In the following formula for creating a confidence interval for a mean where do I get $\displaystyle Z_{\alpha/2}$ from? $\displaystyle \bar{X}\pm Z_{\alpha/2}\frac{\sigma}{\sqrt{n}}$

I know it has something to do with using z-scores and I've been trying to work backwards from examples. For example a confidence interval of 90% should have $\displaystyle Z_{\alpha/2}=1.645$ but when I look up 0.9 I get 0.8159.