What Do You Use To Calculate Power/Voltage Drop?

What tools do you designers use to figure factors like voltage drop and power distribution during designs?

I have never used a calculator or a software tool for this. I always did it manually, and I wasn't especially consistent in calculating values. I just over-engineered (wasted money?) a lot following the precept "When in doubt, make it stout."

There has to be a better, easier method out there.

What tools do you use? Any nifty apps or websites to share?

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