Airflow in CFM = (Engine Cubic Inches x Maximum RPM) ๗ 3456
This formula assumes 100% volumetric efficiency but the real world figure is around 70-80% so it pays to subtract 30-50 cfm from the result to provide a more realistic figure. For example for a high performance 350cid street car|:
350cid x 8000rpm ๗ 3456 = 810cfm
Subtracting 30-50cfm brings this figure back to around 750-780cfm.
Took me under 1 minute to find it from using a search engine - it would probably be quicker for you to try this method next time than waiting for a reply.