 Quandaries and Queries Greg Secondary (10-12) ??? Curious Engineer Question The conversion factor for inches to millimeters 25.4 It strikes me as very odd that it's not some infinite decimal. I could understand this if one system was derived from the other but as I understand it they were derived independently. Is the conversion 25.4 or 25.4000XXXXXXX? If it's just 25.4 then how did that happen? Coincidence just doesn't work for me. Hi Greg, The fact that one inch is exactlt 25.4 millimeters is not a coincidence. The two systems were developed independently and until 1958 an inch was not exactly 25.4 millimeters. In that year a conference of English-speaking nations agreed to unify their standards of length and mass, and define them in terms of metric measures. The standard meter was the length of a metal bar which was kept in France and other units were defined in terms of that meter. In 1960 the meter was redefined in terms of wavelengths of light. You can find more details at A chronology of the SI metric system. Harley Go to Math Central