  F\"{o}rster Resonance Energy Transfer Calculator (FRET$-$Calc) is a program
and web server that analyzes molar extinction coefficient of the acceptor,
emission spectrum of the donor, and the refractive index spectrum of the
donor/acceptor blend. Its main function is to obtain important parameters of
the FRET process from experimental data, such as: (i) effective refractive
index, (ii) overlap integral, (iii) F\"{o}rster radius, (iii) FRET efficiency
and (iv) FRET rate. FRET$-$Calc is license free software that can be run via
dedicated web server (nanocalc.org) or downloading the program executables (for
Unix, Windows, and macOS) from the FRET$-$Calc repository on GitHub. The
program features a user$-$friendly interface, making it suitable for materials
research and teaching purposes. In addition, the program is optimized to run on
normal computers and is lightweight. An example will be given with the step by
step of its use and results obtained.
