Calculations of aperture corrections to infer total CO luminosity. PHANGS–ALMA covers most of the area of active star formation but not the entire disk of each target. We derive an aperture correction to correct from our measured L CO to the global L CO for each target. To do this, we use WISE3 intensity as a template for CO intensity. The left panel shows the motivation for this choice, plotting CO(2−1) intensity as a function of WISE3 intensity at 17″ resolution within the PHANGS–ALMA coverage. A single normalization factor has been derived and applied to correct each galaxy for differences in the median WISE3-to-CO(2−1) ratio. After this normalization, the two intensities show a linear correlation coefficient of 0.88, exhibiting a stunningly tight correlation across three decades in intensity. The right panel shows the derived aperture corrections for WISE3 (red) and the ratio between aperture corrections derived using other templates and that from WISE3. WISE3 represents the clear best option, but if we adopted another reasonable choice like WISE4, stellar mass, or star formation rate, then this would imply ∼10% scatter (gray band) on average and a bias of ≲10%.