MM to Inches Converter

How to Convert MM to Inches

Converting between millimetres and inches is essential for UK tradespeople, DIY enthusiasts, and anyone working with both metric and imperial measurements. The conversion is based on the international standard definition.

MM to Inches Formula

To convert millimetres to inches, divide by 25.4:

Inches to MM Formula

To convert inches to millimetres, multiply by 25.4:

Working with Fractions

Many UK tools and hardware use imperial fractions. Understanding common conversions is essential:

• 1/16" increments: Most imperial rulers are marked in 1/16" divisions
• 1/32" precision: Engineering work often requires 1/32" accuracy
• 1/64" tolerances: High-precision machining uses 1/64" or finer
• Nearest fraction: For practical work, find the nearest standard fraction

The UK's Dual Measurement System

The UK uses a fascinating mix of metric and imperial measurements:

• Legal requirement: UK building regulations officially use metric measurements
• Trade tradition: Many tradespeople still think in imperial for certain tasks
• Metrication: Started in 1965 but never fully completed
• Dual labelling: Many UK products show both metric and imperial measurements
• Generation gap: Older tradespeople often prefer imperial, younger ones metric
• Tool duplication: UK workshops often need both metric and imperial tool sets

Precision and Tolerances

Understanding measurement precision is crucial for engineering and manufacturing:

• Thou (mil): 1 thou = 0.001 inch = 0.0254 mm (engineering precision)
• Micron: 1 micron = 0.001 mm = 0.00003937 inch (ultra-precision)
• Nominal sizes: Many measurements are approximate (like copper pipe sizes)
• Tolerance standards: Engineering drawings specify acceptable variation
• Fit types: Clearance fit, interference fit, transition fit all have specific tolerances

UK Plumbing Measurements

UK plumbing is notorious for mixing metric and imperial:

• Copper pipe: Sizes like 15mm and 22mm are actually nominal bore (internal diameter)
• Outside diameter: 15mm pipe has ~16mm OD, 22mm pipe has ~22.8mm OD
• Imperial origins: 15mm ≈ 1/2", 22mm ≈ 3/4", 28mm ≈ 1"
• Plastic pipe: Modern plastic pipes use true metric measurements
• Compression fittings: Must match the exact pipe size (15mm, 22mm, etc.)
• Thread sizes: Often imperial (BSP - British Standard Pipe thread)

Socket and Spanner Sizes

UK mechanics need to understand both systems:

• Metric sockets: 8mm, 10mm, 11mm, 12mm, 13mm, 14mm, 15mm, 17mm, 19mm, 21mm, 22mm
• Imperial sockets (AF): 1/4", 5/16", 3/8", 7/16", 1/2", 9/16", 5/8", 11/16", 3/4", 13/16", 7/8"
• Near equivalents: 10mm ≈ 3/8", 13mm ≈ 1/2", 19mm ≈ 3/4"
• No exact match: Some sizes exist only in one system
• Classic British cars: Use AF (Across Flats) imperial sizes
• Modern vehicles: Mostly metric, but some American imports use SAE (imperial)

Drill Bit Sizes

UK drill bit sets come in various standards:

• Metric series: 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, 8mm, 9mm, 10mm, etc.
• Imperial series: 1/16", 5/64", 3/32", 7/64", 1/8", 9/64", 5/32", etc.
• Letter sizes: A to Z (American standard, sometimes used in UK)
• Number sizes: #80 to #1 (American standard, rare in UK)
• Tap drill sizes: Specific sizes for threading holes
• Clearance holes: Slightly larger than bolt diameter for easy fit

Wood Thickness Standards

UK timber is sold in metric but often cut to imperial equivalents:

• Planed timber: Finished sizes in mm but based on imperial (e.g., 18mm = 3/4")
• Sheet materials: Plywood, MDF, chipboard in 6mm, 9mm, 12mm, 18mm, 25mm
• Imperial origins: 18mm ≈ 3/4", 12mm ≈ 1/2", 6mm ≈ 1/4"
• Nominal vs actual: Timber dimensions are often nominal (before planing)
• Stud walls: Traditionally 4"×2" (100mm×50mm in metric)

Screw and Bolt Threads

Understanding thread standards is essential:

• Metric threads: M4, M5, M6, M8, M10, M12, etc. (diameter in mm)
• Imperial threads: 1/4", 5/16", 3/8", 1/2", etc.
• BA threads: British Association (0BA, 2BA, 4BA, etc.) - legacy standard
• BSW/BSF: British Standard Whitworth/Fine - older British standard
• UNC/UNF: Unified Coarse/Fine - American standard, common in UK
• Thread pitch: Distance between threads (coarse vs fine thread)

Practical Applications

For DIY Enthusiasts

• Flat-pack furniture: Usually uses metric hardware, but tools may be imperial
• Picture hanging: Wall plugs in metric (5mm, 6mm, 8mm), screws often imperial
• Shelf brackets: Hole spacing may be imperial even on metric shelving
• Door hardware: Mix of both systems - check before buying replacements

For Professional Tradespeople

• Plumbers: Must work with copper (nominal), plastic (metric), and threaded fittings (often imperial)
• Electricians: Cable sizes in mm², backboxes in mm, but conduit sometimes imperial
• Carpenters: Timber in mm, tools often imperial, old buildings use imperial dimensions
• Mechanics: Modern cars metric, classic cars imperial, American imports use SAE

For Engineering

• Technical drawings: Modern UK engineering drawings use metric, older ones imperial
• Tolerance specification: Must be clear whether tolerances are in mm or inches
• Material stock: Steel often sold in imperial sizes (1/2" rod, 1/4" plate)
• Thread standards: Critical to specify metric (M10×1.5) or imperial (1/2"-13 UNC)

For Home Improvement

• Kitchen fitting: Units 600mm wide but worktop thickness 40mm (≈1.5")
• Bathroom: Taps 1/2" or 3/4" BSP, pipes 15mm or 22mm
• Flooring: Thickness in mm (18mm chipboard) but room dimensions sometimes feet
• Windows: Glass thickness in mm, but frame sizes may reference imperial

History of Measurement Standards

Origins of the Inch

The inch has ancient and somewhat arbitrary origins:

• Medieval origins: Originally based on the width of a man's thumb
• 1324 standardisation: King Edward II defined the inch as three barleycorns laid end to end
• 1959 international yard: Inch defined as exactly 25.4mm for international standardisation
• Imperial system: Part of British Imperial System standardised in 1824
• Global use: Still used in USA, UK (partially), and some Commonwealth countries

Origins of the Millimetre

The millimetre is part of the modern metric system:

• French Revolution: Metric system created in 1799 during French Revolution
• Scientific basis: Based on Earth's measurements (metre = 1/10,000,000 from equator to pole)
• Decimal system: Designed for easy calculation using powers of 10
• International standard: Adopted by most countries in 19th-20th centuries
• SI unit: Millimetre is part of the International System of Units (SI)

UK Metrication History

The UK's incomplete transition to metric creates today's dual system:

• 1965: UK government announced metrication programme
• 1970s: Decimal currency introduced, schools taught metric
• 1980s-1990s: Industry gradually converted to metric
• 2000: EU directive required metric-only labelling (with exceptions)
• Incomplete transition: Road signs, beer, milk still use imperial
• Brexit effect: Leaving EU removes pressure for full metrication
• Current status: Dual system likely to continue indefinitely

Why the UK Uses Both Systems

Several factors maintain the UK's dual measurement system:

• Cost: Converting all existing tools, equipment, and infrastructure extremely expensive
• Tradition: Many industries and trades have centuries of imperial experience
• Age gap: Older generations think in imperial, younger in metric
• Partial conversion: Some industries fully metric, others fully imperial, many mixed
• International trade: Must accommodate both metric (Europe) and imperial (USA)
• Cultural identity: Imperial measurements seen as part of British heritage

Interesting MM/Inch Facts

• Exact definition: Since 1959, 1 inch is defined as exactly 25.4mm internationally
• Barleycorn origins: The inch was once defined as three barleycorns laid end-to-end
• Oldest standard: The Royal Cubit from ancient Egypt (524mm) is one of humanity's oldest standards
• Mars Climate Orbiter: Lost in 1999 due to imperial/metric confusion - $327 million mistake
• Human scale: Imperial units often relate to human body (foot, thumb/inch, yard/pace)
• Gimli Glider: Air Canada flight ran out of fuel due to lbs/kg confusion but landed safely
• CD size: A CD is exactly 120mm (4.72") in diameter
• Credit card: Standard credit card is 85.6mm × 54mm (3.375" × 2.125")
• A4 paper: 210mm × 297mm (8.27" × 11.69") - based on √2 ratio
• Vinyl records: 7" (178mm), 10" (254mm), 12" (305mm) - still use inches
• Photography: Film and sensor sizes still use imperial (35mm is an exception)
• Bicycle wheels: Measured in inches (26", 27.5", 29") even on metric bikes

Related Measurement Tools

Need more conversion tools? Try these calculators:

• CM to Inches Converter - Convert centimetres to inches
• Metres to Feet Converter - Convert height and distance
• Universal Unit Converter - Convert all measurement types
• Fraction Calculator - Add, subtract, multiply fractions
• Decimal to Fraction - Convert decimal inches to fractions

Frequently Asked Questions

Can I use a 10mm socket on a 3/8" bolt?

Not reliably. 10mm = 0.394" while 3/8" = 9.525mm. The socket will be slightly loose and may round the bolt head. Always use the correct size socket for the job.

Why are copper pipes sized in mm but called by imperial sizes?

UK copper pipes use imperial nominal bore sizes (internal diameter) but are manufactured to metric outside diameters. A "15mm" pipe is approximately 1/2" bore, but the OD is actually 15mm. It's a legacy system from metrication.

How do I know if a bolt is metric or imperial?

Metric bolts are marked with numbers (e.g., M10), imperial with fractions (e.g., 1/2"-13). Metric thread pitch is in mm (M10×1.5), imperial in threads per inch (1/2"-13 UNC). The head size also differs systematically.

Can I mix metric and imperial fasteners?

Never force fit different standards - you'll damage threads. Some sizes appear close (M12 ≈ 1/2") but thread pitch differs. Always match metric with metric, imperial with imperial.

Why do UK mechanics need two socket sets?

Classic British cars use AF (imperial), modern cars use metric, and American imports use SAE (imperial but different from AF). A complete UK garage needs all three sets for full coverage.

What's the difference between nominal and actual size?

Nominal size is the name/reference size (e.g., "15mm pipe"), while actual size is the measured dimension (e.g., 15mm pipe has 15.88mm OD). Common in plumbing and timber where processing changes final size.

How precise do conversions need to be for DIY work?

For general DIY, rounding to 1 decimal place is fine. For precision furniture or engineering work, use 2-3 decimal places. For professional machining, use the full conversion factor (25.4mm per inch).

Will the UK ever fully convert to metric?

Unlikely in the near future. The dual system is now embedded in trade practices, tools, and infrastructure. Brexit removed EU pressure to complete metrication, and there's strong public attachment to imperial units in certain contexts.