A calibration apparatus includes a calibration section and a conversion section. The calibration section performs calibration with first and second image reading apparatuses which read in-line a color patch formed on paper by an image forming apparatus. The first image reading apparatus includes a first device which reads the color patch and outputs the reading result as a first signal. The second image reading apparatus includes a second device which reads the color patch and outputs the reading result as a second signal having a format different from a format of the first signal. The conversion section converts the first signal into an after-conversion first signal having the format of the second signal. The calibration section performs the calibration in such a way that an output value of the second signal is adjusted to an output value of the after-conversion first signal.