Percolation in Finite Matching Lattices

We derive an exact, simple relation between the average number of clusters and the wrapping probabilities for two-dimensional percolation. The relation holds for periodic lattices of any size. It generalizes a classical result of Sykes and Essam and it can be used to find exact or very accurate approximations of the critical density. The criterion that follows is related to the criterion Scullard and Jacobsen use to find precise approximate thresholds, and our work provides a new perspective on their approach.