What is Crude Oil?
Crude oil — also called petroleum — is a naturally occurring liquid mixture of hydrocarbons found beneath the Earth's surface. It's a fossil fuel formed over millions of years from the remains of ancient marine organisms (mostly algae and zooplankton) that settled on the bottom of seas and lakes, were buried under layers of sediment, and slowly transformed under intense heat and pressure.
In its raw form, crude oil isn't directly usable. It has to be refined into separate products through a process called fractional distillation, where it's heated in a tall column and the different hydrocarbon chains separate out at different temperatures. The result is everything from petrol and diesel to plastics, lubricants, asphalt, and even ingredients in cosmetics and medicines.
How is crude oil formed?
Crude oil forms over tens of millions of years. When ancient marine organisms died, they sank to the seabed and were covered by layers of mud and sediment. As more layers piled on top, the pressure and temperature increased. Over time — typically 10 to 200 million years — this organic matter slowly "cooked" into a waxy substance called kerogen, then into liquid hydrocarbons. The resulting crude oil migrates upward through porous rock until it gets trapped against an impermeable layer, forming the underground reservoirs that drilling rigs tap into today.
What is crude oil used for?
Most people associate crude oil with petrol and diesel — and rightly so, since transport fuels make up roughly 75% of refined output. But crude oil is also the raw material for thousands of other products: jet fuel, heating oil, propane, asphalt for roads, lubricants, plastics, synthetic fabrics like polyester, fertilisers, pesticides, paints, cosmetics, pharmaceuticals, and even chewing gum bases. Modern life is built on petroleum derivatives in ways most people never realise.
Quick Facts About a Barrel of Crude Oil
42
US gallons in one standard oil barrel
159
Litres in one barrel of crude oil
35
UK (imperial) gallons per barrel
~136
Kilograms — the weight of one barrel
Why is a barrel 42 gallons?
The 42-gallon size dates back to 1866, in the early days of the Pennsylvania oil rush. Producers used wooden whisky barrels (40 gallons) and added 2 gallons of "leakage allowance" so buyers could be sure they were getting a full 40 gallons after spillage. The 42-gallon standard stuck and became the international unit for crude oil even though physical wooden barrels stopped being used over a century ago. Today's "barrel" is purely a unit of measurement — crude is transported by pipeline, tanker and rail car.
How much petrol/gasoline comes from one barrel?
About 20 US gallons (75 litres) of gasoline come from a single 42-gallon barrel of crude oil — that's roughly 45% of the refined product. The exact amount varies by refinery and crude type. US refineries are tuned to maximise gasoline output, while European and Asian refineries typically produce more diesel and jet fuel from the same barrel.
What about everything else?
The remaining ~55% becomes diesel (~27%), jet fuel (~9%), hydrocarbon gas liquids like propane (~4%), petrochemical feedstocks for plastics and chemicals (~4%), heavy fuel oil and asphalt (~6%), and a long tail of other products including lubricants, waxes, kerosene and bitumen. Crude oil is so versatile that it's used in everything from medicines and cosmetics to crayons and tyres.
Frequently Asked Questions
A barrel of crude oil is how many gallons?
A barrel of crude oil is exactly 42 US gallons. This is the international standard unit used for trading and pricing crude oil worldwide. In other units, that's 159 litres, 35 UK (imperial) gallons, or 0.159 cubic metres.
How many US gallons are in a barrel of crude oil?
There are 42 US gallons in one barrel of crude oil. The 42-gallon size has been the standard since 1866, when American oil producers used wooden whisky barrels (40 gallons) plus 2 gallons of "leakage allowance" to guarantee buyers received the full amount.
How many litres are in a barrel of oil?
One barrel of crude oil contains exactly 158.987 litres, which is rounded to 159 litres in most reference materials. This equals 42 US gallons or roughly 35 UK (imperial) gallons.
How many cubic metres (m³) in a barrel of oil?
One barrel of crude oil equals 0.159 cubic metres (m³). To convert the other way, 1 cubic metre equals about 6.29 barrels. Cubic metres are commonly used for measuring oil volumes in scientific and industrial contexts, especially outside the US.
How many tons does a barrel of oil weigh?
A standard 42-gallon barrel of crude oil weighs approximately 0.136 metric tonnes (136 kg or 300 lbs). The exact weight depends on the density of the specific crude — lighter crudes like West Texas Intermediate weigh slightly less, while heavier crudes weigh more. For rough conversions, 1 metric tonne of crude oil equals about 7.33 barrels.
How many gallons of gasoline are in a barrel of oil?
A 42-gallon barrel of crude oil produces approximately 20 gallons of gasoline (about 75 litres) when refined in a typical US refinery. This is roughly 45% of the refined output. The exact amount varies depending on the type of crude and how the refinery is configured.
How can a 42-gallon barrel produce 45 gallons of products?
It seems impossible but it's real — it's called "processing gain." When crude oil is refined, the heavy hydrocarbon molecules are broken down into lighter ones. These lighter products take up more volume than the dense crude they came from, creating roughly 7% more liquid by volume. Refineries also add small amounts of other inputs like fuel ethanol and blending agents during the process.
How much does a barrel of oil weigh?
A standard 42-gallon barrel of crude oil weighs approximately 136 kilograms (300 pounds), though this varies based on the density of the specific crude. The density is measured using a scale called API gravity.
Are oil barrels actually still 42 gallons?
The 42-gallon "barrel" is now purely a unit of measurement. Real wooden barrels stopped being used over a century ago — modern crude oil is transported through pipelines, ocean tankers, and rail cars. But the 42-gallon unit stuck because it became the international standard for pricing and trading crude oil.
Why do US refineries make more gasoline than European ones?
US refineries are configured to maximise gasoline output because American consumers drive petrol cars more than diesel. European refineries produce more diesel because diesel cars and trucks dominate that market. The same barrel of crude can be refined into different proportions depending on what the local market needs.
What is in the "other" 5% of a barrel?
The "other" category includes a long list of useful products: lubricants and motor oil, waxes (for candles, food packaging, and crayons), petroleum coke (used in steelmaking), naphtha (a chemical solvent), still gas (burned for refinery energy), and bitumen for waterproofing. Petroleum is in thousands of everyday products you wouldn't immediately associate with oil.
How long does it take to refine a barrel of crude oil?
A modern refinery processes a 30,000-barrel batch of crude in roughly 12 to 24 hours. Refineries operate continuously, 24 hours a day, 365 days a year. From the moment crude is extracted from the ground to the time its products reach consumers, the typical lag is one to three months depending on the destination product.