Example 1
- Direction: inches to mm
- 1 in
25.4 mm
How many mm is 1 inch? 25.4 mm.
Inches to mm
The calculator converts inches to millimetres. One inch is exactly 25.4 mm. Half an inch is 12.7 mm. This is thread and sheet-metal scale, not height in centimeters.
The other page, mm to inches, is on mm to inches. Height: inches to cm.
Result
Enter a length in inches or in millimetres. The result will appear here.
How it works
The calculator multiplies inches by 25.4. 1 inch is 25.4 mm. Half an inch is 12.7 mm. 2 inches are 50.8 mm.
The other way, you divide millimetres by 25.4. 100 mm is 3.937 in.
The international inch is 25.4 mm. The US survey inch from maps is not used here.
This is thread, sheet metal, and a hole. You do not enter 180 cm of height as 180 here.
You may enter zero; the result is then also zero. The calculator does not accept negative numbers.
A comma and a period give the same fraction. 0.25 in is 6.35 mm.
Formula
mm = in × 25.4; in = mm / 25.4
An inch is exactly 25.4 mm. That is the same constant as 2.54 cm.
How to use
- Choose inches to millimetres.
- Enter 1. The result is 25.4 mm.
- Enter 0.5. The result is 12.7 mm.
- The other way, enter 100. The result is 3.937 in.
1 inch is 25.4 mm
An inch is 25.4 mm. Half an inch is 12.7 mm.
- in
- Inch. 1 in = 25.4 mm. This is not a survey inch from a map.
- mm
- Millimetre. 0.5 in = 12.7 mm. 2 in = 50.8 mm.
- 25.4
- The inch constant in millimetres. The same as 2.54 cm.
Examples
Example 2
- Direction: inches to mm
- 0.5 in
12.7 mm
How many mm is half an inch? 12.7 mm.
Example 3
- Direction: inches to mm
- 2 in
50.8 mm
How many mm are 2 inches? 50.8 mm.
Related calculators
Frequently asked questions
How many mm is 1 inch?
Exactly 25.4 mm.
How many mm is half an inch?
12.7 mm.
How many mm are 2 inches?
50.8 mm.
Is this the same constant as inches to cm?
Yes. 25.4 mm is 2.54 cm. This page uses the millimetre scale.
Should I enter 180 cm of height here?
No. 180 cm belongs on inches to cm. This page is for thread and sheet metal.
Is the US survey inch used?
No. The factor is the international inch, 25.4 mm.
Can length be negative?
No. Zero is allowed; after conversion the result is still zero.
How many inches is 100 mm?
3.937 in. That is the other side of the same formula.
Does a comma work?
Yes. 0.25 in is 6.35 mm.
Where do I convert mm to inches?
On the mm to inches page. That page starts from millimetres.
Sources
An inch is 25.4 mm.
Page updated in 2026.