I develop several statistical indices of cratering on planetary surfaces based on Poissonian probability. These cratering formulas, being both analytic and probabilistic, have advantages over numerical or nonprobabilistic approaches in ease of calculation, clarity of interpretation, and evident flexibility. Specific indices developed include the fraction of a planet's surface expected to be cratered N occasions over a given time interval, expected uncertainty in crater coverage, the depth and distribution of developed megaregolith, and the evolution in crater population. For instance, under current conditions, 15% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the median case, ejecta from these impacts would have blanketed the planet to a depth of 313 m. These indices, of course, depend upon asteroid flux and the minimum asteroid size imposed by atmospheric filtering. Analytical formulas make it simple to account for such variations by feeding in their history from current conditions to those present on the early Earth. If, as is thought, Earth's bolide flux rate has decreased by a factor of 10,000 from its formation until now, then 99.99% of the Earth's surface should have been cratered one or more times in the past 3 billion years. In the first 100 Ma of the early Earth, 90% of its the surface would have suffered more than 50 impacts, even considering the protection of the atmosphere. Ramifications of these bombardment statistics pertain to the survivability of crustal fragments and early life forms.