How to calculate resultant bitrate of X264 codec?

[Tuning]

X264 codec has quantizer range from 0 to 51.

Then how should these quantizer values be used to calculate resultant bitrate?
The default value is Q 26 and i think it is lower than required.

Any ideas why there is huge difference in quantizer values of XviD and X264 ?

Thanks.

Tuning