At the critical point in two dimensions, the number of percolation clusters of enclosed area greater than A is proportional to A−1, with a proportionality constant C that is universal. We show theoretically (based upon Coulomb gas methods), and verify numerically to high precision, that C=1/(8`3 p=0.022972037...). We also derive, and verify to varying precision, the corresponding constant for Ising spin clusters, and for Fortuin–Kasteleyn clusters of the Q=2, 3 and 4-state Potts models.