Quantitative precipitation estimates (QPE) based on radar data depends heavily on accurate relationship between radar reflectivity (Z) and rainfall rate (R), i.e. Z = aR^b. Based on 3 years of data for training and 1 year of data for verification, HKO arrived at the following set of a and b values, which were found optimal for Hong Kong and vicinity: Z = 58.53 R ^1.56 HKO is currently reviewing the merits of adjusting a & b values based on real-time radar and rain gauge data. The calibration methods will be summarized and published.