This paper addresses issues associated with the accurate determination of indicated mean effective pressure (imep) for internal combustion engines. Items covered include a comparison of alternative imep equations and the effects of crank angle resolution, signal noise, thermal shock, crank angle phasing and connecting rod length errors. The best imep equation to use has been identified and the magnitude of errors under a wide range of conditions quantified. The results show that inaccuracy in the calculated imep will mainly be caused by thermal shock and errors in the crank angle phasing and transducer sensitivity. In contrast, the effects of coarse crank angle resolution, incorrectly specified connecting rod length, signal noise and integration period error should be relatively small.