Hi,
If I am predicting the code (NBC) as National Building Code in Canada, Then here is the equation for calculating the wind pressure;
For Internal Pressure;
p = Iw * q * Ce * Cgi * Cpi
Iw = Importance Factor,
Rest of the values are available in code it self.
For extrenal pressure formula is almost same, but values will change.
Hope this will help.
Regards
R.K.