Convert MM to Inches

Convert millimetres to inches for engineering and DIY.

Direction

Value in millimetre (mm)

Decimal places

25.4 mm in inch

1 in

Conversion factor

1 mm = 0.039370079 in

Reverse factor

1 in = 25.4 mm

in = mm × 0.039370079 — 25.4 × 0.039370079 = 1

Imperial tooling is sized in binary fractions, so the nearest 16th or 64th of an inch is usually what you need in practice.

Quick reference table.
mm	in
0.5	0.0197
1	0.0394
2	0.0787
3	0.1181
4	0.1575
5	0.1969
10	0.3937
15	0.5906
20	0.7874
25	0.9843
30	1.1811
40	1.5748
50	1.9685
75	2.9528
100	3.937
150	5.9055
200	7.874
500	19.685
1000	39.3701

What does MM to Inches Converter do?

Convert millimetres to inches exactly, with fractional output to the nearest 64th for precision engineering. Also converts in reverse and shows the decimal thou (thousandths of an inch) used in machining.

How to Use MM to Inches Converter

1
Enter a value in millimetresDecimals welcome.
2
Choose the fraction precision64ths for engineering, 16ths for DIY.
3
Read the resultDecimal, fraction and thou.

Key Features

Exact conversion: 1 inch = 25.4 mm
Fractions to the nearest 64th, 32nd, 16th or 8th
Thou (thousandths of an inch) for machining
Two-way conversion in one tool
Reference table including common drill and fastener sizes

Frequently Asked Questions

How many mm is 1 inch?

Exactly 25.4 mm. Every other imperial length derives from this definition, which is what makes the conversions exact rather than approximate.

What is a thou?

One thousandth of an inch, 0.0254 mm, also called a mil. It is the standard precision unit in machining and PCB design, where tolerances are quoted in thou.

Why round to 64ths?

Because imperial tooling comes in binary fractions. Drill bits, wrenches and stock thicknesses are sized in 64ths, 32nds and 16ths, so the nearest fraction tells you which tool to reach for.