Metric vs Imperial Measurements

Three measurement systems are in daily use across the English-speaking world: the metric system (SI), the British Imperial system and the US customary system. Metric and Imperial are frequently treated as the only two, but US customary units are a separate third system that shares names with Imperial units while differing in size for volume and some weights.

The metric system

SI is decimal: every unit is a power of ten away from its base unit, so conversions are shifts of the decimal point. The base units relevant to everyday measurement are the metre, the kilogram, the second and the kelvin, with the litre used for volume.

Imperial vs US customary

Length units are shared exactly — an inch is 25.4 mm in both systems, and a mile is 1,609.344 m. Volume is where they diverge: an Imperial gallon is 4.54609 L while a US liquid gallon is 3.785411784 L. Imperial pints hold 20 fluid ounces, US pints hold 16. The ton also differs: 2,240 lb in Imperial, 2,000 lb in US customary.

Which country uses what

• United States — US customary for everyday use; metric in science, medicine and the military
• United Kingdom — metric officially, but miles, mph, pints of beer, and stone for body weight persist
• Canada — metric officially, with imperial common in construction, cooking and body measurements
• Australia and New Zealand — fully metric since the 1970s, with residual imperial only in niche trades

Key conversion factors