Relay: how to calculate the maximum current and switching current
You don't normally calculate this stuff yourself, because the relay manufacturer doesn't give you all the information you would need.  Instead they give you a whole range of information about currents, voltages and load types.

The ability of a switch to open a circuit reliably over a large number of operations depends on the circuit voltage (how many volts will be across the contacts once they're open), the load current, the contact material, the speed of operation, the gap in the contacts, any other arc quenching measures (blowout magnets,etc).  

You pretty much have to rely on the relay manufacturer to tell you this stuff, or at least the consequences of it.  If they don't then you can assume it's a low-quality or unsuitable relay which will fail, thereby saving you needing to do any further thinking.