What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Statesyours
Same word, different thinggallon
also US gallon
The US liquid gallon, defined as 231 cubic inches — about four fifths of an imperial gallon.
Canada
No local equivalentlitre
also imperial gallon
Canada is metric: fuel and liquids are sold in litres. The imperial gallon survives in older documents and in some recreational contexts.
United Kingdom
No local equivalentlitre
also imperial gallon
Trade is metric. The imperial gallon — about 4.55 litres — persists in fuel economy figures and in older specifications.
Australia
No local equivalentlitre
Fully metric; the gallon is not used in Australian trade at all.
The governing standard, by market
International
SI
The litre is the accepted metric unit of liquid volume; the gallon is not an SI unit.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- How much bigger is an imperial gallon?
- About a fifth, which is more than enough to matter and small enough to pass unnoticed. The imperial gallon is roughly 4.55 litres and the US gallon roughly 3.79 litres, so an imperial gallon is about twenty per cent larger. Paint, sealant, adhesive and fuel are all sold in gallons somewhere, and a coverage rate quoted per gallon in one convention and applied in the other is wrong by that fifth in the direction that matters — under-ordering when an American rate is applied to imperial gallons, over-ordering the other way. The word gives no warning. Any figure taken from a foreign product datasheet is worth converting to litres before it is used.
- Does the difference affect coverage rates?
- Directly, and it is the commonest way this catches somebody out. Paint and coating datasheets quote a coverage — area per unit of product — and the unit is a gallon in North American literature and a litre almost everywhere else. Taking a coverage in square feet per US gallon and applying it to imperial gallons over-estimates how far the product goes by about twenty per cent, which on a large job is the difference between finishing and a second order at a different batch number. The safe practice is to convert the datasheet's coverage into area per litre before doing any arithmetic with it, and to record which convention the original figure came from.
- Are fluid ounces the same?
- No, and confusingly they differ in the opposite direction to the gallons. An imperial gallon contains 160 imperial fluid ounces and a US gallon contains 128 US fluid ounces, so although the imperial gallon is the larger container, the imperial fluid OUNCE is slightly smaller than the US one. That means neither the gallon nor the ounce can be converted by assuming the other is fixed. On site this matters mostly for admixtures, additives and small-quantity products dosed by fluid ounce per unit of mix — where the dosage is small, the tolerance is narrow and the error is invisible until the material behaves unexpectedly.