Capillary rise is the basis of some methods that are widely applied for the determination of contact angles as well as wettabilities of small particles. The equivalent hydraulic radius r(d) in the Classical Washburn equation is assumed to be particle-specific. But it seems that r(d) is not always constant when the type of liquids is different. The new equation with the pressure increment and the hydrostatic effects are theoretically derived based on the Washburn equation, so contact angles of small particles can be measured experimentally independently of r(d). The result shows the validity of the proposed method, and therefore, it becomes possible to accurately measure the wettability of small particles. (C) 2012 Elsevier Inc. All rights reserved.