The relative size of the insulating sheath to electrode area at a micro-disc electrode can lead to significant perturbations in the steady state current observed. A minimum, constant steady state current value is realised once the sheath thickness is greater than twice the radius Delta l > 2r(d). However, as the sheath thickness decreases below this value, the observed current increases. In this paper a theoretical model is presented, allowing for the accurate determination of the outer sheath thickness. The effects of an ultra-thin sheath on steady state currents are demonstrated experimentally and these results are shown to accurately fit with the simulated model developed. Therefore, the model presented here can be used to determine the size of a sheath of unknown thickness. Furthermore, it allows these size effects on the steady state current to be explored. (C) 2015 Elsevier B.V. All rights reserved.