New formulas are suggested for the swarm speed of cap-shaped gas bubbles collectively rising within a gas-solid fluidized bed. Their derivation stems from comparing the added mass of a single bubble and a swarm of bubbles: U-B/U-0 = root c(0)/c. For different flow conditions, there are different trends in the dependence of the added mass on the bubble concentration (voidage). This variance translates into different predictions of the suggested swarm speed formulas. This difference is explained in terms of the geometrical configuration of bubbles inside the bed. The expressions for the added mass coefficient in case of two typical flow regimes are introduced: C = C-0 (1 - epsilon) freely bubbling regime, C = C-0 (1 - epsilon)(2) near slugging regime. (C) 2016 Elsevier Ltd. All rights reserved.