This document specifies the requirements and gives recommendations for the procedures and design criteria used for calculating the required wall thickness of new tubes and associated component fittings for petroleum, petrochemical and natural gas industries. These procedures are appropriate for designing tubes for service in both corrosive and non-corrosive applications. These procedures have been developed specifically for the design of refinery and related process-fired heater tubes (directfired, heat-absorbing tubes within enclosures). These procedures are not intended to be used for the design of external piping. This document does not give recommendations for tube retirement thickness. A technique for estimating the life remaining for a heater tube is described. This document is a supplement to API 530, 7th edition (2015) including the addenda, the requirements of which are applicable with the exceptions specified in this document.