This　paper　proposes　a　simple　closed-form　approximation　of　the　axial　force-bending　moment　interaction　diagram　for　solid　and　hollow　circular　reinforced　concrete　(RC)　cross-sections　with　an　arbitrary　number　of　layers　of　longitudinal　reinforcement.　The　implemented　nonlinear　material　models　are　those　of　the　European　building　code.　Specifically,　the　parabola-rectangular　strain-stress　relation　and　the　stress-block　model　are　considered　for　the　concrete　in　compression.　On　the　other　hand,　the　elastic-perfectly　plastic　behavior　is　assumed　for　the　reinforcing　steel　bars.　The　proposed　semi-exact　closed-form　approximation　involves　one　parameter　only,　which　is　calibrated　by　means　of　a　numerical　optimization　procedure　taking　into　account　a　large　database　of　cross-sections.　Moreover,　an　efficient　procedure　is　developed　for　the　design　of　the　outer　layer　of　longitudinal　reinforcement　in　solid　or　hollow　circular　RC　cross-sections　under　combined　axial　force　and　bending　moment　(the　mechanical　ratio　of　any　other　layers　of　longitudinal　reinforcement　is　considered　as　known　function　of　the　mechanical　ratio　of　the　outer　layer　of　longitudinal　reinforcement).　Finally,　several　numerical　examples　are　included　to　demonstrate　the　accuracy　of　the　approximated　axial　force-bending　moment　interaction　diagram　and　the　effectiveness　of　the　derived　design　procedure.