Hi Lloyd, The units are driven by Product Category Rules (PCR) which state that all GWP numbers need to be expressed in kg of CO2 eq / metric unit of material. For instance concrete, all EPDs have to be expressed in kg of CO2 eq / m3 of concrete. Although metric is also required in the USA, one can reflect their concrete GWP in kg of CO2 / yd3 of concrete in addition to the required metric.
Hope this helps.