Inches to Millimeters Converter

Yes, exactly. The 1959 international yard agreement (signed by the US, UK, Canada, Australia, New Zealand, and South Africa) fixed the international yard at exactly 0.9144 m. Dividing by 36 gives 1 inch = 0.0254 m = 25.4 mm with no rounding. NIST publishes 25.4 mm as the defined, exact value, and every modern CAD package, CNC controller, and ISO drawing standard uses the same factor. Earlier US Survey inch (25.4000508 mm) was retired from federal use on 1 January 2023, so the international inch is now the only standard.

One thou (also called a mil in US machining) is 0.001 inch, which equals 0.0254 mm exactly. Machinists use thou as the everyday unit for tight tolerances: a press fit on a steel shaft might be 1 to 2 thou (0.025 to 0.05 mm), a sliding fit 3 to 5 thou (0.076 to 0.127 mm). Do not confuse thou with the metric millimeter abbreviation; some older European drawings write 'mil' to mean millimeter, which is a different unit by a factor of 1,000. Whenever a US shop drawing says mil, it means thou.

General shop tolerance for milled or turned parts without an explicit callout is around plus/minus 0.005 inch, which equals plus/minus 0.127 mm. That covers most everyday brackets, fixtures, and rough mechanical parts. Precision CNC mills routinely hold plus/minus 0.001 in (0.0254 mm), and surface grinding can reach plus/minus 0.0001 in (0.00254 mm) on hardened steel. ASME Y14.5 and ISO 2768 define the formal tolerance classes for medium and coarse work, but the 0.005 in figure is the practical default a shop will quote when no other tolerance is specified.

The metric system uses powers of 10, so unit conversions inside the system are simple: 1 m = 1,000 mm = 100 cm. Imperial units mix 12 (inches per foot), 3 (feet per yard), and 1,760 (yards per mile), which makes arithmetic, dimensional analysis, and error propagation messier. Every major CAD package defaults to millimeters, every ISO drawing standard specifies millimeters as the default linear unit, and most countries have standardized on metric for industrial work. Inches still appear in US construction drawings, aircraft parts, plumbing pipe sizes, and consumer screen sizes for legacy and supply-chain reasons.

Capability depends on the machine, material, tooling, setup, temperature, and whether the specification refers to positioning accuracy, repeatability, or finished-part tolerance. General CNC work often uses tolerances around 0.025 to 0.1 mm, while specialized processes can be tighter. Use the drawing requirement and verified process capability, not a universal CNC tolerance.