Carbon emission quantities can be calculated from either the volume/mass of fuel or feedstock fed to a process (as applicable) and carbon content of the process or fuel supply, or by directly measuring volume/mass emissions. This Technical Report (TR) provides guidance on the sampling and calculation of carbon content of process or fuel supplies. The API companion technical report, API TR 2571, Fuel Gas Measurement, can be referenced for guidance on measuring the volume/mass of process fuel gas or feedstock, and the API Compendium of Greenhouse Gas Emissions Estimation Methodologies for the Oil and Natural Gas Industry can be reference for guidance on the calculation of emissions. TR 2572 provides guidance and a methodology for the determination of carbon content from hydrocarbon-based petroleum and petrochemical products, and the uncertainty of the average carbon content as calculated from multiple samples taken during a reporting period. This method is intended to make use of industry-accepted mixture property data and test methods with no new or modified test methods introduced in this document. The method is applicable to carbon-content-based reporting or trading for all gaseous and liquid hydrocarbons. This TR provides references and supplemental information on applicable industry practices based on the published resources, existing industry standards, industry-accepted physical constants or properties of hydrocarbons for measurement, sampling, sampling frequency, and analysis of hydrocarbon samples.