We developed a mechanistic model for calculation of bubble volume from orifices in the range from 0.03 mm to 0.193 mm under the constant gas flow conditions in a quiescent liquid. It is known that for such small orifices, the mechanism of bubble formation is highly dependent on the gas momentum force and the liquid inertia force. Accordingly, the model incorporates these forces to calculate the bubble volume in three consecutive stages. Moreover, the model includes the influence of the bubble base expansion and bubble rising induced liquid velocity on the formation of bubbles. Eventually the model is validated with own experimental data using air and deionized water. Experimental validation of the model confirms that the maximum deviation of the model is less than 10%.