John Allen Paulos in his excellent Who’s Counting column at ABC News:
How much oil has leaked into the Gulf of Mexico? Many, including BP, have made estimates and remarked on the difficulty of determining the answer.
As Steve Wereley, an engineering professor at Purdue University, has shown, however, and as many others would have shown had pictures of the leak been released earlier, an approximate estimate is quite easy to come by and indicates a vastly greater oil spill than BP has admitted.
Wereley and a few other scientists, who have come to the same general conclusions, have performed a public service.
Basically, the method for determining oil spillage boils down to common sense and high school (or even middle school) geometry, specifically the formula for the volume of a cylinder.
More here.