Most of the baryonic matter in the Universe is expected to reside outside of galaxies in the low-density intergalactic medium. Fast Radio Bursts (FRBs) have the ability to probe the location of these baryons because the dispersion measure of an FRB is able to account for all of the ionised baryons along the line-of-sight. However, to estimate the baryonic content of the intergalactic medium, we require an accurate dispersion measure - redshift relation for FRBs. In this talk, I will present the dispersion measure - redshift relation in the cosmological hydrodynamic simulation EAGLE, where we have utilised over 1 billion lines-of-sight. I will also discuss the differences between the existing analytic, semi-analytic and hydrodynamic formulations for the dispersion measure - redshift relation. Finally, I will show that using the mean electron density in the intergalactic medium (rather than the mode) significantly underestimates the distance to observed FRBs.