text: 1 barrel (bbl) of petroleum or related products = 42 gallons or A barrel may be called a "drum", but a drum usually holds 55 gallons...Oil has not been shipped in barrels for a very long time since the introduction of oil tanker ships, but the 42-US-gallon size is still used as a unit for measurement, pricing, and in tax and regulatory codes. Although some companies still ship some oil in 55-gallon steel drums. (Volumes for these are still given in 42-gallon "barrels.")...care must be taken to correct for the effects of temperature, which can cause oil to expand or contract..