jats:titleAbstract</jats:title> jats:pThe trapped field in Y–Ba–Cu–O (YBCO) bulk single grain superconductors correlates directly with the critical current density jats:italicJ</jats:italic> jats:subc</jats:sub>, which, in turn, is dependent on the microstructure of the bulk material. It has been shown recently and indirectly that porosity influences jats:italicJ</jats:italic> jats:subc</jats:sub> of these technologically important materials, in addition to the presence of well-researched Yjats:sub2</jats:sub>BaCuOjats:sub5</jats:sub> (Y-211) particles in the bulk sample. In this work, we report the direct impact of porosity on the critical current density of a single grain YBCO bulk superconductor using 3D x-ray computer tomography scans and superconducting quantum interference device measurements. It is concluded that porosity has a considerably more substantial impact on the measured critical current density than Y-211 on the micrometre scale with, predictably, a decrease in porosity leading to an increase in jats:italicJ</jats:italic> jats:subc</jats:sub>. jats:italicJ</jats:italic> jats:subc</jats:sub> is directly proportional to the trapped field, so any method that can reduce porosity, therefore, improves jats:italicJ</jats:italic> jats:subc</jats:sub>, and, subsequently, the trapped field of these technologically important bulk superconductors.</jats:p>