Inch to mm Converter

Convert inches to millimetres with precision. Supports decimal and fractional inch inputs for accurate measurements.

Input Type

Inches (in)

Millimetres (mm)

Quick Conversions

Conversion Formula and Calculation Steps

The standard conversion formula is:

millimetres = inches × 25.4

This relationship is exact, established by international agreement in 1959. One inch equals precisely 25.4 millimetres.

Step-by-Step Conversion Process

Example 1: Converting 5 inches to millimetres

Start with the measurement: 5 inches
Multiply by the conversion factor: 5 × 25.4
Calculate the result: 127 mm
Final answer: 5 inches = 127 millimetres

Example 2: Converting 3/4 inch to millimetres

Convert the fraction to decimal: 3 ÷ 4 = 0.75 inches
Multiply by 25.4: 0.75 × 25.4
Calculate: 19.05 mm
Final answer: 3/4 inch = 19.05 millimetres

Example 3: Converting millimetres to inches

Start with: 100 mm
Divide by 25.4: 100 ÷ 25.4
Calculate: 3.937 inches
Final answer: 100 mm = 3.937 inches

Common Inch to Millimetre Conversions

Decimal Inches to Millimetres

Inches (in)	Millimetres (mm)	Common Application
0.01 in	0.254 mm	Precision engineering
0.1 in	2.54 mm	Small components
1 in	25.4 mm	Standard reference
2 in	50.8 mm	Pipe fittings
3 in	76.2 mm	Construction materials
4 in	101.6 mm	Timber dimensions
5 in	127 mm	Engineering drawings
6 in	152.4 mm	Screen sizes
8 in	203.2 mm	Tablet displays
10 in	254 mm	Tile spacing
12 in	304.8 mm	One foot measurement
24 in	609.6 mm	Monitor screens
36 in	914.4 mm	Fabric widths
48 in	1219.2 mm	Sheet materials

Fractional Inches to Millimetres

Fraction	Decimal (in)	Millimetres (mm)
1/64 in	0.0156 in	0.397 mm
1/32 in	0.0312 in	0.794 mm
1/16 in	0.0625 in	1.588 mm
1/8 in	0.125 in	3.175 mm
3/16 in	0.1875 in	4.763 mm
1/4 in	0.25 in	6.35 mm
5/16 in	0.3125 in	7.938 mm
3/8 in	0.375 in	9.525 mm
7/16 in	0.4375 in	11.113 mm
1/2 in	0.5 in	12.7 mm
5/8 in	0.625 in	15.875 mm
3/4 in	0.75 in	19.05 mm
7/8 in	0.875 in	22.225 mm
1 in	1.0 in	25.4 mm

Imperial and Metric Measurement Systems

The inch belongs to the imperial system, which originated in the British Empire and remains prevalent in the United Kingdom and United States. The millimetre forms part of the metric system, officially known as the International System of Units (SI), adopted by most countries worldwide.

The Inch

System: Imperial / US Customary

Symbol: in or ″

Definition: 1/12 of a foot, 1/36 of a yard

Metric equivalent: Exactly 25.4 mm

Historical origin: Derived from the Roman “uncia” meaning one-twelfth

The Millimetre

System: Metric (SI)

Symbol: mm

Definition: 1/1000 of a metre

Imperial equivalent: Approximately 0.03937 inches

Base unit: Derived from the metre, defined by the speed of light

Historical Context

The inch has ancient origins, with early definitions based on physical references such as the width of a thumb or the length of three barleycorns placed end to end. The current standardised definition of 25.4 millimetres per inch was established through the International Yard and Pound Agreement of 1959, which harmonised measurements between English-speaking nations.

The millimetre emerged as part of the metric system developed in France during the late 18th century. This decimal-based system was designed for simplicity and consistency, with each unit related by powers of ten. The United Kingdom officially adopted the metric system for most applications in 1965, though imperial measurements remain in common usage for certain applications.

UK Measurement Practice: The United Kingdom employs a mixed system where both imperial and metric measurements coexist. Road distances and speeds remain in miles and miles per hour, whilst construction and manufacturing predominantly use metric units. This dual system necessitates frequent conversions between inches and millimetres, particularly in engineering, construction, and manufacturing sectors.

Practical Applications and Industries

Construction and Building Trades

Construction professionals regularly convert between inches and millimetres when working with materials from different manufacturers. British timber often comes in imperial sizes (such as 2×4 inches), whilst fixings and hardware are typically specified in metric dimensions. Accurate conversion prevents costly errors in material ordering and installation.

Engineering and Manufacturing

Precision engineering requires exact conversions between measurement systems. British engineers often work with technical drawings that include both imperial and metric dimensions. CNC machining, tolerance specifications, and component fitting all depend on accurate inch-to-millimetre conversions, where even small discrepancies can compromise product quality.

Electronics and Display Screens

Display screens are universally measured in inches (such as 27-inch monitors or 10-inch tablets), whilst internal components and circuit board dimensions use millimetres. Technical specifications for electronic devices routinely include both measurements, requiring clear conversion standards.

Plumbing and Pipe Fittings

Plumbing systems in the UK often use imperial pipe sizes (1/2 inch, 3/4 inch, 1 inch), whilst pipe wall thicknesses and fitting tolerances are specified in millimetres. Understanding these conversions prevents fitting incompatibilities and system failures.

Fabric and Textiles

Fabric widths commonly appear in inches (45 inches, 60 inches), particularly for imported materials, whilst pattern measurements and seam allowances frequently use metric units. Garment manufacturing and home sewing both require proficiency in converting between these systems.

Measurement Comparison and Visualisation

Very Small Measurements

1/64 inch = 0.397 mm

Thickness of a credit card

Fine mechanical tolerances

Small Measurements

1/4 inch = 6.35 mm

Pencil diameter

Small screw head

Medium Measurements

1 inch = 25.4 mm

Standard button size

Bottle cap diameter

Larger Measurements

12 inches = 304.8 mm

Ruler length (1 foot)

A4 paper width (approximately)

Relative Size Comparisons

Object	Approximate Size (inches)	Approximate Size (mm)
£1 coin diameter	0.87 in	22.5 mm
Credit card thickness	0.03 in	0.76 mm
Standard brick length	8.5 in	215 mm
A4 paper length	11.7 in	297 mm
Smartphone width	2.8 in	71 mm
Door thickness (standard)	1.4 in	35 mm

Frequently Asked Questions

How many millimetres are in one inch?

One inch equals exactly 25.4 millimetres. This conversion factor has been standardised internationally since 1959 and provides the basis for all inch-to-millimetre conversions.

Why is the conversion factor 25.4 and not a round number?

The 25.4 factor results from harmonising two measurement systems with different historical origins. When international standards bodies agreed to define the relationship between imperial and metric units in 1959, they chose 25.4 millimetres per inch as it closely matched existing standards whilst providing an exact conversion.

How do I convert fractional inches to millimetres?

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

Which measurement system is more accurate?

Neither system is inherently more accurate than the other. Both inches and millimetres can express measurements to any required degree of precision. The metric system offers advantages in calculation simplicity due to its decimal basis, whilst the inch remains embedded in many industrial standards and practices.

Do I need to convert measurements when working in the UK?

In many UK contexts, yes. British industry uses a mixed system where both imperial and metric measurements appear. Construction drawings may show dimensions in both systems, imported goods often use different standards, and regulatory requirements may specify particular units for different applications.

What’s the quickest way to approximate an inch-to-mm conversion?

Multiply the inch measurement by 25. This provides a close approximation (within 2% of the exact value). For greater accuracy, multiply by 25.4. For rough estimates in reverse, divide millimetres by 25.

Are screen sizes always measured in inches?

By convention, yes. Television screens, computer monitors, tablets, and smartphones are universally measured diagonally in inches, regardless of the country or manufacturer. A 27-inch monitor measures 685.8 mm diagonally, but is never marketed using the metric measurement.

How precise should my conversions be?

Precision requirements depend on the application. Construction tolerances might allow rounding to the nearest millimetre, whilst precision engineering may require three or more decimal places. Always consider the context and specify tolerances appropriately for your purpose.

Can I use a calculator for inch-to-mm conversions?

Standard calculators work perfectly for these conversions. Simply multiply the inch value by 25.4 for the millimetre equivalent, or divide millimetres by 25.4 to get inches. Most scientific calculators also include unit conversion functions.

What happens if I mix up inches and millimetres?

Confusion between these units can lead to significant errors, as 1 inch is more than 25 times larger than 1 millimetre. In construction, manufacturing, or engineering, such mistakes can result in components that don’t fit, material waste, structural problems, or safety issues. Always verify which unit system your measurements use and convert consistently throughout a project.