Volume measures the amount of space occupied by a substance. Convert between liters, gallons, milliliters, and more.
Volume conversions cover gallons, litres, cups, and fluid ounces, units that trip people up because the same word can mean genuinely different amounts depending on which country's standard is in use. Cooking, fuel economy, and bulk liquid measurements are the main reasons someone lands on this category.
A US liquid gallon equals exactly 3.785411784 litres. An imperial gallon, still used in the UK, equals 4.54609 litres, about 20 percent more liquid for the same word 'gallon.' The two systems never reconciled: the US kept its colonial-era gallon after independence, while the UK redefined its gallon in 1824 to a larger, more precise imperial standard.
The gap matters most when comparing fuel economy. A UK car's rating in miles per imperial gallon looks better than an equivalent US car's miles per US gallon even at identical real-world efficiency, purely because the imperial gallon holds more fuel per unit, so a direct comparison overstates the UK figure by roughly a fifth unless both are converted to the same gallon first.
The cup measure in an American kitchen, the US customary cup, holds exactly 236.5882365 millilitres. Nutrition labels use a different figure: the US legal cup, fixed by FDA regulation at exactly 240 millilitres for consistency across food labeling, a value close to but not identical with the cup sitting in your kitchen drawer. Australia, New Zealand, and many international recipes use a metric cup instead, exactly 250 millilitres, a gap of about 5.6 percent from the US customary cup that adds up fast across a multi-cup recipe.
None of these differences is large enough to ruin a single cup of an ingredient, but they compound. A recipe calling for four cups converted with the wrong definition can be off by nearly 60 millilitres, enough to change a batter's consistency.
The US also maintains a separate dry gallon and dry quart, used historically for produce like grain and berries, and it is not the same volume as the liquid gallon used for milk or fuel. A US dry gallon is about 4.405 litres, larger than the 3.785 litre liquid gallon, because the two systems were standardized independently for different goods. Treating a dry measure and a liquid measure as interchangeable, or assuming a fluid ounce, a volume, and a weight ounce, a mass, are the same thing, is a common source of an ingredient measurement coming out wrong even when every individual conversion factor was applied correctly.