This note provides a conceptually simple derivation of the Kelvin equation, based on the principles of hydrostatics, rather than the more elegant and abstract thermodynamic approach of equating the chemical potential of each of the phases. With the new derivation, the physical picture of a drop in equilibrium with its own vapour is more readily appreciated, and hence the basis of the Kelvin equation more easily understood by those less skilled in the science of thermodynamics. In turn, a number of the potential limitations [Powles, 1985. Journal of Physics A Mathematical and General 18, 1551-1560] of the Kelvin equation become physically obvious, notably the assumption that the liquid is incompressible, the vapour is an ideal gas, and the interfacial tension is independent of the curvature of the interface. In turn, appropriate corrections can easily be introduced as required. (c) 2005 Elsevier Ltd. All rights reserved.