The calibration coefficients of kerma-area product meters significantly depend on the energy spectrum of the x-ray beam. This effect was examined by measuring the calibration coefficients for several radiation qualities in the range generally used in diagnostic x-ray imaging. The intention was to determine the calibration coefficients for other radiation qualities by interpolation between the measured values, relative to one or more suitable parameters. The x-ray tube voltage, total filtration and half-value thickness were examined as possible specifiers of the energy distribution. No single parameter provided an interpolation of calibration coefficients with the accuracy recommended by the ICRU and IAEA, except for a narrow range of radiation qualities. At least two of the parameters are needed to reliably specify the radiation quality for the interpolation of calibration coefficients.