Generally, measurement microphone calibrations consist of a relative frequency response calibration and a sensitivity calibration made at a reference frequency, 250 Hz or 1 kHz. As these frequencies are also the main calibrator operation frequencies, they are the most important calibration frequencies for microphones. The paper describes uncertainty of sensitivity calibrations at these frequencies. DPLA (Danish Primary Laboratory of Acoustics) makes primary calibrations by the pressure reciprocity technique in accordance with IEC 1094-2. The uncertainty of these calibrations influences the secondary calibrations performed by DPLA customers like the B&K Calibration Service Centres which perform comparison calibrations. Within recent years Primary Calibration Laboratories in Europe have taken several steps to reduce their calibration uncertainty. This uncertainty is related to the measurement of electrical transfer impedance of coupled microphones and other physical parameters, in addition to other factors such as calculation software and the operator. Also, the uncertainty of environmental sensitivity correction factors contributes significantly to the overall uncertainty of the measurement chain. Therefore, some coefficients are determined individually. The paper describes the weight of uncertainty components and resulting uncertainties. Today, ISO9000 and IEC/TC29 increase the need for uncertainty analysis.