How to convert inches to millimeters
To convert a measurement in inches to millimeters, multiply the value by 25.4. The conversion factor comes from the relationship between both units and the meter (the SI base unit of length).
For example, to convert 5 in to millimeters: 5 × 25.4 = 127 mm.
What is a inch?
An inch is an imperial unit of length equal to 1/12 of a foot or exactly 25.4 millimeters. Originally defined as the width of a man's thumb, later standardized as three barleycorns laid end to end. Since 1959, defined internationally as exactly 25.4 mm.
Common uses of inches include: Screen and display sizes (TV, phone, monitor diagonals); Screw, bolt, and pipe sizing; Tire rim diameter specifications; Woodworking and carpentry precision.
What is a millimeter?
A millimeter is a metric unit of length equal to one thousandth of a meter (0.001 m) or approximately 0.039 inches. Part of the original metric system, the millimeter derives from the Latin 'mille' (thousand). It became essential during the Industrial Revolution for precision engineering.
Common uses of millimeters include: Engineering tolerances and mechanical parts; Rainfall measurement in meteorology; Paper thickness and printing specifications; Dental and medical precision measurements.
When you need to convert inches to millimeters
Precision manufacturing, bolt sizing, and drill bit selection require exact conversions between inches and millimeters. A quarter-inch bolt is exactly 6.35 mm.
Inches to millimeters worked examples
Here are some common values converted:
1 in = 25.4 mm 5 in = 127 mm 10 in = 254 mm 50 in = 1,270 mm 100 in = 2,540 mm
Frequently asked questions
How many millimeters are in one inch?
One inch (in) equals 25.4 millimeters (mm). Multiply any value in inches by 25.4 to convert to millimeters.
How do I convert inches to millimeters?
Multiply the value in inches by 25.4. For example, 5 in × 25.4 = 127 mm.
How do I convert millimeters to inches?
Multiply the value in millimeters by 0.0393700787, or divide by 25.4. Use the swap button on the converter above to reverse the direction.
Is the inch to millimeter conversion exact?
Yes, the conversion is exact. Since 1959, one inch is defined as exactly 25.4 mm (the international yard and pound agreement), so all metric-imperial length conversions use exact factors.
What is the formula for inches to millimeters?
The formula is: mm = in × 25.4. This is derived from the definition of both units relative to the meter (the SI base unit of length).