If Eval doesn't satisfy you, you might try these. Eval 1.13 is an interactive floating point C-like expression evaluator. You give it an expression with numeric constants and variables and functions, and it churns out a numeric answer. You can use it to immediately evaluate an expression from the DOS prompt, or you can run it interactively, storing variables as you go so that you can do more involved calculations. Or just run Eval without arguments.
Documentation is built-in.
Eval's features include good precision, input and output in any number base, a large number of built-in functions and constants, flexible numeric output format, and ability to save and recall sessions from disk. You can modify Eval to your heart's content. The source code is included with this distribution. The source is in ANSI C and has no conditional compile directives. I have compiled on VMS with minor modifications due to the compiler I used not being ANSI standard.