How many millimeters are in one inch?

By definition there are exactly 25.4 millimeters per inch.


Units of measurements have always caused problems. One inch in the US was not necessarily the same as one inch elsewhere. The need for standards have always been there, but the will to agree upon one standard have not always been present.


Effective July 1, 1959, the United States and countries of the British Commonwealth defined the length of the international yard to be exactly 0.9144 meters. Consequently, the international inch is defined to be equal to exactly 25.4 millimeters.


Where people of different origins work together, they must work with the same units in order to avoid problems. Where people do work together and do not accept a standard for measurements, problems arise. Some times very costly errors happen because one person does calculations in yards and another person uses those calculations as if they were done using meters.
Some times these errors are hardly noticed and does not affect the outcome of anything. At other times, space probes crash violently into a planet because distance measured in feet is interpreted as being meters. A parachute opens in order to slow down the descent too late and the failure is a fact due to lack of standards or the lack of complying to standards.
One of the "recent" mars probes crashed into the planet due to a similar problem.

To convert millimeters into inches, divide millimeters by 25.4.
To convert inches into millimeters, multiply inches by 25.4.