To fully understand the environmental and ecological impacts of the Deepwater Horizon disaster, an accurate estimate of the total oil released is required. We used optical plume velocimetry to estimate the velocity of fluids issuing from the damaged well both before and after the collapsed riser pipe was removed. We then calculated the volumetric flow rate under a range of assumptions. With a liquid oil fraction of 0.4, we estimated that the average flow rate from 22 April 2010 to 3 June 2010 was 5.6 × 10(4) ± 21% barrels/day (1.0 × 10(-1) meter(3)/second), excluding secondary leaks. After the riser was removed, the flow was 6.8 × 10(4) ± 19% barrels/day (1.2 × 10(-1) meters(3)/second). Taking into account the oil collected at the seafloor, this suggests that 4.4 × 10(6) ± 20% barrels of oil (7.0 × 10(5) meters(3)) was released into the ocean.