About Mm to Inches Conversion

The millimeter-to-inch conversion is crucial for anyone working with tools, fasteners, or precision equipment. Mechanics, machinists, and DIY enthusiasts constantly encounter this when specifications use one system but available tools use the other.

Why This Conversion Matters

Drill bits — Finding the right size when only one unit system is available
Fasteners — Matching metric bolts (M3, M5, M8) with imperial wrenches
3D printing — Nozzle sizes, layer heights, and tolerances are typically in mm
Machining — Converting between metric blueprints and imperial tools

The Exact Conversion

One inch equals exactly 25.4 millimeters. This definition was established internationally in 1959, making the conversion mathematically precise.

Practical Application

When a project calls for an 8mm hole but you only have inch drill bits, you need a 5/16" bit (7.94mm) — close enough for most purposes. This converter helps find these equivalents.

How to Convert Mm to Inches

Using This Tool

Enter millimeters in the top field
Read inches (decimal) below
Find the nearest fractional inch using the reference table

Drill Bit Equivalents

Common metric to imperial drill bit matches:

3 mm = 0.118" ≈ 7/64"
5 mm = 0.197" ≈ 13/64"
6 mm = 0.236" ≈ 15/64" or 1/4"
8 mm = 0.315" ≈ 5/16"
10 mm = 0.394" ≈ 25/64" or 3/8"

Socket/Wrench Sizes

For metric bolts, find the nearest inch wrench:

10 mm ≈ 3/8"
13 mm ≈ 1/2"
17 mm ≈ 11/16"
19 mm ≈ 3/4"

The Formula

Inches = mm ÷ 25.4. For quick estimates, divide mm by 25 and you'll be within 2%.

Features

Tool-Focused Reference

The quick reference table shows common drill bit and fastener sizes. Find metric-to-imperial equivalents at a glance.

High Precision

Results shown to 8 decimal places — necessary for machining tolerances where 0.001" matters.

Bidirectional

Need inches to mm? Enter the inch value in the bottom field or tap swap. Convert either direction without leaving the page.

Copy Without Units

The copy function grabs just the number, ready for CAD software, CNC programs, or documentation.

Works in the Workshop

Mobile-friendly — Use on your phone at the workbench
Offline capable — Works after initial load, no internet needed
No installation — Just bookmark and use anytime

Frequently Asked Questions

What is 10 mm in inches?

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

Why do some tools use mm and others use inches?

The US primarily uses imperial units while most of the world uses metric. Many products sold globally (especially from Asia and Europe) use metric, while American-made tools often use inches.

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

Almost, but not quite. 10mm = 0.394" while 3/8" = 0.375". The 10mm socket is slightly larger. It may work in a pinch but risks rounding bolt heads.

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

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

How precise is this conversion?

Mathematically exact. The international definition specifies 1 inch = 25.4 mm precisely. All decimal results use this exact ratio.

Millimeters to Inches