Permeability of hot-mix asphalt (HMA) is an important property that influences asphalt's resistance to moisture damage. All current methods used by pavement engineers rely on measuring vertical permeability. Recently, however, it has been shown that horizontal permeability can be much higher than the vertical because of the anisotropic and heterogeneous nature of air void distribution. Laboratory and field measurements of horizontal permeability require sophisticated equipment and detailed procedures that limit the ability to measure this property routinely. This study developed a procedure for estimating the horizontal permeability of asphalt mixes. The procedure requires simple laboratory measurements of vertical permeability and porosity (percent air voids) of sublayers in a specimen. The development of this simple method was supported by numerical simulations of microscale fluid flow in a wide range of HMA mixtures. The simulations helped in understanding the factors that control vertical and horizontal permeabilities. The developed equation and the numerical simulations confirmed that the horizontal permeability is several times higher than the vertical permeability.