Aim (i) To introduce a new method of quantifying extruded irrigant during root canal irrigation ex vivo. (ii) to evaluate the effect of periapical tissue simulation and pressure equalization and (iii) to determine the effect of needle type, apical preparation size and apical constriction diameter on irrigant extrusion. Methodology Sixteen human single-rooted teeth were sequentially prepared to sizes 25–45, 0.06 taper and mounted on a plastic vial simulating a periapical lesion. The apical constriction diameter was standardized to 0.15–0.35 mm. The vial was filled with distilled water or air and was either open to the environment or closed. A point-conductivity probe was used to determine the volume of extruded irrigant into the vial. NaOCl was delivered by an open-ended or a closed-ended needle at 3 mm short of working length. Results were analysed by two 3-way repeated-measures ANOVAs Results The open-ended needle extruded significantly more irrigant than the closed-ended in the majority of cases (P < 0.002). An increase in the apical size was related to decreased irrigant extrusion (P < 0.024). The effect of constriction diameter was not significant. The water-closed and water-open methods were related to less extrusion than the air-closed and air-open methods, respectively (P < 0.005). Open systems (water-open, air-open) allowed extrusion of larger amounts of irrigant than corresponding closed systems (water-closed, air-closed) (P < 0.005). Conclusions The conductivity probe is a reliable method for real-time quantification of irrigant extrusion ex vivo. Not simulating tissue resistance in ex vivo experiments may lead to significant overestimation of irrigant extrusion.