Fast accurate measurements of air-fuel ratio are desirable in order to provide the best possible transient control of individual cylinders, both in the lean region and during warm-up. This paper compares the accuracy and performance of two algorithms previously reported in the literature. The first algorithm is based on using moments of the cylinder pressure time history. The second algorithm uses the ratio of the cylinder pressure before and after combustion.The accuracy and computational load of each algorithm was determined. The best accuracy achieved was on the order of 3% for equivalence ratio estimates between 0.8 and 1.1. The error based on one pressure cycle measurement was found to be strongly dependent on the measurement noise. Estimate errors as a function of converter accuracy vs the computational loads of averaging a greater number of pressure measurements is examined.The pressure ratio algorithm was found to require the least computational load. The moment algorithm was found to be the least sensitive to measurement noise and to have the best performance in the rich region of operation; but at the cost of an order of magnitude increase in the computational load. Both the algorithms are capable of being executed in real time on automotive engines for each engine cycle with today's microprocessors.