ECDSA x,y coordinate validity verification doesn't seem to work The ECDSA algorithm, secp256k1 for Bitcoin allegedly uses the equation

y ^ 2 = x ^ 3 + 7 mod P

to determine the validity of an alleged point upon the elliptical curve.  Utilizing 
http://web2.0calc.com/

When verifying public key 1 which has the following qualities:

x = 55066263022277343669578718895168534326250603453777594175500187360389116729240

y = 32670510020758816978083085130507043184471273380659243275938904335757337482424

I applied x ^ 3, then added 7, then mod P on that webpage.  Then I square rooted it and got

180964706334543048141953325634608696715.426683454595872

Which is not y.  How am I doing this wrong?  My results suggests that this point is not a valid point on the curve, but everyone knows it is.  Obviously I'm the one who's wrong.

EDIT:  I'd like to further clarify the question.  My question is, how does one actually determine (as an example) the Y value while only having the X value.  Bitcoin does it, the "Bitcoin Address Utility" also does it.  When someone has a compressed key (which only contains the x coordinate) they are able to get the y coordinate as well.  Utilizing the aforementioned webpage calculator does not work and others are saying its "modular root".  Anyone have Python 2.7.7 code which would do this or have a relatively simple way of explaining how to accomplish this whole thing?  Thanks.
The plain text format of this equation only makes sense when you understand the theory behind it.  y ^ 2 = x ^ 3 + 7 mod P means "do all math in this equation using the finite number field with definition P".  It might better be written (y^2 = x^3 +7) mod p.

To keep the math simple, lets simply subtract one side from the other to see if that evaluates to zero.  This lets us avoid roots. Here's code using the Python interactive shell, e.g. python from a command prompt):

&gt;&gt;&gt; x = 55066263022277343669578718895168534326250603453777594175500187360389116729240
&gt;&gt;&gt; y = 32670510020758816978083085130507043184471273380659243275938904335757337482424
&gt;&gt;&gt; p = 115792089237316195423570985008687907853269984665640564039457584007908834671663
&gt;&gt;&gt; (x**3 + 7 - y**2) % p
0