International monitoring of Atlantic mackerel (Scomber scombrus) and Atlantic horse mackerel (Trachurus trachurus), two commercial and ecologically epipelagic fish species in the North Atlantic, is currently done inconsistently. Depending on the country and institute, the length of a specimen is measured as either ‘Fork Length’, ‘Pinched Tail Length’ or ‘Total Length’. This inconsistency increases the uncertainty of results from data analyses that are based on international data compilations. Here we provide conversion tables between all three methods for both species, based on field sampling in Greenlandic and Scottish waters. For both species, we recommend the use of ‘Fork Length’ or ‘Pinched Tail Length’ rather than ‘Total Length’.