We would need to know the geometry of the system. The formula for the moment of inertia about the axis the axis perpendicular to the plane for common geometries are
Solid cylinder: I = (1/2) MR^2
Hollow cylinder: I = MR^2
Solid Sphere: I = (1/12) ML^2
Square Plate: I = (2/3) MR^2 (where R is half the length of the diagonal)
