A semi-empirical cleavage fracture stress (CFS) model, mainly depending on the CFS, σf, has been derived for estimating the ASTM E1921 reference temperature (T0) and demonstrated for ferritic steels with yield strength in the range 400–750 MPa. This requires only instrumented impact test of CVN specimens without precracking and static yield stress data. The T0 estimate based on the CFS model, TQcfs, lies within a ±20°C band, being conservative for most of the steels, but less conservative than TQIGC based on the IGC-procedure (see Nomenclature for definition). Applicability and acceptability of the present calibration curves for highly irradiated steels need further examination.