WebOCalc was written from the start with three goals in mind: it had to be very easy to use, it had to work on your computer whether you use Linux, FreeBSD, Mac OS, or Windows, and it had to be Free as in freedom (Free software, like Linux or Firefox). It also works whether you live in the USA and are stuck with imperial units, or whether you live elsewhere in the world and use metric units.
If you enter a few numbers into WebOCalc (things like wingspan and chord, the weight of your model, the battery voltage, and the desired current draw), the program will tell you how the model will fly, in simple English. It will also suggest some propellers that will work with the model, and appropriate gear ratios matching each propeller. For direct-drive, pick the propeller(s) with a gear ratio near 1.0.
All this means you will know before you build