The Internal Revenue Code prescribes minimum imputed interest rates and time-value-of-money factors applicable to certain loan transactions and estate planning techniques. These rates are tied formulaically to market interest rates. The Internal Revenue Service updates these rates monthly.
These are commonly applicable rates in effect for September 2017:
Short Term AFR (Loans with Terms <= 3 Years) 1.29%
Mid Term AFR (Loans with Terms > 3 Years and <= 9 Years) 1.94%
Long Term AFR (Loans with Terms >9 Years) 2.6%
7520 Rate (Used in many estate planning vehicles) 2.4%
Here is a link to the complete list of rates: https://www.irs.gov/pub/irs-drop/rr-17-17.pdf.