Confidence levels: How to find alpha if I know Z?

If I already know the value of $Z_\frac{\alpha}{2}$, how do I go about finding $\alpha$?

I'm after a formula, I know I can look up a Z table, but I'm making a computer program which needs to calculate alpha. Unless anyone knows any C# commands that will calculate it automatically!

Help will be much appreciated.

Another way of writing $Z_\frac{\alpha}{2}$ is $P(Z > x) + P(Z < -x) = \alpha$, so you get $P(|Z| > x) = \alpha$, where $x$ is your value for $Z_\frac{\alpha}{2}$.

Thanks for the help!

How is $P(Z > x)$ and $P(Z < -x)$ calculated?

What do you mean, as in how to work them out? They're equal to each other so essentially

$2P(Z > x) = \alpha$.

For instance, say your value of $Z_\frac{\alpha}{2}$ was $1.32$, $\alpha = 2P(Z > 1.32)$.

This gives us $\alpha = 2(1 - 0.9066) \Rightarrow \alpha = 0.1868$.