An accurate estimation of cycle-by-cycle in-cylinder mass and the composition of the cylinder charge is required for spark-ignition engine transient control strategies to obtain required torque, Air-Fuel-Ratio (AFR) and meet engine pollution regulations. Mass Air Flow (MAF) and Manifold Absolute Pressure (MAP) sensors have been utilized in different control strategies to achieve these targets; however, these sensors have response delay in transients. As an alternative to air flow metering, in-cylinder pressure sensors can be utilized to directly measure cylinder pressure, based on which, the amount of air charge can be estimated without the requirement to model the dynamics of the manifold. In this work, an air charge estimation algorithm is proposed, which uses the dynamic cylinder pressure trace data at discrete events in the cycle, and by applying thermodynamics and heat transfer relationships, estimates individual cylinder air charge for each cycle at different engine operating points. In addition, a residual gas mass estimator is incorporated in the algorithm. Estimator output is validated and calibrated based on experimentally determined air charge, which is calculated from the amount of injected fuel in each cylinder and individual wide-band sensor data. To investigate the uncertainty in estimated air charge based on the measured and model parameters, uncertainty propagation analysis is performed. This analysis reveals the information about the parameters with major contribution to the uncertainty in estimated air charge.