We study numerical conformal mappings of planar Jordan domains with boundaries consisting of fnitely many circular arcs, also called polycircular domains, and compute the moduli of quadrilaterals for these domains. Experimental error estimates are provided and, when possible, comparison to exact values or other methods are given. We also analyze the rate of convergence as a function of the number of degrees of freedom. The main ingredients of the computation are boundary integral equations combined with the fast multipole method.