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The surface density of a disk is the mass density integrated through its vertical column: $\Sigma (r)={\int }_{-\infty }^{\infty }\rho (r,z)dz$. Its dimensions are mass per area. An axisymmetric annulus of width $dr$ therefore contains $dM=2\pi r\Sigma (r)dr$. This reduces a three-dimensional mass inventory to a radial one, which is useful in viscous evolution of an accretion disk. Distinct vertical profiles can have the same column mass, so $\Sigma$ alone does not determine the vertical structure of an astrophysical disk.

The vertical structure of an astrophysical disk describes how mass density, pressure and temperature vary away from its midplane. For a thin, centrally dominated gas layer, hydrostatic equilibrium gives $dp/dz=-\rho {\Omega }_z^{2}z$. An isothermal ideal gas has $p=c_s^2\rho$, producing a Gaussian density profile with disk scale height $H=c_s/{\Omega }_z$. Disk self-gravity or magnetic support changes that balance.^[ref]