Unit Converters
Inches and Millimetres: Convert Both Directions
Convert inches to millimetres with the exact 25.4 factor, handle fractional-inch dimensions, and separate rounding from manufacturing tolerance.
Use 25.4 for the inch–millimetre relationship
One inch is exactly 25.4 millimetres. Multiply an inch dimension by 25.4 to obtain millimetres, and divide millimetres by 25.4 to obtain inches. For example, 2.5 inches becomes 63.5 mm. The reverse calculation, 63.5/25.4, returns 2.5 inches. The page's linked converter can operate in either direction through its selectors; a route name containing “mm to inches” does not require you to leave the default direction unchanged when the question starts with inches.
Convert a fractional inch as one number
A dimension of 3/8 inch is 0.375 inch, so it equals 0.375 × 25.4 = 9.525 mm. A mixed dimension of 1 3/8 inches is 1.375 inches and equals 34.925 mm. Combine the fraction before entering the numeric field. Do not type the numerator and denominator as separate measurements or mistake 1 3/8 for 1.38. When transferring a dimension from a drawing, preserve whether the fraction describes the entire dimension or just the fractional part of a mixed number.
Treat a preferred metric size as a separate choice
The exact equivalent of 1/2 inch is 12.7 mm. Choosing a nominal 13 mm item is a selection decision, not an exact conversion. The difference is 0.3 mm and may matter for the application. Similarly, 10 mm is approximately 0.3937008 inches; rounding it to a familiar fraction changes the dimension. A converter cannot determine interchangeability from the closest printed number. Compare the specified dimensions, tolerances and relevant fit requirements before substituting one nominal size for another.
Convert tolerance limits consistently
If a drawing gives 2.000 inches ±0.005 inch, the central dimension is 50.8 mm and the tolerance magnitude is 0.127 mm. Both are lengths, so both use the same multiplication factor. The lower and upper limits are 50.673 and 50.927 mm. Converting only the central value while leaving an unlabeled tolerance from the original unit creates an ambiguous specification. Report the chosen rounding rule for the limits, and avoid rounding inward in a way that silently changes the allowable range.
Use exact unit definitions without implying exact measurement
The international inch is exactly 2.54 centimetres, and one foot is exactly 12 inches or 0.3048 metres. These fixed relations let a converter change units without knowing the object being measured. The measurement itself may still be approximate. A tape marked in whole millimetres does not produce a more accurate measurement merely because the converted number has six decimal places. Retain enough digits during the calculation, then report a sensible precision for the original reading and the intended use of the converted dimension.
Keep a mixed measurement separate from a decimal
Feet and inches use a base of twelve inches per foot, whereas decimal feet use tenths and hundredths of a foot. Thus 5 feet 6 inches is 5.5 feet, not 5.6 feet. To combine a mixed measurement, multiply the whole feet by 12 and add the remaining inches. To split a decimal-foot result, take its whole-number part as feet and multiply the fractional part by 12 to obtain inches. If rounded inches reach 12, carry them into the next whole foot rather than writing an unusual twelve-inch remainder.
Choose matching input and output quantities
The linked length converter has a numeric value and From and To unit selectors. Enter a single numerical quantity, not a phrase containing feet and inches; combine mixed units first. Check the output label before copying the result into a drawing or form. A length conversion does not calculate area, circumference or fit. If a problem involves two dimensions, convert each separately. For an area, the length factor must be squared; for volume, it must be cubed. Applying the linear factor once to a square or cubic unit produces the wrong scale.
Calculator for the supported step
Enter the combined length as one number, choose its original unit under From and the requested unit under To, and calculate. The displayed decimal length may need the mixed-unit formatting explained above.
Sources and method
Related guides
Convert feet to metres using 0.3048, combine feet-and-inches measurements correctly, and distinguish ordinary international feet from historical survey units.
Convert height from centimetres to total inches and then feet plus inches, handle rounding carry, and distinguish mixed notation from decimal feet.
Convert a height written in feet and inches into centimetres, decode decimal feet, and show how whole-inch rounding changes a height record.