I'm looking for a calculator 

* that can calculate (basic operations `+`, `-`, `*`, `/` would suffice, more are welcome)
* that can calculate including units (e.g. `2m² + 5000cm²`)
* can convert units (e.g. `m to ft`)
* is gratis
* works on Windows
* starts up quickly and is easy to use

Ideally (but optional)

* is portable

I do not want

* a full-blown mathematics solution like Matlab, Mathematica or similar