The 2 first calculators I used are for "simply supported beam" with support on both ends so PL^3/48EI.Hehe, so you say that if you fix both ends to a wall it will be the same thing when you double the length? And then complain about me being a mechanical engineer? It is far from the same thing... In one case with 1 edge free the formula for deflection is PL^3/(3EI) and where it is 2 fixed edges the formula for deflection is: PL^3/192EI So what you basically are saying with your comment that double length of beam will compensate for that one edge i free means that you think that 2^3 makes up for the 192/3, but 2^3 = 8 and 192/3 = 64 so you are waaaay off. However 4x4x4 = 64, so if you make your beam 24 meters instead it will actually be really close, atleast up to when deformation start to occur.
Can you explain how you calculated when you came to the conclusion that double length and 2 fixed edges give the same deformation?
P = Force at edge of beam when free and at middle when beam is 2 times longer so still 6m from the edge.
L = length of beam
Thus cantilever beam has 2x deflection compared to simply supported beam with 2x span.
I throw the 2 first calculators just to trying to wake you up that you or ghatGPT made gross error somewhere.
Like I wrote: "beam supported from both ends but you get the idea:"