One imperial gallon is exactly 4.54609 litres. It is twenty per cent larger than the US gallon, and unlike the American unit it was defined by water rather than by a cask measure.
About the units
The imperial gallon was created by the Weights and Measures Act of 1824, which swept away a tangle of older English gallons — the wine gallon, the ale gallon and the corn gallon, all different — and replaced them with one unit defined as the volume of ten pounds of distilled water at 62 °F and a specified air pressure. That definition made the imperial system's liquid measures physically reproducible, which the American ones inherited from the wine gallon are not. The value was later fixed at exactly 4.54609 litres to remove any dependence on measuring water. The imperial gallon is subdivided into 4 quarts, 8 pints or 160 fluid ounces.
The exact factor
The exact value is 4.54609 litres. Against the US gallon at 3.785411784 litres the ratio is 1.20095, so an imperial gallon is a little over one and a fifth US gallons. The ten-pound origin is still visible: ten pounds is 4.5359 kg, and a litre of water weighs a kilogram, so 4.54609 litres of water weighs 4.546 kg — close to but not exactly ten pounds, because the original definition specified a temperature and pressure at which water is slightly less dense than at its maximum.
Where you meet this conversion
British and Irish fuel economy figures, which are quoted in miles per imperial gallon and therefore look about twenty per cent better than the American figures for the same vehicle. Older British recipes and industrial specifications use it for liquids. Canada, which metricated in the 1970s, retains it informally and in some agricultural contexts. Aviation fuel in the United Kingdom is uplifted in litres but historically planned in imperial gallons, and mixing the two is a documented cause of fuel exhaustion incidents.