How to calculate conversion?

Conversions (1-per-click) count a conversion for every Ad Words ad click resulting in a conversion within 30 days. This means if more than one conversion happens following a single ad click, conversions after the first will not count.

Another way to say this is that conversions (1-per-click) will count at most one conversion per click. These metrics are useful for measuring conversions approximating unique customer acquisitions (e.g. leads).