An RF Phased array can steer the direction of the beam electronically and it brings about benefits in terms of signal to noise ratio (SNR) and directivity. However, testing the phased array generally requires expensive and high performance RF equipment. This increases production test cost and hampers in-field calibration. We present a low-cost, self-compensating Built-In Self-test (BIST) and calibration solution for RF phased arrays. In our proposed method, we apply a sinusoidal test signal with unknown amplitude to the inputs of two adjacent phased array elements and measure the baseband output signal after down-conversion. Mathematical modeling of the circuit impairments and phased array behavior indicates that by using two distinct input amplitudes, both of which can remain unknown, it is possible to measure the important parameters of the phased array, such as gain and phase mismatch. The BIST circuits are designed and post layout simulations are performed with within-die and die-to-die process variations. Simulations confirm that the BIST circuit provides very accurate results without having to know sinusoidal signal amplitudes or the relation between them. Furthermore, a prototype four-element phased-array PCB was designed and fabricated for verifying our proposed method. With the proposed method, the phase difference between elements can be measured and calibrated with less than 1°error, which would allow for self-monitoring in 6-bit phased array applications.