Zafin ƙarfi yana nufin adadin kuzarin zafi da ke fitowa ko shayarwa lokacin da wani abu mai ruwa ya taru, ko kuma ya zama mai ƙarfi a yanayin zafi akai-akai. Ma'auni ne na makamashin da ake buƙata don canza raka'a na tarin abu daga yanayin ruwa zuwa yanayinsa mai ƙarfi a takamaiman yanayin zafi, ba tare da canjin yanayin zafi ba. Zafin ƙarfi yana nunawa a cikin raka'a na joules kowace gram (J/g) ko adadin kuzari a kowace gram (cal/g).