When analysing the performance of aero-engines with incomplete input data, it is often necessary to estimate the value of the missing parameters. This paper describes the work completed to develop correlations linking various turbojet parameters using a database of nearly 100 data points. The study was limited to small single-spool turbojets with static thrust up to about 8896 N (2000 lbf). For afterburning turbojets, the information for dry operation was used. Two approaches were implemented. A Single-Input-Multiple-Output (SIMO) approach was tried with thrust as the single input variable. This approach met with success, but for only a few cases. A Multiple-Input-Single-Output (MISO) approach was tried with the specific thrust as the single output parameter. This approach worked well only when the number of known input variable values was large. For example, using the correlations derived in this research, a propulsion analyst could estimate the specific thrust from known turbine inlet temperature, fuel-to-air ratio and the pressure ratio with a reasonable degree of confidence. In the process of deriving SIMO and MISO correlations, a number of Single-Input-Single-Output (SISO) correlations were also developed. Some give fine results while others lead to excessive scatter. The paper describes the process of deriving the correlations and reports on the quality of the fits.