Inches To MM Converter

1. Type the inch length the way it is written on the drawing or the part: 3/8, 1 1/4, 1-1/4 or 0.375.
2. Choose how many decimal places you need. The verdict warns when the exact value has more, as 1/64 in (0.396875 mm) does.
3. Read the millimeters. The verdict says whether the value is shown in full and how far it sits from the nearest half millimeter.

What each output is for:

An inch to millimeter conversion rewrites an inch length in millimeters by multiplying by 25.4. The result is exact, because the international inch is defined as exactly 25.4 mm.

NIST Handbook 44 lists 1 inch as exactly 2.54 centimeters, and NIST SP 811 gives 2.54 x 10^-2 meter. The definition dates from 1959, when the United States and other nations fixed the foot at exactly 0.3048 meter. So the inch you measure today is a metric quantity with a round name.

This direction has one big advantage over the reverse. Inch fractions with a bottom number of 2, 4, 8, 16, 32 or 64 always give millimeter values that end. 1/64 in is exactly 0.396875 mm. That is why a drawing converted to mm can keep every digit, while mm to inches almost always needs rounding.

The converter first turns any fraction into a decimal inch, then multiplies by 25.4.

1. Read the fraction. For 1 1/4, divide 1 by 4 to get 0.25, then add the whole part: 1.25 in.
2. Multiply by 25.4: 1.25 x 25.4 = 31.75 mm.
3. Round only for display. 31.75 mm has two decimals, so any setting of 2 or more shows it in full.

There is an exact mental shortcut. 25.4 is 25 plus 0.4, so multiply by 25, multiply by 0.4 and add. For 3 in: 75 + 1.2 = 76.2 mm, with no error at all.

A 3/8 in bolt in a metric drawing. 3 / 8 = 0.375 in. Then 0.375 x 25.4 = 9.525 mm. The nearest half millimeter is 9.5 mm, only 0.025 mm under.

A 1 1/4 in part for 3D printing. 1.25 x 25.4 = 31.75 mm. A file drawn in inches but read as millimeters comes in 25.4 times too small, so the fix is a scale of 2540 percent, not a redraw.

A plus or minus 0.005 in tolerance. 0.005 x 25.4 = 0.127 mm. A dimension of 2.000 in plus or minus 0.005 in becomes 50.8 mm plus or minus 0.127 mm. One thou is exactly 0.0254 mm.

A 12 in steel rule. 12 x 25.4 = 304.8 mm. One foot is therefore 304.8 mm, just under 30.5 cm.

A 5/16 in hole. 0.3125 x 25.4 = 7.9375 mm. This needs four decimals to show in full. For adding or subtracting inch fractions before you convert, use the fraction calculator.

The factor never changes, so the result depends only on what you type and how you display it.

How the Fraction Is Written

1 1/4 and 1-1/4 both mean 1.25 in. The tool accepts both, plus plain decimals. A typo such as 11/4 means 2.75 in, so check the decimal inches result.

Decimal Places Shown

1/64 in is 0.396875 mm, six decimals. At 3 places it shows as 0.397 mm. The verdict flags this so a CAD value is not cut short by mistake. To judge how many digits a measured value really deserves, use the significant figures calculator.

Nominal Sizes That Are Not Real Sizes

Some inch names are labels, not measurements. Pipe is the main case, covered in the next section.

Tolerances

A tolerance converts with the same factor as the size. Plus or minus 0.001 in is plus or minus 0.0254 mm. Convert both the size and its tolerance, not just the size.

Pipe sizes in inches do not convert with 25.4. A nominal pipe size (NPS) is a trade name only loosely tied to the real diameter. The metric partner is the DN number from ISO 6708.

So a 1/2 in pipe is about 21.3 mm across the outside, not 12.7 mm. Use the converter for measured lengths and the pipe table for pipe names. Outside diameters follow the Engineering ToolBox summary of ISO 6708; ASME values can differ by a few tenths of a millimeter.

Machining from an Inch Drawing

Convert every size and tolerance to mm before you set a metric machine. Keep four decimals so a 7.9375 mm bore is not rounded to 7.94 mm.

3D Printing and CAD

Enter part sizes in mm for metric software. A 2 in cube is 50.8 mm on each side. The cm to mm converter helps when a drawing mixes cm and mm.

Buying Metric Parts for an Inch Design

Look at the half-millimeter result. 3/8 in sits 0.025 mm from 9.5 mm, while 1/2 in sits 0.2 mm from 12.5 mm. The first swap may work; the second needs a check.

Common inch fractions and lengths in exact millimeters, with the nearest half millimeter and its gap in mm. Values are computed from 1 in = 25.4 mm. For inch lengths over a foot, the inches to feet converter gives the feet split.

1. Multiplying by 25

Using 25 instead of 25.4 makes every answer 1.6 percent small. A 36 in length comes out 900 mm instead of 914.4 mm, 14.4 mm short.

2. Reading 1 1/4 as 11/4

Without a space or dash, 11/4 is eleven quarters, or 2.75 in. That is 69.85 mm instead of 31.75 mm.

3. Converting a Pipe Name with 25.4

NPS 1/2 is not a 12.7 mm pipe. Its outside diameter is about 21.3 mm.

4. Cutting Decimals Too Early

Rounding 7.9375 mm to 7.9 mm loses 0.0375 mm. On a press fit that can be the whole tolerance.

5. Forgetting to Convert the Tolerance

A size in mm with a tolerance still in inches mixes two units in one line. Convert both.

The conversion is exact, because the inch is defined as 25.4 mm. Only the display is rounded, and the verdict tells you when it is.

Three eighths of an inch is exactly 9.525 mm. Divide 3 by 8 to get 0.375 in, then multiply by 25.4.

Divide the top number by the bottom number, then multiply by 25.4. For 5/16 in: 5 / 16 = 0.3125, and 0.3125 x 25.4 = 7.9375 mm.

One and a quarter inches is exactly 31.75 mm. The fraction is 1.25 in, and 1.25 x 25.4 = 31.75.

No. NPS 1/2 is a nominal name. Its outside diameter is about 21.3 mm, and its metric name is DN 15.

One thou, or 0.001 in, is exactly 0.0254 mm. So 10 thou is 0.254 mm and 100 thou is 2.54 mm.

Twelve inches, or one foot, is exactly 304.8 mm. That follows from 1 ft = 0.3048 m.

The factor is exact, so a terminating inch decimal gives an exact mm value. A fraction such as 1/3 in repeats: it is 8.4667 mm to 4 decimals.

Scale it by 25.4, which is 2540 percent. A model drawn as 2 in wide then prints 50.8 mm wide.

Multiply the tolerance by 25.4, just like the size. Plus or minus 0.005 in is plus or minus 0.127 mm.

Yes. The converter reads a space or a dash between the whole number and the fraction, so both give 31.75 mm.

• Conversion factors for the inch and the mil (NIST SP 811, Appendix B.8).
• Exact inch, foot and mil equivalents in metric units (NIST Handbook 44, 2026, Appendix C).
• The 1959 definition of the international foot (NIST Office of Weights and Measures).
• Nominal pipe size, DN and outside diameters from ISO 6708 (Engineering ToolBox).