Inches to Millimeters

What this tool is used for

• Converting an imperial drawing dimension for a metric machine shop.
• Reading a fractional inch hardware size against metric stock.
• Converting a screen or panel dimension for a metric datasheet.
• Checking whether an imperial fastener matches a metric hole.
• Converting a tolerance from thousandths of an inch into millimetres.

Frequently Asked Questions

Is the factor exact?

Yes — one inch is exactly 25.4 mm by the 1959 international agreement, which follows from the inch being defined as 2.54 cm. It is a definition, not a measurement, so nothing is lost in either direction.

Why do engineers prefer millimetres to centimetres?

Because mm avoids decimals for most manufacturing tolerances, and technical drawings assume mm by default so the unit can be omitted entirely. ISO drawing standards specify millimetres for exactly this reason.

How do fractional inches convert?

Imperial uses binary fractions rather than decimals: 1/16 in is 1.5875 mm, 1/8 is 3.175, 1/4 is 6.35. None are round in metric, which is why converted imperial parts have awkward mm dimensions.

What is a thou or mil?

One thousandth of an inch, exactly 0.0254 mm — the standard unit in PCB design and machining tolerances. Beware the ambiguity: in some European usage "mil" means a millimetre, a factor of about 39 apart.

Why are pipe and screw sizes not real inches?

Because they are nominal trade designations, not measurements. A "1/2 inch" pipe has neither a 1/2 in inner nor outer diameter, so converting the label to millimetres produces a number that measures nothing physical.

How do I convert a fractional inch such as 3/8?

Divide first, then multiply — 0.375 × 25.4 = 9.525 mm. Converting the numerator and denominator separately is the common error, and it produces a value that is wrong by the denominator.

Why do metric and imperial fasteners not interchange?

Because the thread pitch differs as well as the diameter, so a nearly-equal size still cross-threads. An M6 bolt and a 1/4-inch bolt are 0.15 mm apart in diameter and completely incompatible.

Common errors and gotchas

• Converting a fraction approximately, where a thirty-second is 0.79375 mm and rounding loses the fit.
• Confusing millimetres with centimetres, a factor of ten that still looks plausible.
• Assuming a metric fastener is interchangeable because the converted size is close. Thread pitch differs.
• Rounding to whole millimetres on a tolerance, which can be larger than the tolerance itself.
• Converting each dimension separately and then finding the assembly no longer fits.