Convert Inches (in) to Millimeters (mm)

Q: How many millimeters are in one inch?

Exactly 25.4 mm. This is the precise international definition, not an approximation.

1 inch equals 25.4 millimeters.

Formula: mm = in × 25.4

Inch to Millimeter Converter

What Is an Inch?

The inch (in) is a unit of length in the US customary and British imperial systems. Since 1959, it is defined as exactly 25.4 millimeters. The inch is widely used in the US for construction, manufacturing, screen sizes, and hardware specifications.

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 the standard precision unit in engineering, manufacturing, and science worldwide. Most technical drawings, component specs, and machining tolerances are expressed in millimeters.

Inches to Millimeters Formula

The conversion factor is exact by international definition:

millimeters = inches × 25.4 exact

This is the foundational relationship between imperial and metric length — all other inch/foot/yard conversions derive from it.

Worked Example

Convert 3/8 inch (a common drill bit size) to millimeters:

Convert fraction: 3/8 = 0.375
Multiply: 0.375 × 25.4 = 9.525 mm

Quick Estimation

Multiply inches by 25 for a quick approximation (actual factor: 25.4 — within 1.6%). Example: 3 inches × 25 = 75 mm (actual: 76.2 mm). For better accuracy, multiply by 25 and add 2%: 75 + 1.5 = 76.5 mm.

Common Use Cases

Manufacturing and machining: US drawings in inches; international CNC machines use mm. A 1/4-inch bolt = 6.35 mm.
Hardware and fasteners: Imperial drill bits (1/8", 3/16", 1/4") must match metric equivalents (3.175, 4.763, 6.35 mm).
Electronics: PCB component spacing in inches (0.1" pitch = 2.54 mm). Connector pins in metric.
Woodworking: US lumber in inches, European in mm. 3/4" plywood = 19.05 mm (often sold as 18 mm metric).

Frequently Asked Questions

How many millimeters are in one inch?

Exactly 25.4 mm. This is the precise international definition, not an approximation.

How do you convert fractional inches to millimeters?

Convert the fraction to decimal, then multiply by 25.4. Example: 5/16" = 0.3125 × 25.4 = 7.9375 mm. Common: 1/8" = 3.175 mm, 1/4" = 6.35 mm, 1/2" = 12.7 mm.

What is the difference between a mil and a millimeter?

A mil (or thou) = 0.001 inch = 0.0254 mm. A millimeter = 1 mm = 39.37 mils. Mils are used in US electronics for PCB trace widths; mm is the international standard.

Why don't metric and imperial drill bit sizes match exactly?

Imperial sizes (1/8", 3/16") produce non-round mm values (3.175, 4.763). Metric sizes (3 mm, 5 mm) produce non-round inch values. The two systems are offset, so exact matches are rare.

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 Inch to Millimeter Conversions

Inch to Millimeter (Part 1)

Inch	Millimeter	Actions
12 inches	12.7 millimeters
1 inch	25.4 millimeters
2 inches	50.8 millimeters
3 inches	76.2 millimeters

Inch to Millimeter (Part 2)

Inch	Millimeter	Actions
4 inches	101.6 millimeters
5 inches	127 millimeters
10 inches	254 millimeters

Inches to millimeters Conversion Table

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

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

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

Looking for the reverse? Convert Millimeter to Inch

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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.