For a current project I need to calculate the velocity a rocket needs for a circular orbit.

The formula I found:

sqrt(GM / r)
G = Gravitational Constant
M = Mass of Planet
r = Distance from the center of the Planet

That results in this vizzy:

Or, if you can't see the picture:
sqrt((6.67428 * 10 ^ -11 * planet (name of Planet) Mass) / altitude ASL + planet (name of Planet) Radius)

However, using this vizzy I get too high values (like 70.9 Billion m/s for 45 km above Luna).

So, the obvious question:
What am I doing wrong?

Note:
I know you can achieve a circular orbit by comparing Apoapsis and Periapsis - that's what I did on starship. However, I need to actually calculate the velocity this time.

Edit: I just tested the program again (on Luna and Droo) and it suddenly worked. But then, I tried it on Brigo again and got those ridiculously high numbers from before again, meaning it isn't a wrong formula but sth strange about Brigo.
Edit 2: It works now. I have no clue why, but it works.