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Inches to Millimetres Calculator

Convert inches to millimetres for metric drawings, fixings and imported components.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Length

12 in

High confidence

1 inch = 25.4 mm exactly, by international agreement since 1959. Converting a dimension does not make imperial and metric fixings interchangeable — match those by specification.

Conversion factor applied
1 in per in

Anyone working across markets meets this constantly: an American drawing against European sheet goods, or a metric fixing schedule against imperial timber. The factor is exact, so the arithmetic never introduces error. What does cause trouble is assuming that converting a number makes components compatible. An M10 bolt is not a three-eighths bolt, plasterboard sold as twelve and a half millimetres is not the same sheet as half-inch board, and a converted hole diameter will not accept a fastener from the other system without being redrilled. Convert dimensions freely for setting out and cutting; specify fixings, threads and sheet products by their actual designation.

Add the equipment this sizes

This result is a specification — 12 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • International yard and pound agreement (1959): 1 inch = 25.4 mm exactly

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

Is 25.4 exact or rounded?
Exact. Since the 1959 international yard and pound agreement, the inch is defined as precisely 25.4 millimetres. There is no rounding error in this conversion, only in how many decimal places you choose to keep.
How do I enter a fraction?
As a decimal. An eighth is 0.125, a quarter 0.25, three-eighths 0.375, and a sixteenth 0.0625. Multiply the numerator by the sixteenth value if you are working from a tape marked in sixteenths.
Why do metric fixings not match imperial holes?
Because the sizes were never designed to correspond. An M10 bolt is not a 3/8 inch bolt — it is closer to 25/64. Converting the dimension does not make the thread compatible, so match fixings by specification rather than by converted size.