What is half an inch in mm?

Half an inch (0.5 inches) equals exactly 12.7 millimeters.

How do I convert fractions of an inch to mm?

First convert the fraction to a decimal, then multiply by 25.4. For example, 3/8 inch = 0.375 × 25.4 = 9.525 mm.

Inches to Millimeters Converter | in to mm Calculator

Conversion Formula

mm = in × 25.4

To convert inches to millimeters, multiply the inch value by 25.4.

Complete Guide to Inches to Millimeters Conversion

What is a Inches?

An inch (in) is an imperial unit used primarily in the US for everyday and industrial measurements. Fractional inches (1/2", 1/4") are common in American hardware and construction.

What is a Millimeters?

A millimeter (mm) is a metric unit providing greater precision than fractional inches. It is used worldwide in engineering, manufacturing, and scientific applications.

How the Conversion Works

The conversion is exact: 1 inch = 25.4 mm by international definition. This allows perfect conversion between the two systems without rounding errors.

Why This Conversion Matters

Converting inches to millimeters is essential for international manufacturing, 3D printing, CNC machining, and any field requiring precise metric specifications from imperial measurements.

Step-by-Step Conversion Guide

Follow these steps to convert inches to millimeters manually:

Start with your value in inches

Example: You want to convert 1 in

Apply the conversion formula

Use the formula: mm = in × 25.4

Calculate the result

1 in = 25.4 mm

✓

Verify your answer

Use our calculator above to double-check your manual calculation.

Quick Mental Math Tip

To convert inches to millimeters, multiply the inch value by 25.4.

Common Conversions

Inches to Millimeters conversion table
Inches (in)	Millimeters (mm)	Millimeters → Inches
0.25	6.35	0.25 mm = 0.0098 in
0.5	12.7	0.5 mm = 0.0197 in
1 in	25.4	1 mm = 0.0394 in
2 in	50.8	2 mm = 0.0787 in
3	76.2	3 mm = 0.1181 in
4	101.6	4 mm = 0.1575 in
5 in	127	5 mm = 0.1969 in
6	152.4	6 mm = 0.2362 in
10 in	254	10 mm = 0.3937 in
12	304.8	12 mm = 0.4724 in

Did You Know?

The inches and millimeters are among the most commonly converted units worldwide, with millions of daily searches.

This converter is used by thousands of people daily for quick reference. Bookmark it for instant access.

Popular Inches to Millimeters Conversions

1 in= 25.4 mm 2 in= 50.8 mm 5 in= 127 mm 10 in= 254 mm 25 in= 635 mm

When You'll Need This Conversion

Ordering Metric Parts

Convert American specifications to metric when ordering parts from international suppliers.

Drill Bit Sizing

Find equivalent metric drill bits when fractional inch sizes are unavailable or vice versa.

Pipe & Tube Fittings

Convert plumbing and tubing dimensions when mixing imperial and metric components.

3D Printing

Convert model dimensions from inches to millimeters for metric-based slicing software.

Pro Tips for Accurate Conversions

Multiply by 25 for estimates

Using 25 instead of 25.4 is easier for mental math: 2 in × 25 = 50 mm (actual: 50.8 mm).

Common fractional conversions

1/8" = 3.175 mm, 1/4" = 6.35 mm, 3/8" = 9.525 mm, 1/2" = 12.7 mm, 3/4" = 19.05 mm, 1" = 25.4 mm.

Close equivalents

1" ≈ 25 mm, 2" ≈ 50 mm, 4" ≈ 100 mm. These approximations work well for rough estimates.

Common Mistakes to Avoid

Forgetting to convert fractions first

You cannot directly multiply fractions like "3/8" by 25.4. Convert to decimal first.

3/8" = 0.375" → 0.375 × 25.4 = 9.525 mm.

Using 25 for precision work

Using 25 instead of 25.4 causes a 1.6% error that matters in manufacturing and engineering.

For precision: 1" = 25.4 mm exactly, not 25 mm.

History & Background

The 25.4 mm definition of the inch was established internationally in 1959 to standardize measurements between imperial and metric systems in manufacturing and trade.

Global Usage Today

While millimeters are the global standard, many American industries still specify dimensions in inches. The exact 25.4 conversion enables seamless international manufacturing.