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Measuring the gas mass of protoplanetary disks, the reservoir available for giant planet formation, has proven to be difficult. We currently lack a far-infrared observatory capable of observing HD, and the most common gas mass tracer, CO, suffers from a poorly constrained CO-to-H$_2$ ratio. Expanding on previous work, we investigate if N2H+, a chemical tracer of CO poor gas, can be used to observationally measure the CO-to-H$_2$ ratio and correct CO-based gas masses. Using disk structures obtained from the literature, we set up thermochemical models for three disks, TW Hya, DM Tau and GM Aur, to examine how well the CO-to-H$_2$ ratio and gas mass can be measured from N2H+ and C18O line fluxes. Furthermore, we compare these gas masses to independently gas masses measured from archival HD observations. The N2H+ (3-2)/C18O (2-1) line ratio scales with the disk CO-to-H$_2$ ratio. Using these two lines, we measure 4.6e-3 Msun < Mdisk < 1.1e-1 Msun for TW Hya, 1.5e-2 Msun < Mdisk < 9.6e-2 Msun for GM Aur and 3.1e-2 Msun < Mdisk < 9.6e-2 Msun for DM Tau. These gas masses agree with values obtained from HD within their respective uncertainties. The uncertainty on the N2H+ + C18O gas mass can be reduced by observationally constraining the cosmic ray ionization rate in disks. These results demonstrate the potential of using the combination of N2H+ and C18O to measure gas masses of protoplanetary disks.