How Many Gallons Are in a Barrel of Oil?
Conversion of gallons to barrels of oil and barrels of oil to gallons is a relatively easy one because both gallons and barrels are units of volume, making their conversion relatively easy. We say relatively easy since deep down, a barrel is not only a unit of volume but a unit of volume under certain conditions.
Not to mention that the unit "barrel" was and still is used for measuring volume and that there are "dry barrels", "fluid barrels", "oil barrels", etc. In this article, we are discussing the "oil barrel".
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Conversion Formulas
Gallons are a nonSI measure of volume, with two different units  US Gallon and Imperial Gallon:
1 US Gallon = 128 US Fluid Ounces (fl oz) = 3.78541 liters → 1 liter = 0.26417 US Gallon
1 Imp Gallon = 160 Imp Fluid Ounces = 4.54609 liters → 1 liter = 0.219969 Imp Gallon
Actually, 1 US gallon is defined as having exactly:
1 US gallon = 3.785411784 liters
Also, an oil barrel (or barrel of oil) is a nonSI measure of volume, defined as:
1 oil barrel = 42 US gallons
Hence, one oil barrel equals:
1 oil barrel = 42 US gallons = 158.987294928 liters = 34.97231575 Imp gallons
Or for short:
1 oil barrel = 42 US gallons ≈ 35 Imp gallons ≈ 159 liters
However, one also should mention that the standard oil barrel is the amount of oil that at a standard pressure of 101.325 kPa and temperature of 60°F (15.6 °C) occupies a volume of exactly 1 oil barrel  this is important when trading with oil, however, not all oil types expand or shrink equally as the temperature and pressure increase or decrease.
But, for most of us, it is important to remember that 1 barrel of oil equals 42 US gallons, which is a little less than 159 liters.
Thus, in order to convert volume given in gallons into volume barrels of oil and vice versa, use this formula:
1 oil barrel = 42 US gallons = 34.97231575 Imp gallons
And forget about pressures, temperatures, oil density, and similar.
Gallons to Oil Barrels and Oil Barrels to Gallons Conversion Examples
If You have 4.7 gallons (US/Imp) that would be:
Volume_{(oil barrels)} = 4.7 / 42 = 0.1119047 barrels of oil
Volume_{(oil barrels)} = 4.7 / 34.97231575 = 0.13439201 barrels of oil
Similarly:
 1000 US Gallons = 23.809 barrels of oil
 500 Imp Gallons = 14.297 barrels of oil
 20 barrels of oil = 840 US gallons = 699.44 Imp gallons
etc.
Gallons to Oil Barrels and Oil Barrels to Gallons Conversion Calculator
In order to convert gallons to oil barrels and oil barrels to gallons, feel free to use this conversion calculator  write the value that You have and click 'Calculate' to convert it:
Barrels of Oil to US Gallons 
Barrel of Oil: US Gallons: 
US Gallons to Barrels of Oil 
US Gallons: Barrels of Oil: 
Barrels of Oil to Imperial Gallons 
Barrels of Oil: Imperial Gallons: 
Imperial Gallons to Barrels of Oil 
Imperial Gallons: Barrels of Oil: 
Gallons to Oil Barrels and Oil Barrels to Gallons Conversion Charts
Here are some gallons to oil barrels and oil barrels to gallons quick conversion charts to aid You with units' conversions:

 


For more values, please, use the conversion calculators.