This is solely an Euler-Bernoulli beam [Wikipedia] leading into simple "Moments sum to zero" calculator.
It knows nothing of design codes (eg the "EN 1990" series of
the "Eurocodes").
What this calculator does is: [links are to Wikipedia]; {are the input variables}
* Use SI units *
You can use indicial notation
eg 123.45mm == 1.23453e-1m == 123.45e-3m
My calculation is based on a "sharp" right-angled-corner shape.
In the real world Rectangular Hollow Sections have curved corners.
The deviation caused by this from real sections is small - but there.
Authoritative tabulated data for commercial sections including
important characteristic which cannot be readily calculated are
presented as
The Blue Book - Steel for Life
[external link]
which is considered a vital information for most working with steel.
The program might allow you to rapidly explore concepts, but
The Blue Book - Steel for Life
[external link] needs to be your reference.
(R. Smith, 24Jul2023, 03Aug2023 (title))