How do I get a formula to tell the maximum load I can place at the center of a beam that is supported on both ends, of any given lenght if I have a stack of said beams, measuring tape and some set weights.

As in - say I measure and cut a beam with the lenght of 3 meters and keep adding weights to until it cracks. I now know the weight a 3 meter beam can take - how do I turn this in to a formula to calculate how much load a 5m beam can take?

Also, same but for uniformly distributed loads?