Length

mm to Inches Converter

Convert millimeters to inches and back instantly — type in either box, with a reference chart for the sizes people look up most.

How to convert mm to inches

One inch is defined as exactly 25.4 millimeters, so converting either direction is a single step:

inches = mm ÷ 25.4
To go the other way: mm = inches × 25.4

For example, 100 mm ÷ 25.4 = 3.937 inches, and a 12-inch ruler is exactly 304.8 mm long. Type a value into either box above and the paired field, the 0–200 mm progress bar and the chart all update instantly — clear a box and the other clears too.

mm to inches conversion chart

These are the millimeter values people convert most often, from a single millimeter up to 200 mm. Handy for reading engineering drawings, screen and print specs, or hardware sizes that list only metric measurements:

Millimeters (mm)	Inches (in)	Inches (approx.)
1 mm	0.0394"	~1/32"
5 mm	0.1969"	~3/16"
10 mm	0.3937"	~3/8"
25 mm	0.9843"	~1"
50 mm	1.9685"	~2"
100 mm	3.9370"	~4"
150 mm	5.9055"	~6"
200 mm	7.8740"	~8"

Why 25.4 is the exact factor

Since the international yard-and-pound agreement of 1959, the inch has been fixed at exactly 25.4 millimeters, which makes this conversion precise rather than approximate. Because 1 cm = 10 mm, you could convert millimeters to centimeters first and then to inches, but dividing straight by 25.4 is quicker and just as exact. As a rough mental estimate, 25 mm is very close to one inch.

Where millimetre precision actually matters

Millimetres are the working unit wherever small differences carry consequences. Engineering drawings are dimensioned in mm almost universally, including in countries that otherwise use imperial, because a millimetre is a convenient smallest-meaningful-unit for machined parts — fine enough to specify a tolerance, coarse enough to avoid decimals everywhere.

The same applies to anything that has to fit. Plumbing and pipe fittings, bearing sizes, camera lens filter threads, watch strap widths, phone screen bezels and PCB layouts are all specified in mm. When a spec sheet gives you millimetres and your tape measure reads inches, the conversion is not optional — a 2 mm error on a 20 mm part is a 10% error, and parts that should slide together will not.

Screw, drill and fastener sizes

Hardware is where mm and inches collide most often in ordinary life, because metric and imperial fasteners look nearly identical and are not interchangeable. A few of the closest near-misses are worth knowing:

Metric	In inches	Nearest imperial
3 mm	0.118"	1/8" (0.125")
6 mm	0.236"	1/4" (0.250")
10 mm	0.394"	3/8" (0.375")
13 mm	0.512"	1/2" (0.500")
19 mm	0.748"	3/4" (0.750")

Note how close 19 mm and 3/4" are — 0.002" apart, near enough that a socket of one will usually turn a fastener of the other. But 10 mm and 3/8" differ by 0.019", which is enough to round off a bolt head if you force it. That is the practical value of converting: knowing which pairs are genuinely interchangeable and which only look it.

Reading a ruler in both units

On a dual-scale ruler the metric edge is divided into millimetres, with every tenth marked as a centimetre. The imperial edge is divided into fractions — halves, quarters, eighths and sixteenths. One sixteenth of an inch is 1.5875 mm, so the two scales almost never line up, which is why measurements taken on one edge should be converted rather than eyeballed across to the other. If you need a fractional inch from a millimetre figure, divide by 25.4 and then round to the nearest sixteenth: 10 mm is 0.394", which is closest to 6/16" (0.375") or 7/16" (0.4375") — noticeably between the two, and a good illustration of why precision work stays in one system.

Frequently asked questions

How many inches is 1 mm?

1 mm equals 0.03937 inches. Divide the millimeter value by 25.4 to get inches.

How many mm is 1 inch?

1 inch equals 25.4 mm exactly, fixed by the international yard-and-pound agreement of 1959.

How do I convert inches to mm?

Multiply the number of inches by 25.4 to get millimeters.

Is mm to inches the same formula as cm to inches?

They are related but not identical. Since 1 cm = 10 mm, you can convert mm to cm then to inches, but dividing millimeters directly by 25.4 is faster.

Is a 19 mm socket the same as 3/4 inch?

Almost — 19 mm is 0.748" and 3/4" is 0.750", just 0.002" apart, so one will usually turn the other. 10 mm and 3/8" are not: they differ by 0.019", enough to round off a bolt head.

How do I convert mm to a fraction of an inch?

Divide the millimetres by 25.4, then round to the nearest sixteenth. One sixteenth is 1.5875 mm, so the two scales rarely line up exactly.