The best replacement for mathcad is matlab.

The best alternatives to mathcad are rated as following:
★★★★★  matlab
★★★★  mathematica
★★★  maple
★  wolfram
  tedds
  tk solver
  scilab
  smath studio

Your alternative for mathcad is not listed? Suggest!
Speaking about mathcad, one probably shouldn't forget about smartsketch.

Other search terms related to mathcad and their ratings are:
★★★★★ smartsketch
You have in mind a companion for mathcad and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of mathcad in all years. Click on graph for more.
mathcad
matlab
mathematica