For a two-jet final state, a useful measure of acoplanarity is
where  are the azimuthal angles of the final state jets with respect to the beam line.
An alternative measure of acoplanarity which is infrared safe and which works for broad jets of many particles is given by
where  are the momenta of the final state particles and  are the components of these momenta perpendicular to a plane chosen such that A is minimized.
In the case of two coplanar final state particles, the plane which minimizes A would contain the paths of both particles and the beamline, and A would equal 0.