A differential amplifier frequent use is the amplification of the voltage difference between its input signals, while rejecting the common-mode level. However, the output common-mode level cannot be zero. The operational amplifier technological limitations, as well as the outside resistor tolerances let the common-mode voltage to make it to the amplifier output as an output error. As a consequence, the amplifier output voltage is the input signal difference times gain, plus the output common-mode voltage.
Based on the resistor tolerances, this calculator will show the output common-mode voltage, Vocm, and the total voltage, Vout, at the differential amplifier output. Vocm will alter the ideal output voltage, which can be calculated when the resistors are perfectly matched.