In the SI system of units, angular momentum is measured in kg.m2.sâˆ’1. In contrast, linear momentum is measured in units of kg.m.sâˆ’1 so the two are not compatible and cannot be added.
To calculate the angular momentum of a rigid body or system, a point called the origin is chosen. For convenience this might be on the axis of rotation of the rigid body or the center of mass of the system. The distance from the origin to each element of the rigid body or system is determined. This is then multiplied by the transverse component of the linear momentum of each part. The angular momentum of the rigid body or system is the vector sum of all the elements. (wikipedia)