Based on sensitive CO measurements from HERACLES and H i data from THINGS , we show that the azimuthally averaged radial distribution of the neutral gas surface density ( \Sigma _ { HI } + \Sigma _ { H 2 } ) in 33 nearby spiral galaxies exhibits a well-constrained universal exponential distribution beyond 0.2 \times \mbox { r$ { } _ { 25 } $ } ( inside of which the scatter is large ) with less than a factor of two scatter out to two optical radii r _ { 25 } . Scaling the radius to r _ { 25 } and the total gas surface density to the surface density at the transition radius , i.e. , where \Sigma _ { HI } and \Sigma _ { H 2 } are equal , as well as removing galaxies that are interacting with their environment , yields a tightly constrained exponential fit with average scale length 0.61 \pm 0.06 r _ { 25 } . In this case , the scatter reduces to less than 40 % across the optical disks ( and remains below a factor of two at larger radii ) . We show that the tight exponential distribution of neutral gas implies that the total neutral gas mass of nearby disk galaxies depends primarily on the size of the stellar disk ( influenced to some degree by the great variability of \Sigma _ { H 2 } inside 0.2 \times \mbox { r$ { } _ { 25 } $ } ) . The derived prescription predicts the total gas mass in our sub-sample of 17 non-interacting disk galaxies to within a factor of two . Given the short timescale over which star formation depletes the H _ { 2 } content of these galaxies and the large range of r _ { 25 } in our sample , there appears to be some mechanism leading to these largely self-similar radial gas distributions in nearby disk galaxies .