You don't calculate this parameter.
You can either examine it in a test circuit or easier you read the pdf where it is explicitly given by a device manufacturer.
circuit simulators can help u out..
in .op analysis , normally vth of mos comes in output.
also some standard parameter are there in simulators to dump vth value
i know that it is specified by device manufacturer but how he gets to know it , there must be a way to calculate from where he gets the value and specify in datsheet ...in brevity, i would like to know the methods for calculating Vt
well i am looking for device level info but certainly wont mind getting some updates on other one as ic level will involve some more effects which need to be taken into consideration.