A basic parameter in two-phase gas-liquid flow is the size of the bubbles. From the point of view of calculating the interfacial area for mass transfer, the size of the bubbles is as important as the holdup. Many empirical expressions for bubble diameter have been proposed. The purpose of this paper is to see whether recent work on bubble diameter in developed flow in pipes can also be used for bubble columns. The new expression for bubble diameter, which is based on an analogy with the structure of single-phase turbulent flows, is simpler to use and comparable in accuracy with the previous empirical expressions, in spite of not having any adjustable constants in it.