The international inch is defined exactly as 25.4 mm
At one time an ‘inch’ was a imprecise measurement. Often each town or region would have its own version, an this itself would change from time to time. Measures were, and are, used in trade, and traditionally, in times of shortage or plenty, the prices of goods change. However, in the medieval era, it was frequently the custom to fix the price for a given named quantity, and then change the quantity as supply dictated. Thus, the price of cloth might be consistently a certain amount per inch or per yard, but the inch would be increased or decreased according to need.
This began to change with the increase of industry and communications, and the growth of science in the 17-19th century. These developments centered on France and Britain, leading to the rise of two competing systems. The System International, which was invented in the 1790’s in Paris, and the Imperial System, which was defined by the British Parliament in 1835. France threw out its old system and started afresh with meters, centimeters, kilograms, etc. Britain decreed new fixed and standard definitions for its traditional units. Thus an inch became a uniform and consistent length throughout the Empire. In each nation, accurate samples – standard rulers – were sent out to the provinces to be used and copied.
For the next century, scientists in each nation competed to increase the precision of their standards, but by the 1950’s it was clear that the French had won. The British scientists proposed that the Imperial inch be redefined in terms of the more precise meter.
This left one oddity. The United States had broken away from the British Empire before this movement began. Neither did they adopt the widely used French System International. America continued to use the pre-Imperial British inch, along with other units. So in 1957 an international conference was called. The US and the British Commonwealth countries attended, and it was agreed through compromise, that a new standard inch, the International Inch, would be used and defined to be exactly 25.4 mm
Thus is the story of the size of an inch.