There are 42 gallons in a barrel of Oil. Though barrels are no longer used to ship oil, in America they are still the unit of measurement in commerce. The size was determined by the practices of Pennsylvania oil companies. They shipped oil to market by wagon or train in open wine barrels that held 48 gallons. By the time they reached the market only 42 gallons was left because of spillage and therefore the measurement became 42 gallons.