Elo Calculator - strictly implemented FIDE Rating Regulations
...There is a Windows only version (Windows 7 or higher, .NetFramework 4.7.2 or higher (you don't need to worry about this with a current Windows 10 or 11)) ... and a Java version (Java 17 or higher).
SZChess will be suite of chess applications covering every aspect of chess using computers. The development will be carried out in Java to ensure maximum portability.