How do I install NumPy under Windows 8.1 ? Similar questions/answers on overflow hasn't helped.

Have you tried

python -m pip install numpy

and as you are using pyCharm you can go to:

1. ctrl-alt-s
2. click "project:projet name"
3. click project interperter
4. double click pip
5. search numpy from the top bar
6. click on numpy
7. click install package button

and if it doesnt work : check this