Inches to MM Converter

How to Use This Tool

1 inch = 25.4 millimeters — exactly, with no rounding. That’s rare in the imperial-metric system, where most conversions are approximations.

The inch became exact in 1959. That’s when it was formally redefined as precisely 25.4mm.

Formula

mm = inches × 25.4
inches = mm ÷ 25.4

Worked example: 12 inches × 25.4 = 304.8 mm.
Reverse: to go from mm to inches, divide by 25.4. So 100mm ÷ 25.4 = 3.94 inches.

Inches to millimeters — fractions and common decimals

The table below covers fractions, whole inches, and common decimals.

Inches	Millimeters
1/16″	1.59 mm.
1/8″ (.125″)	3.18 mm.
3/16″	4.76 mm.
1/4″ (.25″)	6.35 mm.
3/8″ (.375″)	9.53 mm.
1/2″ (.5″)	12.7 mm.
5/8″ (.625″)	15.88 mm.
3/4″ (.75″)	19.05 mm.
7/8″	22.23 mm.
1″	25.4 mm.
1.5″	38.1 mm.
2″	50.8 mm.
2.5″	63.5 mm.
3″	76.2 mm.
3.5″	88.9 mm.
4″	101.6 mm.
4.5″	114.3 mm.
5″	127 mm.
5.5″	139.7 mm.
6″	152.4 mm.
6.5″	165.1 mm.
7.5″	190.5 mm.
8″	203.2 mm.
8.5″	215.9 mm.
9.5″	241.3 mm.
10″	254 mm.
12″ (1 ft)	304.8 mm.
13″	330.2 mm.
18″	457.2 mm.

Millimeters to inches — the sizes people search for most

This second table flips the direction, for the mm values people look up most.

Millimeters	Inches
6.5 mm	0.26 in.
8 mm	0.31 in.
10 mm	0.39 in.
12.5 mm	0.49 in.
48 mm	1.89 in.
54 mm	2.13 in.
75 mm	2.95 in.
76 mm	2.99 in.
85 mm	3.35 in.
95 mm	3.74 in.
105 mm	4.13 in.
110 mm	4.33 in.
115 mm	4.53 in.
127 mm	5 in. (exactly)
130 mm	5.12 in.
135 mm	5.31 in.
140 mm	5.51 in.
155 mm	6.10 in.
165 mm	6.50 in.
175 mm	6.89 in.
190 mm	7.48 in.
210 mm	8.27 in.
220 mm	8.66 in.
250 mm	9.84 in.
256 mm	10.08 in.
270 mm	10.63 in.
285 mm	11.22 in.
305 mm	12.01 in.
320 mm	12.6 in.
360 mm	14.17 in.
380 mm	14.96 in.
420 mm	16.54 in.
700 mm	27.56 in.
800 mm	31.5 in.

Notice 127mm converts to exactly 5 inches. That’s one of the rare round numbers on both sides of this conversion — worth remembering as a benchmark.

Why this conversion matters more in some fields than others

Construction and consumer goods use inches-to-mm constantly. Parts specified in one system — US screws, lumber, phone screens — often need to fit into or be described in the other.

Machining and engineering tolerances are especially sensitive to this. A 1/4″ bolt measures 6.35mm, not 6mm and not 6.5mm. Mixing up “close enough” metric hardware with imperial specs is a classic source of stripped threads and mismatched parts.

Jewelry sizing runs into the same issue at a smaller scale. A ring component listed as 18mm and one listed as 0.7 inches look close, but they aren’t identical — 0.7″ actually works out to 17.78mm, close enough to throw off a snug fit.

A common mix-up

People confuse millimeters and centimeters when reading a converted result. 25.4mm and 2.54cm describe the same length, but glancing at “25.4” and assuming it’s centimeters makes the object look ten times too long.

Always check the unit label, not just the number. A related mix-up: reading “165mm” as “16.5mm,” or the reverse, by misplacing a decimal point.

Double-check against the reference tables above if a converted number looks unexpectedly large or small for the object you’re measuring.

Converting fractions of an inch without a calculator

Convert the fraction to a decimal first, then multiply by 25.4. Here are the common ones:

1/16 = 0.0625
1/8 = 0.125
3/16 = 0.1875
1/4 = 0.25
3/8 = 0.375
1/2 = 0.5
5/8 = 0.625
3/4 = 0.75
7/8 = 0.875

For 3/8″: 0.375 × 25.4 = 9.525mm.

For a rough mental shortcut, multiply by 25 instead of 25.4. That lands you within about 1.6% — close enough to sanity-check a precise calculation, but not close enough for a machining tolerance.

Frequently Asked Questions

How big is 8mm in inches?
About 0.315 inches — a bit under 1/3 of an inch. Common for small hardware like watch pins and some drill bit sizes.

How thick is 5mm in inches?
About 0.197 inches, just under 1/5 of an inch — a common thickness for phone cases, craft foam, and some glass panels.

How long is 12mm in inches?
About 0.472 inches, just under 1/2 inch — a frequent socket wrench and bolt-head size.

Is 1 inch closer to 25mm or 25.4mm?
It’s exactly 25.4mm. Some rougher references round to “about 25mm,” but 25.4 is the precise, internationally defined value.

How many mm is 1/16 of an inch?
1.5875 mm — useful for reading fine machinist or jewelry measurements.

How do I convert a fraction of an inch to mm without a calculator?
Convert the fraction to a decimal first (1/4 = 0.25), then multiply by 25.4. For 3/8″: 0.375 × 25.4 = 9.525mm.