How many millimeters are in 1 inch?

1 in equals 25.4 mm. This is an exact conversion factor defined by NIST.

How do I convert inches to millimeters?

Multiply the value in in by the conversion factor. For example, 10 in = 254 mm. Use our calculator above for instant results.

Is the inches to millimeters conversion exact?

Yes, this conversion is mathematically exact based on the international definition of these units (NIST SP 811).

Can I convert inches to millimeters offline?

Yes! OmniConvert runs entirely in your browser — no internet connection needed after the page loads. Your data never leaves your device, ensuring complete privacy.

What is the difference between inches and millimeters?

Inches (in) and Millimeters (mm) are both units of length but belong to different measurement systems. 1 in = 25.4 mm. Use the converter above to switch between them instantly.

Inches to Millimeters (in → mm) — Free Online Converter

in → mm Reference Table

in	mm
0.001 in	0.0254 mm
0.002 in	0.0508 mm
0.003 in	0.0762 mm
0.004 in	0.1016 mm
0.005 in	0.127 mm
0.006 in	0.1524 mm
0.007 in	0.1778 mm
0.008 in	0.2032 mm
0.009 in	0.2286 mm
0.01 in	0.254 mm
0.02 in	0.508 mm
0.03 in	0.762 mm
0.04 in	1.016 mm
0.05 in	1.27 mm
0.06 in	1.524 mm
0.07 in	1.778 mm
0.08 in	2.032 mm
0.09 in	2.286 mm
0.1 in	2.54 mm
0.2 in	5.08 mm
0.3 in	7.62 mm
0.4 in	10.16 mm
0.5 in	12.7 mm
0.6 in	15.24 mm
0.7 in	17.78 mm
0.8 in	20.32 mm
0.9 in	22.86 mm
1 in	25.4 mm
1.5 in	38.1 mm
2 in	50.8 mm
2.5 in	63.5 mm
3 in	76.2 mm
3.5 in	88.9 mm
4 in	101.6 mm
4.5 in	114.3 mm
5 in	127 mm
5.5 in	139.7 mm
6 in	152.4 mm
6.5 in	165.1 mm
7 in	177.8 mm
7.5 in	190.5 mm
8 in	203.2 mm
8.5 in	215.9 mm
9 in	228.6 mm
9.5 in	241.3 mm
10 in	254 mm
11 in	279.4 mm
12 in	304.8 mm
13 in	330.2 mm
14 in	355.6 mm
15 in	381 mm
16 in	406.4 mm
17 in	431.8 mm
18 in	457.2 mm
19 in	482.6 mm
20 in	508 mm
25 in	635 mm
30 in	762 mm
35 in	889 mm
40 in	1016 mm
45 in	1143 mm
50 in	1270 mm
55 in	1397 mm
60 in	1524 mm
65 in	1651 mm
70 in	1778 mm
75 in	1905 mm
80 in	2032 mm
85 in	2159 mm
90 in	2286 mm
95 in	2413 mm
100 in	2540 mm
120 in	3048 mm
125 in	3175 mm
130 in	3302 mm
140 in	3556 mm
150 in	3810 mm
160 in	4064 mm
175 in	4445 mm
180 in	4572 mm
200 in	5080 mm
225 in	5715 mm
250 in	6350 mm
275 in	6985 mm
300 in	7620 mm
350 in	8890 mm
400 in	10160 mm
450 in	11430 mm
500 in	12700 mm
600 in	15240 mm
700 in	17780 mm
750 in	19050 mm
800 in	20320 mm
900 in	22860 mm
1000 in	25400 mm
1200 in	30480 mm
1250 in	31750 mm
1500 in	38100 mm
1750 in	44450 mm
2000 in	50800 mm
2500 in	63500 mm
3000 in	76200 mm
4000 in	101600 mm
4500 in	114300 mm
5000 in	127000 mm
6000 in	152400 mm
7000 in	177800 mm
7500 in	190500 mm
8000 in	203200 mm
9000 in	228600 mm
10000 in	254000 mm
15000 in	381000 mm
20000 in	508000 mm
25000 in	635000 mm
50000 in	1270000 mm
75000 in	1905000 mm
100000 in	2540000 mm

Values based on NIST SP 811 conversion factors

Why Convert Inches to Millimeters?

Converting between inches (in) and millimeters (mm) is one of the most common length conversions. Whether for engineering, cooking, travel, or science, knowing how to quickly convert in to mm saves time and prevents errors.

Length conversions between in and mm are fundamental in construction, manufacturing, and international trade. Architects rely on precise conversions when adapting building plans between metric and imperial standards — a 1% error on a 30-meter wall translates to 30 cm of misalignment. Travelers frequently convert between kilometers and miles for road trips, while online shoppers need length conversions to verify clothing sizes, furniture dimensions, and screen diagonals across international retailers. In athletics, track distances (100m, 1500m, marathon) must be precisely converted for record comparisons across different measurement traditions.

How to Convert Inches to Millimeters

To convert inches to millimeters, use the following formula:

mm = in × 25.4

Example: 1 in = 25.4 mm

For example, 5 in = 127 mm, 10 in = 254 mm, and 100 in = 2540 mm. For larger values, 1000 in = 25400 mm. Conversely, 1 mm = 0.03937007874 in. Our calculator above performs this conversion instantly with full precision — no rounding errors.

Common Use Cases

Half-inch pipe diameter (plumbing): 0.5 in = 12.7 mm
Quarter-inch audio jack: 0.25 in = 6.35 mm
6-inch smartphone screen diagonal: 6 in = 152.4 mm
One-inch sensor (camera): 1 in = 25.4 mm

Common Mistakes to Avoid

Confusing similar units: confusing nautical miles with regular miles, or yards with meters.
Decimal errors: moving the decimal point the wrong way (e.g., 100 cm = 1 m, not 10 m).
Approximation: assuming an inch is exactly 2.5 cm (it's 2.54 cm).

Pro Tips

Use 'approx' for mental math: 1 km is roughly 0.6 miles (or 2/3 for quick estimates).
Visualize: A meter is about a guitar length; an inch is a thumb width.
Ruler check: Most rulers show cm and inches together—great for quick checks.

What is a Inch?

The inch (in) is a unit of length in the Imperial and US customary systems. Historically defined as the width of a thumb or three barleycorns, it is now exactly defined as 25.4 millimeters (0.0254 meters) since 1959.

Inches are widely used in the US, UK, and Canada for short lengths. Common uses include screen sizes (TVs, monitors), pipe diameters, and paper sizes.

What is a Millimeter?

The millimeter (mm) is a metric unit of length equal to one-thousandth of a meter (0.001 m). It is a standard unit in the International System of Units (SI).

Millimeters are used for precision engineering, manufacturing dimensions, rainfall measurement, and paper sizes (ISO 216). It represents small scale accuracy.

Inches → Millimeters

Inches to Millimeters Formula