A calibration technique for a vector network analyzer (VNA) enabling calibration standards to be included internal to the VNA. To calibrate the VNA utilizing the internal calibration standards, error terms a, b and c of two two-port error boxes E are defined between the measurement ports and the reflectometer of the VNA wherein a=-det(E), b=e00 and c=e10. Error terms a, b and c are determined by measuring external calibration standards with known reflection coefficients connected directly to the measurement ports. Reflection coefficients for internal calibration standards are then determined using the error terms a, b and c to enable future automatic calibrations. To measure S-parameters of an arbitrary device under test (DUT), one embodiment of the present invention uses the Ferrero technique to measure a reciprocal thru with estimated S.sub.21 characteristics connected between ports A and B to determine an additional error term .alpha. for the error boxes E, where .alpha.=e01.sub.A /e.sub.01B. In another embodiment a thru is measured which is not required to be reciprocal or to have known S.sub.21 characteristics, both alone and in series with the DUT.