The definition of tissue–air ratio (TAR) is based on the concept of primary dose. To determine TAR, both in‐phantom and in‐air ionization measurements are utilized. To convert ionization in the phantom into dose and that in air into primary dose, correction factors must be applied to chamber readings in both geometries. Due to difficulties in selecting proper correction factors, TAR is subject to systematic error. The error comes from two sources of uncertainty: (1) Primary dose cannot be measured. Therefore approximate methods, such as in‐air ionization measurements, are used. (2) Detectors of ionization are of finite dimensions and they are inhomogeneous. In this study, analytical expression for a systematic error is derived. Because in this derivation systematic error is an accumulative error, it is no longer necessary to convert ionization, both in air and in phantom, into a dose when calculating TARs. A method of determining systematic error is described. This method is based on the ability to produce accurate zero‐field data in photon beams by means of a linear extrapolation technique. Using60Co γ radiation in water as an example, it is shown how to generate TAR data free of systematic error. A possibility of determining TARs for therapeutic x rays is discussed.