More about Impact
Usually forex calendars provide users with color-coded indicators representing impact intensities. These representations are most likely derived from subjective observation.
Our Impact, or rather post-event market volatility values are derived by calculating the maximum absolute price excursion that the related composite currency makes within an hour after the event for each instance of event and then averaging those values across each event. This allows us to say, in a nutshell, that event X has moved a maximum of Y% on average historically across all component cross-currencies that make up the related composite currency.
The caveat of this method is that often times low impact events are released at the same time as high impact events and therefore their Impact values become skewed, so some common sense is needed to interpret this information.