Y Cyfrifiad yw’r brif ffynhonnell wybodaeth ar gyfer y gyfran o bobl sy’n medru’r Gymraeg, ond defnyddir yr Arolwg Cenedlaethol i gadw llygad ar dueddiadau ymhlith oedolion rhwng pob Cyfrifiad. Mae’n holi cwestiynau am agweddau at yr iaith, hyder pobl yn ei defnyddio a gallu pobl i’w defnyddio.
While the Census is the main source of information about the proportion of people who can speak Welsh the National Survey is used to monitor trends for adults between censuses and asks questions about confidence and attitudes towards the language as well as ability.