Millimeters to Inches Converter

1 inch = 25.4 mm · 1 mm = 0.0393701 inches

Length & Distance

How to Use This Tool

Millimeters to inches converter

Divide millimeters by 25.4 to get inches. That conversion factor is exact. One inch has been defined as precisely 25.4 millimeters since 1959, when the international yard-and-pound agreement locked in the value. Every calculation on this page uses that definition.

Millimeters to inches quick reference

Millimeters Inches Fraction (approx.)
1 mm 0.0394 in ~1/25 in
2 mm 0.0787 in ~5/64 in
3 mm 0.1181 in ~1/8 in
4 mm 0.1575 in ~5/32 in
5 mm 0.1969 in ~13/64 in
6 mm 0.2362 in ~15/64 in
8 mm 0.3150 in ~5/16 in
10 mm 0.3937 in ~25/64 in
12 mm 0.4724 in ~15/32 in
15 mm 0.5906 in ~19/32 in
20 mm 0.7874 in ~25/32 in
25 mm 0.9843 in ~63/64 in
25.4 mm 1.0000 in 1 in
30 mm 1.1811 in ~1 3/16 in
40 mm 1.5748 in ~1 37/64 in
50 mm 1.9685 in ~1 31/32 in
75 mm 2.9528 in ~2 61/64 in
100 mm 3.9370 in ~3 15/16 in
150 mm 5.9055 in ~5 29/32 in
200 mm 7.8740 in ~7 7/8 in
300 mm 11.8110 in ~11 13/16 in

For values not listed, type them into the converter above or divide by 25.4.

How to convert millimeters to inches

The formula

inches = millimeters / 25.4

Since 25.4 is the exact definition, there is no rounding in the conversion factor itself. Any rounding happens only when you choose how many decimal places to keep in the result.

Example 1: Bolt diameter

An M6 bolt has a nominal diameter of 6 mm:
6 / 25.4 = 0.2362 inches
The closest standard imperial drill bit is 15/64 inch (0.2344 in), which is why M6 bolts do not fit cleanly into a 1/4-inch hole (0.25 in).

Example 2: Phone thickness

A phone that is 7.8 mm thick:
7.8 / 25.4 = 0.307 inches
Just under a third of an inch.

Example 3: Plywood sheet

A 12 mm plywood panel:
12 / 25.4 = 0.4724 inches
This is close to but not quite 1/2 inch (0.5 in), which matters when fitting panels into frames built to imperial dimensions.

When you need millimeters to inches

Hardware and fasteners

Metric bolts, screws, and nuts are sized in millimeters. If you are working with imperial tools or threading into imperial-sized holes, you need the inch equivalent to pick the right tap or drill bit. An 8 mm bolt converts to 0.315 inches, which sits between 5/16 in (0.3125) and 21/64 in (0.3281). Knowing this prevents stripped threads and loose fits.

3D printing and machining

Most 3D printer slicers work in millimeters, but some US-made CNC machines and shop drawings use inches. A part designed at 50 mm wide needs to be entered as 1.9685 inches if the machine reads imperial. Getting the conversion wrong by even a fraction of a millimeter can ruin a tolerance fit.

Electronics and components

Component datasheets from Asian and European manufacturers list dimensions in millimeters. US circuit board layouts often use mils (thousandths of an inch). A 2.54 mm pin spacing equals exactly 0.1 inches (100 mil), which is why through-hole components still fit US-standard breadboards.

Jewelry

Ring widths, gemstone sizes, and chain links are measured in millimeters worldwide. US jewelers sometimes need inch equivalents for custom settings or legacy tooling. A 6 mm wide wedding band is about 15/64 inch, and a 2 mm chain link is roughly 5/64 inch.

Finding the nearest fractional inch

Imperial measurements in the US often use fractions (1/8, 3/16, 1/4, etc.) rather than decimals. After dividing millimeters by 25.4, you get a decimal inch value. To find the nearest fraction, multiply the decimal by the denominator you want. For 64ths: multiply by 64, round to the nearest whole number, and put that over 64.

For example, 10 mm = 0.3937 inches. Multiply 0.3937 by 64 = 25.2, which rounds to 25. So 10 mm is approximately 25/64 inch. For most woodworking and general construction, 32nds or 16ths are precise enough. For machining, 64ths or decimal inches are standard.

Tips for accurate conversions

  • Use 25.4, not 25. Rounding the factor introduces a 1.6% error. At 100 mm, that is the difference between 3.937 inches (correct) and 4.0 inches (wrong by nearly 2 mm).
  • For quick mental estimates, remember that 25 mm is almost exactly 1 inch. So 50 mm is about 2 inches, 75 mm is about 3, and so on. Add roughly 1.5% for the precise value.
  • When working with fractional inches, convert the fraction to a decimal first, then compare. 3/8 inch = 0.375 in = 9.525 mm. A 10 mm measurement is slightly larger than 3/8 inch.
  • Avoid chain conversions (mm to cm to inches). Go directly from mm to inches by dividing by 25.4. Each intermediate step can introduce rounding errors.

Frequently asked questions

How many inches is 1 mm?

1 mm = 0.03937 inches. This is roughly 1/25 of an inch.

How many mm is 1 inch?

1 inch = 25.4 mm exactly. This is a defined value, not a measurement.

How do I convert mm to inches?

Divide the millimeter value by 25.4. For example, 50 mm / 25.4 = 1.9685 inches.

What is 10 mm in inches?

10 mm = 0.3937 inches, which is approximately 25/64 of an inch.

Is 25 mm the same as 1 inch?

Close but not exact. 1 inch = 25.4 mm, so 25 mm is 0.9843 inches. The difference (0.4 mm) matters in precision work like machining, though it is negligible for rough measurements.

What is the difference between mm and inches?

Millimeters are metric units (1/1000 of a meter). Inches are imperial/US customary units. One inch is much larger than one millimeter: it takes 25.4 mm to make a single inch. Most of the world uses millimeters; the US primarily uses inches.

How do I convert mm to fractional inches?

First divide by 25.4 to get decimal inches. Then multiply by the fraction denominator (16, 32, or 64), round to the nearest whole number, and write it as a fraction. For 8 mm: 8 / 25.4 = 0.315 in. 0.315 x 32 = 10.08, which rounds to 10. So 8 mm is approximately 10/32, or 5/16 inch.

Why is the conversion factor 25.4?

The 1959 international yard-and-pound agreement defined one yard as exactly 0.9144 meters. Dividing that down: 1 yard = 36 inches, and 0.9144 meters = 914.4 mm. So 914.4 / 36 = 25.4 mm per inch. The number is exact by definition.

Measurement guides

Formula & Explanation

Since 1 inch is defined as exactly 25.4 mm, the reverse factor is 1/25.4 = 0.0393700787... inches per mm. We display the rounded value 0.0393701 in the formula for readability; the actual computation uses full precision.

Frequently Asked Questions

How do I convert millimeters to inches?

To convert millimeters to inches, multiply the millimeters value by 0.0393701. For example, 1 millimeters = 0.0393701 inches. Our converter above does this calculation instantly as you type.

What is the formula for millimeters to inches?

The formula is: inches = millimeters u00d7 0.0393701. This is a linear conversion, meaning the ratio is constant regardless of the value being converted.

Is this millimeters to inches converter accurate?

Yes. Our converter uses the internationally recognized conversion factor of 1 millimeters = 0.0393701 inches, which is the standard accepted by NIST and most engineering references. Results are rounded to a sensible precision to avoid floating-point noise.

Can I convert inches back to millimeters?

Yes. The converter is bidirectional u2014 type in either field and the other updates automatically. To convert inches to millimeters, divide the inches value by 0.0393701, or simply use the swap button (u21c4) between the two fields.

Why would I need to convert millimeters to inches?

Common reasons include international travel (different countries use different measurement systems), cooking recipes from other regions, engineering and construction work, scientific calculations, and understanding product specifications listed in unfamiliar units.

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