MM to Inches Converter

Common questions

How many inches is 1 mm?

0.0393700787402 inches, which is 1 divided by 25.4. Ten millimetres is 0.393700787402 in, 100 mm is 3.93700787402 in and 1,000 mm is 39.3700787402 in: the digits are the same and only the point moves, because the whole conversion is a single division. Going the other way, 25.4 mm is exactly 1 inch and 12.7 mm is exactly half of one.

What is the formula for mm to inches?

Inches = millimetres / 25.4. The inch has been exactly 2.54 centimetres since the 1959 international agreement on the yard and the pound, so 25.4 mm to the inch is a definition rather than a measurement, and nothing about it drifts with time or country. This converter divides by that exact figure instead of multiplying by a rounded 0.03937, which is why 25.4 mm comes back as 1 and not as 0.99999.

Is the decimal exact, or is it rounded?

The definition is exact: 1 mm is exactly 5/127 in. The unit ratio repeats because 127 is a prime other than 2 or 5, but an input can cancel it: 25.4 mm is exactly 1 in and 12.7 mm exactly 0.5 in. Decimal-rational arithmetic preserves the original scalar, then rounds outputs beyond twelve significant digits even if their exact decimals terminate.

How do I turn the decimal into a fraction of an inch, like sixteenths?

Calculate sixteenths directly from original millimeters as n = round((mm / 25.4) × 16), then reduce n/16 if desired. For 10 mm the unrounded count is about 6.2992, giving 6/16 = 3/8 in. That is exactly 0.375 in or 9.525 mm, 0.475 mm short of the original 10 mm. A nearest-sixteenth result can be exact when the source already lies on that grid; for example 12.7 mm is exactly 8/16 = 1/2 in. This converter displays decimals rather than doing the grid rounding or accepting fraction input syntax.

Can I convert inches back to millimetres here?

Yes. The swap button between the two boxes reverses the pair, so the page becomes inches to millimetres: 1 in gives 25.4 mm and 3.5 in gives 88.9 mm. The two unit menus stay open as well, so the same number will go from millimetres to feet, metres, yards or thou without leaving the page.

Where are thou, mils and the other engineering units?

Under the answer, the Uncommon units switch adds seven more: nanometre, micrometre, thou, furlong, astronomical unit, light year and parsec. Use a source’s explicitly supplied unit; a drawing or trade context does not determine which unit it means. One thou is a thousandth of an inch, exactly 0.0254 mm, so 1 mm is 39.3700787402 thou and 0.5 mm is 19.6850393701 thou.

Why does the page open on millimetres and inches when I last converted something else?

This focused page opens on Millimeter and Inch, while preserving the main converter’s saved unit pair. When storage is usable, a saved value and uncommon-unit preference can travel between pages. Its suffix must still match From. Valid deliberate scalar edits can save those fields after a short delay; invalid or incomplete drafts preserve the saved record. Unusable or conflicting records remain until you use Forget saved settings, which keeps the current conversion on screen. Your input and converted values are calculated in the browser and are not uploaded by the converter. The definitions arrive with the page, so conversion needs no external data lookup and keeps working offline after the page has loaded. The core conversion needs no account. Copy result copies the displayed headline without digit-grouping commas; each visible conversion row has its own Copy button. Displayed and copied values round to twelve significant digits. If native copying fails, a complete selectable summary identifies the input, selected pair and conventions and includes every unit in the category, including uncommon units.