Calculate Back EMF from Torque Constant
OK I'm going to turn the comments into an answer since the question raises a surprisingly subtle point, and one that is most easily grasped if you consistently use the SI system rather than traditional units.

Because a motor translates electrical power into mechanical power (and vice-versa in generator mode) it must obey the conservation of energy.

So (ignoring friction, resistive and other losses) power in = power out.

Or, voltage * current = rotational speed * torque.

Rearranging, Voltage/Speed = Torque/Current.

Torque/Current (Nm/A) is known as the torque constant Kt.

Speed/Voltage (rad/s/volt) is known as the speed constant Kv (commonly seen is RPM/V but here expressed in SI units.

So, given the torque constant for a motor, the speed constant is also known, and presumably its inverse is known as the back EMF constant in some circles (though I haven't personally ever seen that).

EDIT : Following Gregory Kornblum's comment : who says it's the same power? The principle of conservation of energy.

Now clearly this is the simplest, most ideal situation - as I said above, ignoring all losses. You can define anything any way you like, but the most generally useful approach is to start with the simplest ideal situation, then separately account for energy losses until you have a satisfactory model for your purpose.