About Inches to Mm Conversion

Converting inches to millimeters is crucial for mechanics, machinists, and DIY enthusiasts working with mixed tool systems. When imperial specifications meet metric tools, this conversion bridges the gap.

Why This Matters

Wrenches and sockets — Finding metric equivalents for 1/4", 3/8", 1/2" sizes
Automotive work — Many cars use both metric and imperial fasteners
Machining — Converting between measurement systems for precision work
Pipe fittings — Matching imperial pipes with metric connectors

The Exact Conversion

One inch equals exactly 25.4 millimeters. This is the international definition, making every conversion mathematically precise.

Practical Application

A 1/2" bolt needs a 12.7 mm wrench, but you'll likely use a 13 mm since that's the nearest common metric size. Knowing these close matches saves time in the shop.

How to Convert Inches to Mm

Using This Tool

Enter inches in the top field (use decimals for fractions)
Read millimeters appearing below
Find the nearest standard metric size

Common Fractional Inches

Convert fractions to decimals first:

1/4" = 0.25" = 6.35 mm
3/8" = 0.375" = 9.53 mm
1/2" = 0.5" = 12.7 mm
5/8" = 0.625" = 15.88 mm
3/4" = 0.75" = 19.05 mm
1" = 25.4 mm

Socket/Wrench Equivalents

Nearest metric matches:

1/4" → 6 mm (close fit)
3/8" → 10 mm (close fit)
1/2" → 13 mm (close fit)
3/4" → 19 mm (very close)

The Formula

mm = inches × 25.4. For fractions, convert to decimal first: 3/8" = 0.375" × 25.4 = 9.525 mm.

Features

Tool-Focused Reference

The quick reference shows common wrench and socket sizes with their metric equivalents. Find the right tool fast.

High Precision

Results to 8 decimal places for machining tolerances where 0.001 mm matters.

Bidirectional

Convert mm to inches by entering in the bottom field. Essential when reading metric specs but using imperial tools.

Workshop Ready

Use at the workbench — Works on mobile at the job site
Offline capable — Functions without internet after loading
No installation — Bookmark and use anytime

Copy Function

Copy the exact value for CNC programming, CAD software, or documentation.

Frequently Asked Questions

What is 1 inch in mm?

Exactly 25.4 mm. This is the precise international definition established in 1959.

Can I use a 13 mm socket on a 1/2" bolt?

Usually yes, but be careful. 1/2" = 12.7 mm, so a 13 mm socket is slightly loose. It may work but risks rounding the bolt head. A 12.7 mm or 1/2" socket is better.

Why do some cars have both metric and imperial bolts?

American and Japanese manufacturers historically used different systems. Even within the same car, engine components might use metric while body parts use imperial (or vice versa).

What's the closest metric wrench to 9/16"?

9/16" = 14.29 mm. A 14 mm wrench is slightly small; 15 mm is slightly large. Neither is a perfect fit — this is one of the harder sizes to substitute.

Is the conversion always exact?

Yes. Since 1959, 1 inch is defined as exactly 25.4 mm. All decimal results are mathematically exact based on this definition.

Inches to Millimeters