The accuracy of the Attitude Determination and Controls System (ADCS) for CubeSat-class satellites is crucial to their required space-flight functionality. For such satellites, with a magnetic-based attitude system (i.e. involving magnetometers and torque coils); a Tri-Axial Square Helmholtz Cage serves as a ground-based simulation and testing platform to both verify and validate the desired pointing accuracy of the satellite. The University of Illinois' Tri-Axial Square Helmholtz Cage is known as the HC3. The calibration of such a system is critical to validating the magnetic-based ADCS of a small satellite. The calibration and characterization process of the effective output magnetic field generated by the HC3 is discussed in this paper. For the validation of magneticbased attitude systems, the center of the HC3 was required to be calibrated to within 0.5% error in the magnitude of the magnetic field and to within 0.5° of angular error in the magnetic field, both with respect to the commanded value. Results demonstrated that at the center of the HC3, the calibrated model produced within 0.3% error in magnitude of the magnetic field and within 0.4° of angular error in the magnetic field, again both with respect to the commanded value; therefore, satisfying the requirements at the center of the HC3. The calibrated model, at the center of the HC3, performed about an order of magnitude better than the un-calibrated model for most test cases.