By observing the thickness of the film of oil and its appearance on the surface of the water, it is possible to estimate the quantity of oil spilled. If the surface area of the spill is also known, the total volume of the oil can be calculated.

Oil spill model systems are used by industry and government to assist in planning and emergency decision making. Of critical importance for the skill of the oil spill model prediction is the adequate description of the wind and current fields. There is a worldwide oil spill modelling (WOSM) program.

	Film thickness		Quantity spread
Appearance	in	mm	gal/sq mi	L/ha
Barely visible	0.0000015	0.0000380	25	0.370
Silvery sheen	0.0000030	0.0000760	50	0.730
First trace of color	0.0000060	0.0001500	100	1.500
Bright bands of color	0.0000120	0.0003000	200	2.900
Colors begin to dull	0.0000400	0.0010000	666	9.700
Colors are much darker	0.0000800	0.0020000	1332	19.500

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Oil Spills: K-12 Experiments & Background Information