was derived based upon the concept of representative volume (RVE). The results show that the uncertainties of the estimates depend upon the size of blocks, measurement area, and level of V f. The analytical solution has been verified via numerical simulation. The latter was carried out by first generating spherical blocks to various level of Vf, which was then sampled by 2D probes in obtaining the Vf estimates and the associated uncertainties. Application examples are given at the end.