The gain measurement of an antenna is usually performed by the gain comparison technique. This tecnique requires that an antenna, whose gain is know, be provide. In the microwave band this antenna is usually a standard horn whose gain has been calibrated in a previous measurement. So, the first step is to measure the absolute gain the standard horn, which is often performed by determining the transmission loss versus separation between two identical horns. If the test environment is compatible with the measurements to be madde, and if the test range has sufficient length to adequately supress interation between antennas, one is only left the problem of the determination of the gain corrections factors. This problem is just the objective of the present work, and it will be discussed for the case of conical horns. By using the near field power transmission formula, and by assuming the field in the aperture horn to be the same as the TE11 mode of a circular waveguide but with a spherical phase front, the gain correction factors have been computed as a function of the horn dimension and test range length. The factors were presented under the form of a set of curves.