Radar Cross Section (RCS) is measure
of the amount of electric field scattered
by an object due to the electric fields incident on that object.
The RCS in three-dimensional space is defined as :
RCS = 4*pi*(r^2)*(|Scattered Electric Field|^2)/(|Incident Electric Field|^2)
The units of RCS are square meters in SI. The incident electric field intensity is measured at the target, and the scattered electric field intensity is measured at an observation point in space away from the target. r is the radial distance from the target to the observation point.
It is important to point out that electromagnetic scattering is a very complicated topic, and is very difficult to describe accurately without advanced analysis techniques. In general, the RCS of a target object is highly dependent on both the orientation of the object to the incident field, the orientation of the reciever with respect to the object, and the frequency of the wave.
The RCS will typically vary by many orders of magnitude as the orientation of the target and the radar frequency is changed. Because of this, the RCS is typically shown in decibel square meters (dBsm), calculated as
RCS (dBsm) = 10.0 * log10(linear RCS)
Therefore, it is incorrect to state that an object has a single definite value of RCS, since this value is dependent on many different variables. Average RCS over some angular sweep is generally specified for objects just as aircraft and missiles, however, RCS can vary by as much as 30-40 dB as angle changes.
You can buy software that actually numerically predicts the RCS of computer-generated objects at http://www.radarsoftware.com.