Actually the Impact Factor (IF) is provided by Thomson Reuters by means of the Journal Citations Report (JCR). Most of ACM journals are indexed there, and they have an impact factor computed. The impact factor varies from field to field. Hence it is important to compare Impact Factors from journals of the same field. You can google impact factor of a specific journal and you will probably find it :). (e.g. the Impact factor of ACM TOSEM is 1.17 for year 2015 https://www.researchgate.net/journal/1049-331X_ACM_Transactions_on_Software_Engineering_and_Methodology)