In radiation protection, the quantity ambient dose equivalent, in short H*(10), is very important in the field of area dosimetry, because this quantity is used to estimate the effective dose to human beings, whose permissible limits are restricted by law. The conversion coefficient from air kerma to H*(10), valid for a 137Cs photon field, is essential as it enters the realization of the quantity H*(10) as well as every (directly or indirectly) related calibration. The basic calibration factor of any dose rate instrument is usually related to a calibration in a 137Cs photon field, which underlines the importance of the conversion coefficient discussed in this article. Because different values of this coefficient are used in different calibration facilities, a harmonized best value will be proposed based on detailed investigations on the origin of the values recommended in the past. The newly recommended value differs by almost 1% from the value used by most institutes, at the moment, which is large in comparison with the typical uncertainties in primary fields. The goal of this article is to interpret known ICRU/ICRP data adequately, not to calculate new data.