Free MM to Inches Converter

Convert millimeters to inches and back. One inch is exactly 25.4 mm, so the result is precise — type in either box for an instant answer.

Millimeters (mm)

Inches (in)

Common values

Millimeters	Inches
1 mm	0.0394 in
5 mm	0.1969 in
10 mm	0.3937 in
25 mm	0.9843 in
50 mm	1.9685 in
100 mm	3.937 in
1000 mm	39.3701 in

Quick answer

To convert millimeters to inches, divide the number of millimeters by 25.4, because one inch is exactly 25.4 mm. For example, 100 mm ÷ 25.4 = 3.94 inches. To go back, multiply inches by 25.4.

Formula & method

inches = millimeters ÷ 25.4

millimeters — length in mm
25.4 — millimeters in one inch (exact)

Inverse: millimeters = inches × 25.4.

Examples

Example 1: 25.4 millimeters

Input: 25.4 mm
Result: 1 inch
Why: 25.4 ÷ 25.4 = 1, since an inch is defined as 25.4 mm.

Example 2: 10 millimeters

Input: 10 mm
Result: 0.3937 in
Why: 10 ÷ 25.4 = 0.3937.

Example 3: 100 millimeters

Input: 100 mm
Result: 3.937 in
Why: 100 ÷ 25.4 = 3.937.

When to use this tool

Reading engineering, woodworking, or 3D-printing dimensions in the other unit.
Converting screw, bolt, or part sizes.
Following plans that mix metric and imperial.

Common mistakes

Confusing millimeters with centimeters; 10 mm = 1 cm, so 1 inch is 25.4 mm but 2.54 cm.
Using 25 instead of the exact 25.4.
Reading the decimal of inches as a fraction directly — 0.5 in is a half inch, but 0.3937 in is not 39/100 of anything tidy.

Frequently asked questions

＋ How do I convert mm to inches?

Divide the millimeters by 25.4. The result is exact because an inch is defined as exactly 25.4 mm.

＋ How many millimeters are in an inch?

Exactly 25.4 millimeters.

＋ What’s the difference between mm and cm to inches?

There are 10 mm in a centimeter. So an inch is 25.4 mm or 2.54 cm — divide mm by 25.4, or cm by 2.54.

＋ How do I convert inches back to millimeters?

Multiply inches by 25.4, or just type a value into the inches box here.

＋ Is this accurate enough for engineering?

Yes. The 25.4 factor is exact, and the tool shows several decimals; round to the precision your work requires.

✓ Free to use
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✓ Formula and method shown above

Last updated: 2026-06-05How we calculate Report a correction Disclaimer

Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.