The oil barrel is a volume unit — nothing more. It is a common source of confusion precisely because people slide from "a barrel of oil" (a volume) to "the energy in a barrel of oil" (a completely different, estimated quantity). The tool keeps these strictly apart.
The exact definition
The oil barrel is defined as exactly:
1 barrel = 42 US gallons = 158.987294928 L
This is a pure, exact volume identity. You can convert a barrel to US gallons, cubic metres or litres with no assumptions at all, just as you would any other volume.
US gallon, not imperial gallon
The "42 gallons" in the definition means US gallons specifically. This matters, because the US gallon and the imperial (UK) gallon are noticeably different:
- US gallon ≈
3.785 L - Imperial gallon ≈
4.546 L
Using one gallon where the other is meant is roughly a 20% error, which would then poison every barrel and fuel-volume result downstream. The tool therefore keeps us_gallon and imperial_gallon as separate units, and defines the barrel via US gallons explicitly. A bare "gallon" is treated as ambiguous and disambiguated (defaulting to US, always labeled) rather than silently guessed.
Because the barrel is defined via US gallons, the tool converts freely within the volume dimension but never assumes the barrel "contains" any particular energy. That step needs a fuel, a density and a heating value, and it is treated as a separate calculation with its own, non-exact answer.
Volume is not energy
A barrel is a container size; how much energy that volume holds depends on what is in it. The energy content of one physical barrel of crude oil is a separate, estimated quantity that varies with the crude grade — it is not baked into the volume definition. A light, high-value crude and a heavy one occupy the same 42-gallon barrel but carry different amounts of energy, and this tool records a calorific value for generic crude (IPCC, 42.3 MJ/kg with a 40.1–44.8 range) but no DENSITY for it. Without a density there is no way from a barrel to the kilograms that value is stated per, so a physical barrel reports exactly that gap instead of quoting an energy figure nobody can trace.
Do not confuse the volume "barrel" with the energy-equivalence unit "barrel of oil equivalent" (boe): they live in different dimensions. The barrel is a volume; the boe is a fixed energy convention. See barrel vs boe for that distinction, which is one of the classic pitfalls in energy reporting, and sources for the provenance behind every figure this tool does quote.