Millimeters to Inches (mm to in) Converter

Millimeters to Inches

The millimeter-to-inch conversion is crucial for anyone working with tools, fasteners, or precision equipment. Mechanics, machinists, and DIY enthusiasts hit it whenever specs use one system but the available tools use the other.

One inch equals exactly 25.4 mm, so mm = inch × 25.4 and inches = mm ÷ 25.4. This definition was set internationally in 1959.

Why It Matters

Drill Bits

Find the right size when only one unit system's bits are on hand.

Fasteners

Match metric bolts (M3, M5, M8) with imperial wrenches.

3D Printing

Nozzle sizes, layer heights, and tolerances are usually given in mm.

How to Use This Converter

Enter millimetres

Type the mm value in the top field. Decimal inches appear instantly below.

Read inches

The result updates live to 8 decimal places for machining tolerances.

Find the fraction

Use the reference table to match the nearest fractional inch.

Conversion Formula

inches = mm ÷ 25.4 • Quick estimate: divide mm by 25 to be within about 2%.

Drill Bit & Fastener Equivalents

mm	Inches	Nearest fraction
6 mm	0.236"	~1/4"
8 mm	0.315"	~5/16"
10 mm	0.394"	~3/8"
19 mm	0.748"	~3/4"

Features

Instant As-You-Type Results

The result updates the moment you enter a number — no button to press, no page reload.

One-Click Swap

The swap button between the two fields reverses the direction instantly, so you can convert back without retyping.

Copy Without the Unit

Each field has its own copy button that grabs only the number, ready to paste into forms, spreadsheets, or documents.

Searchable Unit Picker

Each dropdown has a search box — switch to any other length unit and use it as a general-purpose converter.

Workshop Precision

Tool Matching

When a project needs an 8 mm hole but you only have inch bits, a 5/16" bit (7.94 mm) is close enough for most jobs.

High Precision

Results to 8 decimal places matter for machining where 0.001" counts in CAD and CNC programs.

Works in the workshop, stays private. Mobile-friendly at the workbench, runs offline after loading, and keeps your project dimensions on your device.

Frequently Asked Questions

What is 10 mm in inches?

10 mm equals 0.3937 inches, which is closest to 25/64" or roughly 3/8" (0.375").

Why do some tools use mm and others inches?

The US mainly uses imperial while most of the world uses metric. Products sold globally (especially from Asia and Europe) tend to be metric, while American-made tools are often in inches.

Can I use a 10 mm socket instead of a 3/8" socket?

Almost, but not quite. 10 mm = 0.394" while 3/8" = 0.375", so the 10 mm socket is slightly larger and may round bolt heads — fine in a pinch, not ideal.

What's the difference between M6 and 1/4" bolts?

M6 means a 6 mm thread diameter; 1/4" = 6.35 mm, so M6 is slightly smaller. The threads also differ — metric and imperial threads are incompatible.

How precise is this conversion?

Mathematically exact. The international definition fixes 1 inch = 25.4 mm precisely, so all decimal results use this exact ratio.

Millimeters to Inches