Question. Context: The Deepwater Horizon oil spill (also referred to as the BP oil spill / leak, the BP oil disaster, the Gulf of Mexico oil spill, and the Macondo blowout) is an industrial disaster that began on 20 April 2010, in the Gulf of Mexico on the BP-operated Macondo Prospect, considered to be the largest marine oil spill in the history of the petroleum industry and estimated to be 8 % to 31 % larger in volume than the previous largest, the Ixtoc I oil spill. The U.S. government estimated the total discharge at 4.9 million barrels (210 million US gal; 780,000 m). After several failed efforts to contain the flow, the well was declared sealed on 19 September 2010. Reports in early 2012 indicated that the well site was still leaking.

Question: how much oil was spilled in the deep water horizon spill?
Answer. 4.9 million barrels ( 210 million US gal ; 780,000 m )
What step-by-step reasoning justifies that answer?
Reasoning: To answer the question, consider the following: The U.S. government estimated the total discharge at 4.9 million barrels (210 million US gal; 780,000 m).