I dunno if anything is open source here... but the most important parameters are available on Javascript applications all over the place.

https://saturnpcb.com/saturn-pcb-toolkit/

I don't think this tool is open source, but via inductance, various heat-equations, trace-width calculations and the like are available on this tool.

Its my understanding that higher-end PCB software integrates tools like these directly into the PCB-designers.

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KiCAD has simple calculators like the SaturnPCB.com one (though with fewer features). I'm not sure if more advanced calculators exist though.