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Mathematics & calculation

Convert mm to inches (and inches to mm)

Two-way millimeters ↔ inches converter: exact factor (1 in = 25.4 mm), fractions of an inch (1/4, 3/8, 1/16), formulas, a conversion table and real examples (1/4 in bolt, 13 mm wrench, 205/55 R16 tire, 17-inch wheel).

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Conversion table

mm to in conversion
mmin
1 mm0.03937 in
2 mm0.07874 in
5 mm0.1969 in
10 mm0.3937 in
20 mm0.7874 in
50 mm1.9685 in
100 mm3.93701 in
1,000 mm39.3701 in
in to mm conversion
inmm
1 in25.4 mm
2 in50.8 mm
5 in127 mm
10 in254 mm
20 in508 mm
50 in1,270 mm
100 in2,540 mm
1,000 in25,400 mm

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How does this tool work?

This converter turns millimeters into inches and inches into millimeters: type a value in either field and the other updates instantly. It is the scale of small technical measurements: the diameter of a screw or drill bit, the size of a wrench or socket, the width of a tire, the diameter of a wheel, the thickness of a board or sheet metal, or the dimensions on a US drawing given in fractions of an inch.

The international inch is exactly 25.4 mm. The 1959 international yard and pound agreement fixed 1 yard = 0.9144 m, so 1 inch = 0.9144 m ÷ 36 = 25.4 mm. In the other direction, 1 mm ≈ 0.0393701 inch.

To go from inches to millimeters, multiply by 25.4; to go from millimeters to inches, divide by 25.4. US dimensions are often written as fractions: convert the fraction to a decimal first (3/8 = 0.375), then multiply: 0.375 × 25.4 = 9.525 mm. Each sixteenth of an inch is 1.5875 mm.

For a quick estimate, divide millimeters by 25 and take off about 1.6%: 100 mm ≈ 4 in (exact value 3.937). The other way, multiply inches by 25 and add a little: 17 in ≈ 425 mm (exact value 431.8).

Beware of nominal sizes: a "1/2 inch" pipe has an outside diameter of about 21.3 mm (0.84 in), not 12.7 mm, and a "28-inch" bike wheel has a bead seat diameter of 622 mm (about 24.5 in). Another trap: the "mil" or "thou" is one thousandth of an inch, or 0.0254 mm, and has nothing to do with the millimeter.

How to use this tool

  1. Enter a measurement

    In millimeters or in decimal inches; for a fraction such as 5/16, type 0.3125.

  2. Read the conversion

    The other field recalculates on every keystroke, with enough decimals for machining work.

  3. Check the chart

    Below the converter, the table lists the most common diameters and fractions of an inch, as well as 15 to 18-inch wheels.

Mm ↔ inches conversion formulas

The inch is exactly 25.4 mm, which also makes it easy to convert fractions of an inch and pixel densities.

Inches to millimeters

mm = in × 25.4

Exact factor. For a fraction n/d of an inch: mm = (n ÷ d) × 25.4.

Millimeters to inches

in = mm ÷ 25.4 ≈ mm × 0.0393701

The 0.0393701 factor is rounded.

Screen density

ppi = 25.4 ÷ p

Pixels per inch from the pixel pitch p in millimeters.

mm
length in millimeters
in
length in international inches (1 in = 25.4 mm)
p
pixel pitch, the distance between two pixels in millimeters
ppi
pixels per inch

Example: a 205/55 R16 tire has an overall diameter of 16 × 25.4 + 2 × 205 × 0.55 = 406.4 + 225.5 = 631.9 mm, or about 24.88 inches.

Use cases

Fasteners and tools

Find the metric equivalent of a screw, drill bit or socket given in fractions of an inch, or check that a wrench fits.

Cars and bikes

Read tire and wheel sizes, which mix millimeters (width) and inches (diameter).

Drawings and spec sheets

Convert the dimensions of a plan, a manual or a 3D-printed part designed in inches.

Electronics and screens

Go from pixel pitch in millimeters to pixels per inch, or convert the thickness of a circuit board.

Examples

1/4 inch bolt

0.25 × 25.4 = 6.35 mm, close to an M6 screw but not interchangeable.

Input

0.25 inch

Result

6.35 mm

13 mm wrench

13 ÷ 25.4 ≈ 0.512 in, very close to a 1/2 in wrench (12.7 mm), which may still round off the nut.

Input

13 mm

Result

0.512 inch

Tire width

In 205/55 R16, 205 is the width in millimeters: 205 ÷ 25.4 ≈ 8.07 in.

Input

205 mm

Result

8.07 inches

17-inch wheel

17 × 25.4 = 431.8 mm wheel diameter.

Input

17 inches

Result

431.8 mm

Headphone jack

3.5 ÷ 25.4 ≈ 0.138 in; it is often called a 1/8 in jack (3.175 mm), which is only an approximation.

Input

3.5 mm

Result

0.138 inch

Tips and best practices

  • Handy reference: 1/16 in = 1.5875 mm, 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 1/2 in = 12.7 mm, 3/4 in = 19.05 mm.
  • To turn a decimal result into a fraction, multiply it by 16 (or 64) and round: 0.512 × 16 ≈ 8.2, so about 8/16 = 1/2 in.
  • Metric and inch wrench sizes only match approximately: 19 mm ≈ 3/4 in (19.05 mm) works, but 13 mm and 1/2 in much less so.
  • To add or simplify fractions of an inch, the fraction calculator is better suited.

Sources and references

Frequently asked questions

How many millimeters are in an inch?

One inch is exactly 25.4 mm. This value comes from the 1959 international agreement that fixed the yard at 0.9144 m.

How do I convert mm to inches?

Divide the millimeters by 25.4. For example, 10 mm ≈ 0.394 in, 19 mm ≈ 0.748 in and 100 mm ≈ 3.937 in.

What is 1/4 inch in mm?

1/4 inch = 0.25 × 25.4 = 6.35 mm. Likewise 3/8 in = 9.525 mm and 1/2 in = 12.7 mm.

What does 205/55 R16 mean on a tire?

205 is the tire width in millimeters (≈ 8.07 in), 55 the sidewall height as a percentage of that width (112.75 mm) and 16 the wheel diameter in inches (406.4 mm).

What is a mil or a thou?

It is one thousandth of an inch: 1 mil = 0.0254 mm. One millimeter is therefore about 39.37 mils. The unit is used for plastic film thickness and machining tolerances.

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