Imperial gallons to liters (imp gal to L)

One imperial gallon is exactly 4.54609 liters. That is a definition and a surprisingly recent one: the imperial gallon was originally the volume of ten pounds of water at a specified temperature and pressure, and the UK's Weights and Measures Act 1985 replaced that with the round metric figure outright. So the factor is exact by legislation rather than by measurement.

Worked through: 10 imp gal x 4.54609 = 45.4609 L.

Going the other way, one liter is 0.2199692483 imperial gallons.

For mental arithmetic: Multiply by 4.5 and add 1%.

The whole reason anyone searches this is that there are two gallons and they differ by about 20%. The US gallon is 3.785411784 liters and the imperial gallon is 4.54609, because the US kept the old English wine gallon of 231 cubic inches while Britain redefined its own in 1824. The practical consequences are everywhere: a US pint is 473 ml and an imperial pint 568, and US and UK fuel economy figures in 'miles per gallon' are not comparable: 40 mpg imperial is 33 mpg US for the same car.

Precision is worth matching to the input. A measurement given as 1 imp gal carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.