if there a way/method/formula to convert an analogRead value to a voltage value?

I think I could use the 'map' function to do the conversion. So I guess I'm wondering if the analog to digital converter is accurate enough to do this? I'm assuming that the value of 1023 would be 5V? 511 would be 2.5V? ect...

I just want to know the input voltage value of a piezo element when I 'tap' it or 'knock' on it.

Thanks