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Inches to millimeters converter

Convert inches to millimeters, both ways.

Type inches and read the exact millimeters, or type millimeters to convert back to inches. The result updates live, with centimeters, meters and feet for context and a one-click copy.

Your value

in
mm

Edit either box. The formula is mm = inches times 25.4, and inches = mm divided by 25.4.

1 inch in millimeters
0 mm
25.4 mm

What this means

    Common inch to mm values

    InchesMillimetersCentimetersTypical use

    How the conversion works

    Conversion Factor

    1 inch equals exactly 25.4 millimeters. To convert inches to mm, multiply by 25.4. This is the international inch, used in virtually every modern document and machine spec.

    Engineering and Manufacturing

    Most international engineering uses millimeters. US fractional-inch dimensions (1/8 in, 5/16 in, etc.) often need conversion to mm for international suppliers and machinists. Always specify whether dimensions are 'nominal' or 'precise.'

    Common Reference

    1 in = 25.4 mm. 1/2 in = 12.7 mm. 1/4 in = 6.35 mm. 1/8 in = 3.175 mm. 3 in = 76.2 mm. 12 in = 304.8 mm.

    Common questions

    How many mm is 1 inch?

    Exactly 25.4 mm. This is a defined conversion, not measured.

    Convert 1/8 inch to mm?

    1/8 in = 0.125 in × 25.4 = 3.175 mm. Useful for drill bit sizing.

    Why is exactly 25.4?

    Since 1959 the international yard has been defined as 0.9144 m, making 1 in = 25.4 mm exactly. Adopted to unify US and UK standards.

    All conversion runs in your browser. Nothing is uploaded. 1 inch is exactly 25.4 mm by international definition, so these results are precise, not approximate.