Census tracts are small, relatively permanent statistical subdivisions designed to be relatively homogeneous with respect to population characteristics.
I recommend the Federal Financial Institutions Examination Council, FFIEC, Geocoding System for determining census tract. This site helps financial institutions meet their legal requirements to report information on applications.
Each location will have a state code, a country code, and a tract code as listed on the screen shot below. For the census tract where AU is located the full number is 11-001-000901. We drop the leading zeros and add a decimal before the next zero so the tract is 9.01