Data about the flowability of fly ashes are important for the design of hoppers of fly ash collection equipment and fly ash storage silos. Fly ashes from sixteen grate-fired combustion plants (first-stage and second-stage de-dusting fly ash) were examined for flow relevant properties. The flowability category of the coarser first-stage fly ashes was cohesive to easy-flowing, while for the second-stage fly ashes the flowability category was very cohesive to cohesive. A good correlation was found between the flowability of the fly ashes and the mass median diameter of their particle size distribution. The correlation of the flowability with the bulk density or with the angle of repose was less significant. No correlation was found with the other parameters investigated. Moreover, the effective angle of internal friction, the wall friction angle and bulk density can be expressed as a function of the mass median diameter. Thus, the particle size is the most important parameter for the flowability of the fly ashes.