MM to Inches Converter

Convert millimeters to inches, or inches to millimeters, using the exact definition: 1 inch = 25.4 mm. Handy for drill bits, wrenches, bolts and technical drawings.

FreeNo sign-upInstant resultsUpdated: October 22, 2026

Converter

mm → inin → mm

Millimeters

Inches

1 in

25.4 mm = 1 in

Conversion factor

1 mm = 0.03937 in

Rounded to 2 decimal places: 1 in
Rounded to 4 decimal places: 1 in
Full value: 1 in

Quick chart: millimeters to inches

1 mm = 0.03937 in

3 mm = 0.1181 in

5 mm = 0.1969 in

6 mm = 0.2362 in

8 mm = 0.315 in

10 mm = 0.3937 in

12 mm = 0.4724 in

16 mm = 0.6299 in

20 mm = 0.7874 in

25.4 mm = 1 in

50 mm = 1.97 in

100 mm = 3.94 in

1 inch is exactly 25.4 mm. Tools and hardware often use fractional inches (1/4, 3/8…): 1/4 in = 6.35 mm.

Keep analyzing

For estimates only. Double-check the numbers before making important decisions.

MM to inches formula

inches = millimeters ÷ 25.4
millimeters = inches × 25.4

1 in = 25.4 mm exactly

To convert a fractional inch, turn the fraction into a decimal first (3/8 = 0.375), then multiply by 25.4: 0.375 × 25.4 = 9.525 mm.

Fractional inches to mm chart

Fraction	Decimal inches	Millimeters
1/16 in	0.0625 in	1.5875 mm
1/8 in	0.125 in	3.175 mm
3/16 in	0.1875 in	4.7625 mm
1/4 in	0.25 in	6.35 mm
5/16 in	0.3125 in	7.9375 mm
3/8 in	0.375 in	9.525 mm
7/16 in	0.4375 in	11.1125 mm
1/2 in	0.5 in	12.7 mm
5/8 in	0.625 in	15.875 mm
3/4 in	0.75 in	19.05 mm
7/8 in	0.875 in	22.225 mm
1 in	1 in	25.4 mm

Common metric sizes in inches

Millimeters	Inches	Closest fraction
3 mm	0.1181 in	just under 1/8 in
5 mm	0.1969 in	just over 3/16 in
6 mm	0.2362 in	just under 1/4 in
8 mm	0.315 in	just over 5/16 in
10 mm	0.3937 in	just over 3/8 in
13 mm	0.5118 in	just over 1/2 in
19 mm	0.748 in	just under 3/4 in

Close isn't the same as equal. A 13 mm socket is 0.3 mm larger than a 1/2 in bolt head, enough to round it off under torque.

How much precision to use

The conversion factor is exact, so the result is only rounded for display. In machining and engineering, two or three decimals are typical for millimeters and four for inches (often called "ten-thousandths"). The calculator shows 2 and 4 decimal versions plus the full value.

Common uses

Matching metric and SAE wrenches, sockets and drill bits.
Reading plans and parts made in countries that use the metric system.
Pipe, tubing and hose diameters.
Jewelry, bead and wire sizes.

Example

A 10 mm drill bit is 0.3937 inches, slightly larger than 3/8 in. A 3/4-inch wrench is 19.05 mm.

10 ÷ 25.4 = 0.3937 in
3/8 in = 0.375 in, so 10 mm is about 0.019 in larger
3/4 in = 0.75 in
0.75 × 25.4 = 19.05 mm

Common mistakes

Mixing up millimeters and centimeters. The factor is 25.4 for mm and 2.54 for cm.
Rounding to the nearest fractional inch without checking the part's tolerance.
Assuming metric and SAE tools are interchangeable because the sizes are close.

Frequently asked questions

How many millimeters are in an inch?

Exactly 25.4 mm.

How many inches is 10 mm?

10 mm is 0.3937 inches, a little more than 3/8 in (0.375 in).

How do I convert a fraction of an inch to mm?

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

Is 1/2 inch the same as 13 mm?

No. 1/2 in is 12.7 mm, so 13 mm is 0.3 mm larger. They're close, but not interchangeable for a snug fit.

How many mm is 1/4 inch?

1/4 inch is exactly 6.35 mm.

Keep going with the next step.