<jats:title>ABSTRACT</jats:title> <jats:p>Radiocarbon (<jats:sup>14</jats:sup>C) concentrations in the oceans are different from those in the atmosphere. Understanding these ocean-atmospheric <jats:sup>14</jats:sup>C differences is important both to estimate the calendar ages of samples which obtained their <jats:sup>14</jats:sup>C in the marine environment, and to investigate the carbon cycle. The Marine20 radiocarbon age calibration curve is created to address these dual aims by providing a <jats:italic>global-scale</jats:italic> surface ocean record of radiocarbon from 55,000–0 cal yr BP that accounts for the smoothed response of the ocean to variations in atmospheric <jats:sup>14</jats:sup>C production rates and factors out the effect of known changes in <jats:italic>global-scale</jats:italic> palaeoclimatic variables. The curve also serves as a baseline to study regional oceanic <jats:sup>14</jats:sup>C variation. Marine20 offers substantial improvements over the previous Marine13 curve. In response to community questions, we provide a short intuitive guide, intended for the lay-reader, on the construction and use of the Marine20 calibration curve. We describe the choices behind the making of Marine20, as well as the similarities and differences compared with the earlier Marine calibration curves. We also describe how to use the Marine20 curve for calibration and how to estimate Δ<jats:italic>R</jats:italic>—the localized variation in the oceanic <jats:sup>14</jats:sup>C levels due to regional factors which are not incorporated in the <jats:italic>global-scale</jats:italic> Marine20 curve. To aid understanding, illustrative worked examples are provided.</jats:p>