Rectangular Hollow Section beam calculator
This is solely an
EulerBernoulli 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}
I {dimensions}
→
Z
→
M_{y} {σ_{y}}
→
M=FL or M=FL/4 {L}
* Use SI units *
You can use indicial notation
eg 123.45mm == 1.23453e1m == 123.45e3m
My calculation is based on a "sharp" rightangledcorner 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.
Stepbystep guidance for finding the maximum load and deflection
just before onset of yielding (bending) for a "regular" RHS
If you have a "regular" commercial Rectangular Hollow Section beam the
"steel properties"

"RHS material Young's modulus"  any structural steel, regardless
of yield stress, can be taken as having a Young's Modulus "E" of
210GPa  for which you enter 210e9

"RHS material yield stress"  the steel will be specification "S355",
with a yield stress of 355MPa  for which you can enter 355e6
(in the rare case of the beam having a higher yield stress steel the
answer will be conservative ("to the safe side")).
Beam dimensions, known by measurement, are entered here in the SI unit
of metres. Hence "100mm" is entered as 100e3, etc.
A representative example features in the mechanical tests in
Weld test  weld at centre of centrallyloaded simplysupported beam
(the presence of the buttweld (seamweld) is irrelevant to the
calculation).
The commercial section is the nominal 100x50x8mm RHS.
The accurately measured wall thickness of 7.8mm is used in this
calculation and the linked example. The difference between nominal
and accurately measured wall thickness is withintolerance and
negligible (too small to matter) given all other uncertainties like
the effect of corner radius, etc.
For the 100x50x7.8mm RHS which is 0.6m between bearers (endsupports),
enter
 100e3 for Height
 50e3 for Width
 7.8e3 for Wallthickness
 0.6 for beam length
then
 355e6 for yield stress
 210e9 for Young's modulus
Your "real" input form should then look something like this:
Clicking on "Submit", the answer includes:

Second Moment of Area = 2.443193e6m^4 (244.31934592cm^4) [244cm^4]

Moment = 17346.6735603 (N.m) [17.3kNm]

For a "simple beam" (it's supported at the ends, with a point central
load), that central load on reaching the yield stress is
115644.490402Newtons [116kN]

The deflection is 0.00101428571429metres (1.01428571429mm) [1.0mm]
"The Blue Book" gives for a commercial 100x50x8mm RHS a Second Moment
of Area of 230cm^4. Which agrees with the calculated 244cm^4,
allowing for the real commercial section having rounded corners, which
slightly reduces the Second Moment of Area
Moment resistance "Mc,y,Rd" is given as 21.8kNm. Which is somewhat
higher than calculated "Moment" of 17.3kNm. That calculated "Moment"
is "Moment resistance" given the "Maximum stress" has been set to the
yield stress of 355MPa.
The calculated "Maximum stress" and "deflection at the middle of the
beam" are "extrinsic", dependent on the length of the beam, so are not
tabulated in "The Blue Book".
But are seen, with "arithmetic workings", in the
linked page.
In that page "F_max=115644 N" [116kN] and "y=1.014286e03 m" [1.0mm] 
which agree with output of the online beam calculator.
The arithmetic in this online beam calculator to find the deflection
as well as the load is thought to be novel (?), and is
presented here. In one of the pages
linked from there
is discussion showing that it is very safe to assume that any
Rectangular Hollow Section encountered in structural steelwork has a
yield stress of 355MPa.
Please use the
Contact form
to alert me if you know of any case where this is not correct.
(R. Smith, 24Jul2023, 03Aug2023 (title), 12Feb2024 (worked example), 13Feb2024 (eg. form))