Engine performance diagnostics play a major role in keeping track of gas turbine condition and performance and identification of possible errors or faults. One of the key performance indicators used in engine performance diagnostics is the Hot Day Exhaust Gas Temperature Margin (EGTM). Ambient humidity affects the EGTM, however humidity is not measured on current civil aircraft and consequently introduces inaccuracy in the reported EGTM. Research has been performed investigating the effect of humidity on turbofan performance, to provide corrections for the reported EGTM for ambient humidity. Test-cell corrections, simulations and historical data have been reflected and were found to be very comparable. Corrections have been developed increasing reported EGTM accuracy. However not negligible, the overall effect of humidity on the inaccuracy of the EGTM is found to be small. It is therefore concluded that correcting the EGTM for humidity is possible but will increase EGTM accuracy only slightly.