Convert Millimeters (mm) to Inches (in)

Q: How many mm is 1 inch?

Exactly 25.4 mm. This is the international standard since the 1959 yard and pound agreement between six nations.

Q: What is 10 mm in inches?

10 mm = 0.3937 inches — between ⅜" (9.525 mm) and 13/32" (10.319 mm). In practice, a 10 mm socket is the closest metric equivalent to ⅜".

1 millimeter equals 0.0393701 inches.

Formula: in = mm × 0.0393701

Millimeter to Inch Converter

What Is a Millimeter?

The millimeter (mm) is a metric unit of length equal to one thousandth (1/1000) of a meter. It is the global standard for precision measurements in engineering, manufacturing, electronics, and medical imaging. Technical drawings, bolt specifications, and CNC machining all use millimeters.

What Is an Inch?

The inch (in or ") is an imperial/US customary unit defined since 1959 as exactly 25.4 millimeters. It is the primary small-measurement unit in the US for hardware (bolt sizes, pipe fittings), construction, and consumer products.

Millimeters to Inches Formula

The conversion is based on the exact 1959 definition:

inches = millimeters ÷ 25.4 exact

Since 1 inch is defined as exactly 25.4 mm, this division is the precise conversion. No rounding of the conversion factor is needed.

Worked Example

Convert 10 mm (a common bolt head size) to inches:

Divide: 10 ÷ 25.4 = 0.3937 inches

So 10 mm ≈ 25/64 inch — between ⅜" (9.525 mm) and 13/32" (10.319 mm).

Quick Estimation

25 mm ≈ 1 inch. Divide mm by 25 for a fast estimate (overshoots by 1.6%). Example: 50 mm ÷ 25 = 2 inches (exact: 1.969"). For better accuracy, divide by 25.4.

Common Millimeters to Inches Conversions

Millimeters	Inches	Context
1 mm	0.0394 in	Mechanical pencil lead / thin wire
3 mm	0.118 in	Standard eyeglass screw
5 mm	0.197 in	Allen key / small bolt
6 mm	0.236 in	Standard drill bit (close to ¼")
8 mm	0.315 in	Socket wrench size (close to 5/16")
10 mm	0.394 in	Common bolt size (close to ⅜")
12 mm	0.472 in	Plywood thickness (close to ½")
19 mm	0.748 in	Close to ¾ inch (19.05 mm)
25.4 mm	1.000 in	Exactly 1 inch (definition)
50.8 mm	2.000 in	Exactly 2 inches

Which Is Bigger: Inch or Millimeter?

An inch is exactly 25.4 times larger than a millimeter. One inch = 25.4 mm. A millimeter is roughly the thickness of a credit card, while an inch is about the width of a thumb.

When You Need This Conversion

Hardware and fasteners: Metric bolts (M6, M8, M10) must often match imperial tools. An M10 bolt head (10 mm) requires a socket close to ⅜" (9.525 mm) — but not exact, so use a 10 mm socket.
Electronics and PCB design: Component pin spacing is often 2.54 mm (0.1") — the standard DIP pitch. PCB trace widths in mm convert to mils (1 mil = 0.0254 mm).
Woodworking: Metric plywood (12 mm, 18 mm) needs comparison with imperial sizes (½", ¾"). 18 mm = 0.709" — slightly thinner than ¾" (19.05 mm).

Frequently Asked Questions

How many mm is 1 inch?

Exactly 25.4 mm. This is the international standard since the 1959 yard and pound agreement between six nations.

How do I convert mm to inches?

Divide by 25.4. Example: 50 mm ÷ 25.4 = 1.969 inches ≈ 2 inches. For a quick estimate, divide by 25.

What is 10 mm in inches?

10 mm = 0.3937 inches — between ⅜" (9.525 mm) and 13/32" (10.319 mm). In practice, a 10 mm socket is the closest metric equivalent to ⅜".

What is the difference between mm and mil?

A "mil" (or "thou") = 1/1000 of an inch = 0.0254 mm. So 1 mm = 39.37 mil. Mils are used in US electronics and thin-film measurement; mm is the metric equivalent.

Can I use metric and imperial tools interchangeably?

Sometimes, but not recommended. Close pairs like 10 mm / ⅜" or 19 mm / ¾" can strip fasteners because they are close but not exact. Always use the correct system for the fastener.

Fractional Inches to Millimeter Chart

Common fractional inch values and their equivalents in mm. Useful for machining, woodworking, drill bits, and precision measurements.

Fractions of an inch (1/64″ to 1″)

Fraction (in)	Decimal (in)	mm
1/64″	0.0156	0.3969
1/32″	0.0313	0.7938
3/64″	0.0469	1.1906
1/16″	0.0625	1.5875
5/64″	0.0781	1.9844
3/32″	0.0938	2.3813
7/64″	0.1094	2.7781
1/8″	0.1250	3.1750
9/64″	0.1406	3.5719
5/32″	0.1563	3.9688
11/64″	0.1719	4.3656
3/16″	0.1875	4.7625
13/64″	0.2031	5.1594
7/32″	0.2188	5.5563
15/64″	0.2344	5.9531
1/4″	0.2500	6.3500
17/64″	0.2656	6.7469
9/32″	0.2813	7.1438
19/64″	0.2969	7.5406
5/16″	0.3125	7.9375
21/64″	0.3281	8.3344
11/32″	0.3438	8.7313
23/64″	0.3594	9.1281
3/8″	0.3750	9.5250
25/64″	0.3906	9.9219
13/32″	0.4063	10.319
27/64″	0.4219	10.716
7/16″	0.4375	11.113
29/64″	0.4531	11.509
15/32″	0.4688	11.906
31/64″	0.4844	12.303
1/2″	0.5000	12.700
33/64″	0.5156	13.097
17/32″	0.5313	13.494
35/64″	0.5469	13.891
9/16″	0.5625	14.288
37/64″	0.5781	14.684
19/32″	0.5938	15.081
39/64″	0.6094	15.478
5/8″	0.6250	15.875
41/64″	0.6406	16.272
21/32″	0.6563	16.669
43/64″	0.6719	17.066
11/16″	0.6875	17.463
45/64″	0.7031	17.859
23/32″	0.7188	18.256
47/64″	0.7344	18.653
3/4″	0.7500	19.050
49/64″	0.7656	19.447
25/32″	0.7813	19.844
51/64″	0.7969	20.241
13/16″	0.8125	20.638
53/64″	0.8281	21.034
27/32″	0.8438	21.431
55/64″	0.8594	21.828
7/8″	0.8750	22.225
57/64″	0.8906	22.622
29/32″	0.9063	23.019
59/64″	0.9219	23.416
15/16″	0.9375	23.813
61/64″	0.9531	24.209
31/32″	0.9688	24.606
63/64″	0.9844	25.003
1″	1.0000	25.400

Whole inches (2″ to 12″)

Inches	mm
2″	50.800
3″	76.200
4″	101.60
5″	127.00
6″	152.40
7″	177.80
8″	203.20
9″	228.60
10″	254.00
11″	279.40
12″	304.80

Bold rows indicate common fractions (1/8, 1/4, 3/8, 1/2, etc.). The conversion factor 1 inch = 25.4 mm is exact by international definition.

Common Millimeter to Inch Conversions

Millimeter to Inch (Part 1)

Millimeter	Inch	Actions
1 millimeter	0.0393701 inches
5 millimeters	0.19685 inches
10 millimeters	0.393701 inches

Millimeter to Inch (Part 2)

Millimeter	Inch	Actions
25 millimeters	0.984252 inches
50 millimeters	1.9685 inches
100 millimeters	3.93701 inches

Millimeters to inches Conversion Table

Reference table with common millimeters to inches conversions. All values calculated with high precision.

Millimeters to Inches Table
0.01 to 50

Millimeters	Inches	Actions
0.01 mm	0.000393701 in
0.025 mm	0.000984252 in
0.05 mm	0.0019685 in
0.1 mm	0.00393701 in
14 mm	0.00984252 in
12 mm	0.019685 in
1 mm	0.0393701 in
2 12 mm	0.0984252 in
5 mm	0.19685 in
10 mm	0.393701 in
25 mm	0.984252 in
50 mm	1.9685 in

Millimeters to Inches Table
100 to 10000000

Millimeters	Inches	Actions
100 mm	3.93701 in
250 mm	9.84252 in
500 mm	19.685 in
1000 mm	39.3701 in
2500 mm	98.4252 in
5000 mm	196.85 in
10000 mm	393.701 in
25000 mm	984.252 in
50000 mm	1968.5 in
100000 mm	3937.01 in
250000 mm	9842.52 in
500000 mm	19685 in
1000000 mm	39370.1 in
2500000 mm	98425.2 in
5000000 mm	196850 in
10000000 mm	393701 in

Inches to Millimeters Table
0.0001 to 12

Inches	Millimeters	Actions
0.0001 in	0.00254 mm
0.00025 in	0.00635 mm
0.0005 in	0.0127 mm
0.001 in	0.0254 mm
0.0025 in	0.0635 mm
0.005 in	0.127 mm
0.01 in	0.254 mm
0.025 in	0.635 mm
0.05 in	1.27 mm
0.1 in	2.54 mm
14 in	6.35 mm
12 in	12.7 mm

Inches to Millimeters Table
1 to 100000

Inches	Millimeters	Actions
1 in	25.4 mm
2 12 in	63.5 mm
5 in	127 mm
10 in	254 mm
25 in	635 mm
50 in	1270 mm
100 in	2540 mm
250 in	6350 mm
500 in	12700 mm
1000 in	25400 mm
2500 in	63500 mm
5000 in	127000 mm
10000 in	254000 mm
25000 in	635000 mm
50000 in	1270000 mm
100000 in	2540000 mm

Looking for the reverse? Convert Inch to Millimeter

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Conversion factors verified against NIST, BIPM, ISO 80000-3 1 inch = 2.54 cm by international agreement (1959). Last reviewed: March 2026

By Adilson Fernandes

Reviewed by Tiago Fernandes

All unit conversions on CoolConversion use conversion factors defined or documented by internationally recognised standards bodies (such as ISO and NIST), including both SI and non-SI units.