motor inductance > max rps

Hi,
I am new in the forum, did a small search and couldn´t find anything to my problem:
I just cannot get any further regarding your guide about choosing the right steppermotor (https://duet3d.dozuki.com/Wiki/Choosing_and_connecting_stepper_motors#Section_Inductance) in the section "Inductance":
I am currently trying to recalc your "max rev speed"example and getting strange results: The given example INDUCTANCE: (2 * 12) / ((360 / 1,8) * pi * 4 * 1,5) = 0,006366197724 (?) and more confusing (2 * 20 * 2 * 12) / ((360 / 1,8) * pi * 4 * 1,5) = 0,2546479089 (?)
Also posted this question on the website as comment. What am I doing wrong?
Hope somebody will help!

There is a calculator to woroi this out at http://reprapfirmware.org/.

Wow, that was fast, thanks so much!
But to further educate myself, can you tell me why I am getting those strange results even when I pick the numbers provided on the website (doublechecked)?
(I am trying to setup a calculationsheet where those values are taken into another calculation...and that is taken into another calculation...) 
Your motor inductance is 4mH not 4H.

@dc42 THANKS! ah  of course...
so I just divide that value by 1000 and should be fine (https://en.wikipedia.org/wiki/Henry_(unit))