A motor nameplate sells you power. The coupling, the gearbox and the shaft key all care about torque. They are the same fact at different speeds, and one relation crosses between them.

$T = \dfrac{P}{2\pi N}$, read aloud T equals P over two pi N. $T$ is the shaft torque in newton-metres; $P$ is the mechanical power at the shaft in watts; and $N$ is the rotational speed in revolutions per second. That last one is where people lose marks, so say it twice: the SI form wants rev per second, and every nameplate in the plant quotes rev per minute.

Fold both conversions in once and you get the working form: with $P$ in kilowatts and $N$ in rpm, $T = \dfrac{9549\, P}{N}$ newton-metres. The 9549 is just $\dfrac{60 \times 1000}{2\pi}$ — a thousand watts per kilowatt, sixty seconds per minute, and $2\pi$ radians per revolution, all in one number. Its imperial cousin, $T = \dfrac{5252\, \mathrm{hp}}{N}$, is the same arithmetic in a different set of clothes.

The habit worth building: torque and speed trade against each other at fixed power. Put a gearbox in and halve the speed, and the torque doubles — which is why the low-speed shaft is always the fat one, and why a slow motor of the same rating is a bigger, heavier, more expensive machine than a fast one.