Source iteration and fission matrix methods are two techniques used for Monte Carlo K calculations. There are three different, but correlated, estimators [Bri88] that can be used to estimate K using the two algorithms. They are collision, absorption and track length estimators. These estimators are used to estimate K for a particular batch. Averaging over all the active batches gives the final estimate of K for each of the three estimators. Active batches are the ones that are used to accumulate K after the fundamental mode source distribution has been reached.