We report the design of a millimeter-sized parallel plate free-air ionization chamber 10 (IC) aimed at determining the absolute air kerma rate of an ultra-soft X-ray beam (E = 1.5 keV). The size of the IC was determined so that the measurement volume satisfies the condition of charged-particle equilibrium. The correction factors nec-essary to properly measure the absolute kerma using the IC have been established. Particular attention was given to the determination of the effective mean energy 15 for the 1.5 keV photons using the PENELOPE code. Other correction factors were determined by means of computer simulation (COMSOL and FLUKA). Measure-ments of air kerma rates under specific operating parameters of the lab-bench X-ray source have been performed at various distances from that source and compared to Monte-Carlo calculations. We show that the developed ionization chamber makes it 20 possible to determine accurate photon fluence rates in routine work and will consti-tute substantial time-savings for future radiobiological experiments based on the use of ultra-soft X-rays.