In the analysis of particle size distribution (PSD), evaluation of the uncertainty of the percentile values is often required. Both theoretical and data processing methods are applicable to estimate standard deviations of percentile values. In this paper, a theory, applicable both in paramedic and non-parametric ways, is introduced. As a practical example, theoretical results based on a lognormal distribution are compared to those obtained by bootstrap method. Excellent agreement is demonstrated between the theory and the direct data processing method. (C) 2019 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan.