The system of imperial units or the imperial system (also known as British Imperial) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined (until 1959) and reduced. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, although as of 2011 the United Kingdom had only partially adopted it.

Since 1959, the US and the British yard have been defined identically to be 0.9144 metres, to match the international yard. Metric equivalents in this article usually assume this latest official definition. Before this date, the most precise measurement of the Imperial Standard Yard was 0.914398416 metres.

In 1824, the United Kingdom adopted a unit similar to the ale gallon, known as the imperial gallon. It was based on the volume of 10 lb of distilled water weighed in air with brass weights with the barometer standing at 30 in Hg at a temperature of 62 °F (17 °C). In 1963 this definition was refined as the space occupied by 10 pounds (4.5 kg) of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 g/mL against weights of density 8.136 g/mL. This works out to 4.546096 L, or 277.4198 cu in. The Weights and Measures Act of 1985 switched to a gallon of exactly 4.54609 L (approximately 277.4194 cu in).