The BHCA for a switch can not be calculated, in terms of the GSM switch, the BHCA is the capability of the switch to process the number of call attemps.
When you make the traffic calculation and you get the BHCA, the your HBCA must be lower than the BHCA of the switch.
You can calculate the Erlangs and BHCA of one network, the you need to know some values:
The numbers of subscribers, the average trafic per subscriber (ussualy 0,1 erlang), the Holding Time and the grade of service (GOS), with these inputs ant the righ toll you can get erlang, Ports, Trunk and BHCA