Example 1
100 mm -> 3.9370 in on the caliper.
Mm to inches
Type a length in millimetres or in inches and pick the direction. The calculator divides by 25.4. 100 mm is 3.9370 in. The US survey inch from maps is not used here.
Threads and sheet. Height and tape: centimeters and inches. Feet: meters and feet.
Result
Enter a value. The result shows up here.
How it works
The international inch has been exactly 25.4 mm since 1959. 100 mm on a caliper is 3.9370 in. 1 in back is 25.4000 mm. A4 paper is 210 mm wide, 8.2677 in. 10 mm is 0.3937 in, not a clean 3/8 inch. 2 in on a screw is 50.8000 mm. A 6 in ruler is 152.4000 mm.
A socket is often marked in inches, while an M8 screw is in millimeters. Type 100 and leave mm. A 3.5 mm jack is 0.1378 in, close to 1/8 (3.175 mm), but not equal. 12 in is 304.8000 mm, which is 1 ft, but feet are not printed here. 1.5 mm sheet uses the same formula.
Pick the direction from the list: mm to inches or inches to mm. A comma and a period both work. Typed zero is allowed. A minus is rejected. Four decimals stay, because 1/16 in is 1.5875 mm and rounding to zero loses a thread. The switch sets mm or inches on an empty field and does not rewrite a typed 100.
Centimeters are next door: 10 mm = 1 cm, but you stay in mm here. Type a 180 height on the cm page, not as 180 mm. Feet live on the meters page when someone gives 1 ft instead of 12 in.
3/8 in is 9.525 mm. 10 mm is 0.3937 in. Close by a rough estimate, too far on a caliper. The survey inch from old US maps is not used here.
Threads, sheet, and jacks share the same 25.4 mm inch. Do not mix 180 mm with 180 cm, and do not treat a 3.5 mm jack as exactly 1/8.
How to use
- Type a length from a caliper or a drawing, for example 100 or 3.5.
- Pick mm to inches when you measured metric, or inches to mm when you have an inch rule.
- Click Calculate. 100 mm gives 3.9370 in. A4 width, 210 mm, gives 8.2677 in.
- The result is decimal, not a 25/64 fraction. 10 mm is 0.3937 in, not exactly 3/8.
- For height, open centimeters. Here it stays the millimeter from a thread or a sheet.
Formula
in = mm / 25.4
mm = in × 25.4
International inch, not survey. 12 in = 1 ft, but feet are not printed here.
mm and in from the 25.4 factor
1 in = 25.4 mm. 100 mm is 3.9370 in. A 3.5 mm jack is 0.1378 in, not exactly 1/8 in. Survey inch is not used here.
- mm
- Millimetre. 100 mm of rod, 3.5 mm jack, 210 mm A4 side. 180 mm is not 180 cm.
- in
- International inch, 25.4 mm. 3.9370 from 100 mm. 12 in is a foot, but the result stays in in.
Real-life examples
Example 2
1 in -> 25.4000 mm exactly.
Example 3
10 mm -> 0.3937 in, not a clean 3/8.
Example 4
2 in -> 50.8000 mm on a bolt.
Example 5
210 mm -> 8.2677 in of A4 paper.
Example 6
6 in -> 152.4000 mm on a ruler.
Example 7
12 in -> 304.8000 mm, which is 1 ft.
Ways to use this converter
- You convert a screw diameter from millimeters when the socket is marked in inches.
- You check 1.5 mm sheet against a US spec written in fractional inches.
Frequently asked questions
Is 1 inch exactly 25.4 mm here?
Yes, since 1959. The “about 25” shortcut is too loose for a thread. The US survey inch from maps is not used here.
Is 10 mm 3/8 inch?
10 mm is 0.3937 in. 3/8 in is 9.525 mm. Close by a rough estimate, too far on a caliper. The calculator does not round to a fraction of an inch.
How many inches is A4 paper?
Width 210 mm is 8.2677 in. Height 297 mm is 11.6929 in. You convert each side on its own, with the same 25.4.
Is 12 in a foot on this page?
12 in is 304.8000 mm, which is 1 ft by definition. The result is still in mm or in, with no foot label. Feet live on another calculator.
How is this different from centimeters and inches?
Same family, another scale. Threads and sheet metal go in mm. Height is usually in cm. 180 mm is not 180 cm.
Is a 3.5 mm jack 1/8 inch?
1/8 in = 3.175 mm. 3.5 / 25.4 is 0.1378 in. Close in a shop, not equal in the formula.
Can length be negative?
Zero mm is allowed and gives zero in. A minus is usually a typing mistake, not a negative thread. The calculator rejects that entry.
What does the header switch do?
On an empty field, metric sets mm and US sets inches. It does not rewrite a typed number, only the label.
How many mm are 2 in and 6 in?
2 in is 50.8000 mm. 6 in is 152.4000 mm. 1 in is 25.4000 mm. Same factor 25.4.
Does 1/16 inch vanish when you round?
1/16 in is 1.5875 mm. That is why the calculator keeps four decimals, so a thread does not vanish behind a “1.6 mm” sign.
Knowledge sources
The converter multiplies your number by an SI factor. NIST SP 330 and BIPM define the units, not your reading.
Page updated in 2026.