If it's an axial flow fan,it shouldn't have a duct attached to it these type fans don't operate very efficiently against static pressure, if it,s a squrrel type fan with no discharge duct then you have no static pressure and if you have attached duct(resistance) then I suggest you read this:
If you want to calculate it, you will need to evaluate the path the replacement air takes getting into the room. Then make a curve of dp verses air flow into the room. Overlay the fan curve and where the two lines intersect will be the calculated static pressure in the room.