EngineeringPaper.xyz is a web app for engineering applications with free open-source code:

EngineeringPaper.xyz is a web app for engineering calculations that handles unit conversion/checking automatically and also supports plotting, solving systems of equations, and documenting your calculations (see the official blog for many examples). It’s easy to share your calculations by creating a shareable link that anyone can open and build off of. Additionaly, you can save and open your files locally if you prefer not to save to the cloud. EngineeringPaper.xzy runs on Mac, Windows, Linux, and ChromeOS and works on all of the major browsers. Additionally, EngineeringPaper.xyz is designed to run well on Android and iOS devices. Launch EngineeringPaper.xyz in your browser to try it out.

From the tutorial:

EngineeringPaper.xyz is a free and open source Mathcad® alternative for engineering calculations that automatically handles unit checking and unit conversions, can solve systems of equations, and supports plotting. You can also easily save and share your work by creating a shareable link. EngineeringPaper.xyz is a web app that doesn’t require you to create an account. The tutorial video below will get you started using this powerful tool that runs on any operating system including macOS, Windows, Chrome OS, and Linux. The table at the bottom of this page lists several example sheets that walk you through some of the more advanced capabilities of EngineeringPaper.xyz. If your interested getting involved with the development of EngineeringPaper.xyz, visit the EngineeringPaper.xyz GitHub project site.