This paper addresses issues associated with the accurate determination of mass fraction burned (MFB) in gasoline engines. Items covered include an evaluation of the accuracy of alternative MFB models and the effects of errors in the absolute pressure referencing, crank angle phasing and assigned compression ratio. The implications of using crank angle averaged pressure data and varying the crank angle resolution and number of engine cycles are also covered.The well known Rassweiler and Withrow MFB model was found to produce the best results in comparative tests with simulated and experimental pressure data. Absolute pressure referencing offset caused the largest error in the calculated MFB and burn angles, particularly at low engine load. Calculated data at the extreme ends of the MFB curve were shown to be most sensitive to measurement errors and noise. Burn angles calculated from cycle averaged pressure data were generally similar to the mean values calculated from individual cycles. One degree crank angle resolution and 150 cycles are considered to be adequate for the calculation of burn angle statistical data.