This article studies the use of capacitive sensors for in situ calibration of the piezoscanner of a scanning tunneling microscope (STM). Nonlinearities, hysteresis, and creeps of piezoactuators are the main sources of the uncertainty in the tip position while scanning the sample surface to be imaged. In order to compensate for these effects in the reconstruction of the surface topography, capacitive sensors have been coupled to a piezoscanner of a STM operating in air. This scanner provides x, y, z displacements up to about 130 mum X 130 mum X 30 mum, and consists of bimorph plates mounted in a parallelogram arrangement. The capacitive sensors are inserted in a differential configuration to minimize capacitance variations due to changes in humidity and temperature of the air and to compensate for thermal drifts of the structure. As a performance test, the piezoscanner displacements were independently measured by means of a differential interferometer. A linearity within 3% and a deviation within 0.3% from the expected capacitance versus displacement dependence were achieved for scanner range up to 130 mum, with a resolution of 5 nm. Perspective improvements of the present setup are also discussed.