LengthWise

Millimeters to Inches Converter

Divide by 25.4 — the precision conversion for engineering, fasteners, and materials.

millimetres (mm)

inches (in)

millimetres → inches

25 mm = 0.984 in

Type in either box — the other updates to match.

millimetres to inches conversion table

Fastener and sheet-material sizes; 25.4 mm is exactly one inch.

millimetres (mm)	inches (in)
1 mm	0.039 in
2 mm	0.079 in
3 mm	0.118 in
5 mm	0.197 in
6 mm	0.236 in
8 mm	0.315 in
10 mm	0.394 in
12 mm	0.472 in
15 mm	0.591 in
20 mm	0.787 in
25 mm	0.984 in
25.4 mm	1 in
30 mm	1.181 in
50 mm	1.969 in
100 mm	3.937 in
250 mm	9.843 in
500 mm	19.685 in

What you entered

millimetres: 25 mm

1
Conversion factor
1 mm = 0.0394 in= × 0.0394
2
Multiply by the value
25 × 0.0394= 0.9843 in

Result

inches: 0.984 in

25 mm equals 0.984 in, because one millimetre is 0.039 in.

Small measurements in millimetres and inches

One inch is exactly 25.4 mm, so dividing by 25.4 converts the other way. These rows cover the sizes where millimetres are normally used.

What it is	mm	in
Sheet of paper thickness	0.1	0.004
Credit card thickness	0.76	0.030
Standard screw diameter	6	0.236
Pencil diameter	7	0.276
Phone thickness	8	0.315
AA battery diameter	14.5	0.571
Plywood sheet thickness	18	0.709
Ruler width	25	0.984

Millimetres dominate engineering, joinery and electronics because a millimetre is a practical smallest unit - the equivalent fractional inches are awkward, as the 0.276 and 0.709 rows show. Fractional inches are more usual in imperial practice: 18 mm is 0.709 inches, closest to 23/32, which is why imperial plywood comes in sizes like 3/4 inch rather than a metric equivalent. For a rough conversion, divide by 25 and subtract about 1.5%.

Converting millimetres to inches means dividing by 25.4, since an inch is exactly 25.4 millimetres. This is the same definition that gives 2.54 centimetres per inch, just expressed at a finer scale, and it is exact rather than approximate.

Millimetres are the working unit of engineering and manufacturing across most of the world, because they let people express precise dimensions without decimals. A part specified as 6 mm is cleaner to write and read than 0.6 cm or 0.00600 m. Inches, meanwhile, remain standard in American manufacturing and in certain globally standardised products regardless of country. That mismatch means anyone working with hardware, machining, 3D printing, or construction materials converts between the two constantly.

Fasteners are where this bites hardest, and the danger is that metric and imperial sizes are often close enough to seem interchangeable while being incompatible. A 6 mm bolt is 0.2362 inches, sitting between the ¼-inch (0.25) and 7/32-inch sizes — near enough that a wrong socket may grip well enough to round off a fastener head, but not near enough to hold safely under load. The same trap exists with thread pitches, where a metric and an imperial thread can start together and then bind. Matching by conversion arithmetic alone is not sufficient; the standard has to match too.

One more difference in convention: imperial engineering often expresses fine dimensions in thousandths of an inch, called thou or mil, where one thou is 0.0254 mm. A tolerance of ±0.005 inches is ±0.127 mm. When reading imperial drawings, watch for decimal inches rather than fractions, since the two notations appear on the same drawings and mean different things.

Close is not compatible for fasteners

A 6 mm bolt is 0.2362 inches, close to but not the same as ¼ inch (0.25). Near-matches are dangerous: a wrong-standard socket may grip well enough to round off a head, and mismatched thread pitches can start cleanly then bind. Match the standard, not just the converted number.

Thou, mil, and decimal inches

Imperial engineering often works in thousandths of an inch — one thou is 0.0254 mm, so a ±0.005 inch tolerance is ±0.127 mm. Imperial drawings mix fractional and decimal inch notation, and the two mean different things, so read carefully before converting.

Frequently asked questions

How many millimetres are in an inch?

Exactly 25.4 mm. It follows directly from the inch being defined as exactly 2.54 cm, so the conversion carries no uncertainty at all.

What is 25 mm in inches?

25 mm is 0.9843 inches — just under one inch. The one-inch mark sits at 25.4 mm, which is why 25 mm and one inch are often loosely treated as equal but genuinely are not.

Can I substitute a metric bolt for an imperial one?

Not safely on the basis of a converted number alone. Metric and imperial fasteners have different thread standards as well as different diameters, so a near-match can start threading and then bind or strip. Match the standard, not the arithmetic.

What is a thou or a mil?

One thousandth of an inch, equal to 0.0254 mm. It's the standard unit for tolerances on imperial engineering drawings — a ±0.005 inch tolerance means ±0.127 mm.