Precise measurements of the inner diameter (ID) of nanocapillaries are of interest to many areas of study due to the high sensitivity of gas and liquid flows with respect to the radius of the capillary. Applications ranging from biological transport to electronics manufacture require knowledge of fluid transport properties through passages with nanoscale dimensions. In the microscale, volume flow and fluid velocity are quadratic and squared functions of the capillary radius, respectively. While these relationships hold for most fluids in the nanoscale, the dependence on the ID is extreme with small variations in diameter leading to significant variations in flow properties. The current approach for measuring nanocapillary ID (i.e., FESEM) is time consuming and typically requires a conductive coating. A simple, nondestructive test to accurately determine the diameter of nanocapillaries based on bubble point was developed. The apparatus consists of a high pressure gas source connected to a nanopipette where the tip is located under the surface of a liquid. Bubble rate is determined as a function of pressure using an optical microscope. By using the Laplace relationship between surface tension induced pressure and capillary radius one can define inner diameter of nanocapillaries.