In this paper, we present a method for accurately calculating the Error Vector Magnitude (EVM) of OFDM transmitters based on their IQ mismatch, IIP3, noise, AM/AM and AM/PM distortion. The effects of these impairments are correlated. Thus, modeling and analyzing them in isolation results in large errors. We derive the analytical relation of the received symbol when all types of impairments are present at once and compute EVM based on this overall relation. This method helps test engineers to compute the EVM based on already measured parameters and eliminates the need to develop and set-up EVM measurements. Simulations and hardware measurements show that this calculation can be done with less than 1% error for a large range of EVM values. This error level has been shown to be within the uncertainty bounds of EVM measurements.