In this paper hydrogen adsorption capacity of carbon fibers is presented. These fibers are synthesized by pyrolysis of cotton fibers. Forty five varying parameters are applied for the pyrolysis of cotton to get carbon fiber of varied specific densities. Specific densities are calculated by (i) water displacement method and (ii) tapping method. Maximum hydrogen adsorption capacity is found to be 1.246 wt% if specific density value obtained by water displacement method is considered; whereas specific density obtained from tapping method showed 7.076 wt% hydrogen adsorption by the same carbon fibers. This paper imposes a question that since the measurement of density of carbon by these two method give different results which one should be accepted or is there a need to develop another method. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.