Using Advanced Camera for Surveys-Hubble Space Telescope images to yield continuum-subtracted photometric maps in Hα of the Sbc galaxy M51 and the dwarf irregular galaxy NGC 4449, we produced extensive (over 2000 regions for M51 and over 200 regions for NGC 4449) catalogs of parameters of their H II regions: their Hα luminosities, equivalent radii, and coordinates with respect to the galaxy centers. From these data we derived, for each region, its mean luminosity-weighted electron density, langne rang, determined from the Hα luminosity and the radius, R, of the region. Plotting these densities against the radii of the regions, we find excellent fits for langne rang varying as R -1/2. This relatively simple relation has not, as far as we know, been predicted from models of H II region structure, and should be useful in constraining future models. Plotting the densities against the galactocentric radii, r, of the regions, we find good exponential fits with scale lengths of close to 10 kpc for both galaxies. These values are comparable to the scale lengths of the H I column densities for both galaxies, although their optical structures, related to their stellar components, are very different. This result indicates that to a first approximation the H II regions can be considered in pressure equilibrium with their surroundings. We also plot the electron density of the H II regions across the spiral arms of M51, showing an envelope which peaks along the ridgelines of the arms.