How big is 1 inch?

Answer 1

An American inch is 25.4mm. A British Imperial Inch is very slightly less.

add. It is more than 50 (1959) years since there was any difference between US and Imperial inches.

In 1959, the USA, the UK, Canada, Australia and South Africa reached an agreement on the International Yard and an International inch is one thirty-sixth of this. The British Imperial Yard is the length of a lump of platinum kept at the Royal Observatory and the Imperial inch is one thirty-sixth of this. The Imperial Yard is approximately one four-thousandth (0.025%) shorter than the International Yard. This is largely irrelevant since when the United Kingdom requires an exact measurement, it uses metres.