History: A barrel of oil is 42 US gallons and resulted from 19th-century Pennsylvania oil producers standardizing on a 42-gallon wooden “tierce” to avoid leakage and ensure manageable transport weight (around 300 lbs or 136 kg) for horse-drawn wagons. Adopted officially in 1866, this size was chosen because it was a standard shipping container for other liquids like fish, whiskey, and molasses, offering a convenient, readily available and portable vessel for early crude oil. Read more