We have developed and characterized a force measuring system that is able to measure small forces down to 50 nN. The force measuring system and the characterizing method are described, thoroughly tested and analysed. The force measuring system consists of a linear positioner, a bending cantilever and an optical detection system for detecting the bending of the cantilever displacement. Characterization was carried out with a microbalance and a laser interferometer. The characterized measurement range of the system is from 50 nN to 1.5 mN. The performance of the system is limited by the linearity of the positioner and the noise of the optical sensor detecting the cantilever bending. The characterization system was found to be adequate for force sensors in the nanonewton range. The results are traceable to mass and acceleration of gravity, which makes the measurements reliable and comparable to other force measurements and applications.