The Carbon Calculation Tool 4.0, CCTv40.exe, is a computer application that reads publicly available forest inventory data collected by the U.S. Forest Service's Forest Inventory and Analysis Program (FIA) and generates state-level annualized estimates of carbon stocks on forest land based on FORCARB2 estimators. Estimates can be recalculated as new inventory data become available. The input set of FIA data fi les available on the Internet (as well as some older inventory fi les used to fi ll in gaps) are summarized by the application, converted to carbon stocks, and saved as part of a state or substate level "survey summary" fi le. This is used to produce state-level and national tables with annualized carbon stocks and fl ux (or net stock change) beginning with the year 1990. This user's guide includes instructions for use, example data sets, and a discussion of methods and assumptions.