Inches to Millimeters (in to mm) Converter

Inches to Millimeters

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

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

Why This Matters

Wrenches & Sockets

Find metric equivalents for 1/4", 3/8", and 1/2" tool sizes.

Automotive Work

Many cars mix metric and imperial fasteners — know the close matches.

Machining & Pipes

Convert between systems for precision work and pipe fittings.

How to Use This Converter

Enter inches

Type the inch value in the top field — use decimals for fractions (3/8" = 0.375).

Read millimetres

The mm result appears live to 8 decimal places for machining tolerances.

Match the nearest size

Compare to the reference table to find the closest standard metric socket or bit.

Conversion Formula

mm = inches × 25.4 • Fractions: convert to decimal first. 3/8" = 0.375" × 25.4 = 9.525 mm.

Fractional Inch & Socket Equivalents

Fraction	Millimetres	Nearest metric
1/4"	6.35 mm	6 mm
3/8"	9.53 mm	10 mm
1/2"	12.7 mm	13 mm
3/4"	19.05 mm	19 mm

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

High Precision

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

Substitute With Care

A 13 mm socket on a 1/2" (12.7 mm) bolt is slightly loose — it can round the head, so prefer the exact fit.

Workshop-ready and private. Works on mobile at the workbench, runs offline after loading, and keeps your project dimensions on your device.

Frequently Asked Questions

What is 1 inch in mm?

Exactly 25.4 mm — the precise international definition established in 1959.

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

Usually yes, but carefully. 1/2" = 12.7 mm, so a 13 mm socket is slightly loose and may round 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 one car, the engine might be metric while body parts are 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 and 15 mm slightly large — neither is a perfect fit, making it one of the harder sizes to substitute.

Is the conversion always exact?

Yes. Since 1959, 1 inch is defined as exactly 25.4 mm, so all decimal results are mathematically exact.

Inches to Millimeters