How to calculate the torque constant for a BLDC motor
The equation for the torque constant of a BLDC motor: \$K_{\tau} = \frac{60}{2{\pi}K_{v(RPM)}} = \frac{1}{K_{v(SI)}} \$, where \$K_{\tau}\$ is the torque constant in \$\frac{N \cdot m}{A}\$, and \$K_v\$ is the speed constant, in either rpm or rad/s.
The speed constant given to you is almost certainly in rpm, not rad/s, so you would use the first equation with the unit conversion factor. You can verify the speed constant with an encoder/resolver, another motor, and an oscilloscope. Spin the test motor up to a constant velocity using the other motor (applying a constant voltage/duty cycle to the drive motor likely sufficient). Measure the induced backemf voltage from test motor with your oscilloscope, and the steady-state velocity of the test motor with the encoder/sensor (or by using frequency detection on your oscilloscope and scaling it based on the number of pole pairs). The steady state speed divided by the induced backemf will be the the Kv value of the motor (and will likely be more accurate than the one provided by the motor manufacturer/seller).