RTCalcExpress is a Delphi component for calculating expression, entered in Run-Time. It is possible to add any quantity of the FDU functions. These functions can be as without arguments and to have the unlimited quantity of the arguments. The arguments suppose any level of an enclosure. A maximal length of the expression corresponds to the length of the Delphi data type AnsiString. The expression can be divided on substrings which will be calculated one after another.