The accessible pore space for Lennard-Jones gas molecules adsorbed in smooth-walled carbon nanopores in the Henry’s law region is determined for slit-shaped, cylindrical, and spherical pores. The size of this space, over which the total potential is nonpositive, depends on pore geometry and number of wall layers. Pores with small surface mean curvature are more accessible than those with larger surface mean curvature. Adding layers to the wall helps to increase the accessible pore space, but this effect is small after the second layer. Limits are determined for the smallest pores having accessible space. For the slit-shaped pore with a single wall, the minimum pore half width is the same as the width of the inaccessible layer at the wall of a single graphene sheet, which is less than the Lennard-Jones solid–fluid collision diameter. In contrast, the minimum radius of a spherical pore with accessible space is the Lennard-Jones solid–fluid collision diameter.