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How to convert inches to millimeters 
Understand the relationship between inches and millimeters. 
One inch is internationally accepted as being equivalent to 25.4 millimeters. 
Written as an equation, this relationship is represented as : 1 in = 25.4 mm this official, standardized measurement has been set on international records since 1959.
Both inches and millimeters are units of measurement. Inches belong to the english system of measurement, while millimeters belong to the metric system of measurement.