Millimetres
0 mm
Exact conversion
Imperial to Metric Conversion Tool
Accurate Inch to MM Converter for Engineering & Construction
Converting inches to millimetres is essential for international engineering, construction, and manufacturing projects. Our inches to millimetres calculator provides instant conversions between imperial and metric measurements following NIST standards and international metrology specifications used in 2026.
The conversion factor is precisely defined: 1 inch = 25.4 millimetres exactly. This relationship was established by international agreement and is used worldwide for accurate dimensional conversions in engineering drawings, construction specifications, and manufacturing tolerances.
Convert between imperial and metric measurements
The inch to millimetre conversion is based on the international inch definition established in 1959. Our inches to millimetres calculator uses the exact conversion factor accepted by BIPM (International Bureau of Weights and Measures) and national standards organizations worldwide.
📐 Official Conversion Factor:
| Inches | Millimetres (mm) | Centimetres (cm) | Common Use |
|---|---|---|---|
| 1/16" | 1.5875 mm | 0.16 cm | Fine measurements |
| 1/8" | 3.175 mm | 0.32 cm | Sheet thickness |
| 1/4" | 6.35 mm | 0.64 cm | Bolt sizes |
| 1/2" | 12.7 mm | 1.27 cm | Pipe dimensions |
| 3/4" | 19.05 mm | 1.91 cm | Plumbing fittings |
| 1" | 25.4 mm | 2.54 cm | Standard reference |
| 2" | 50.8 mm | 5.08 cm | Lumber nominal |
| 3" | 76.2 mm | 7.62 cm | Drainage pipes |
| 4" | 101.6 mm | 10.16 cm | Wall studs |
| 6" | 152.4 mm | 15.24 cm | Foundation depths |
| 8" | 203.2 mm | 20.32 cm | Block dimensions |
| 12" | 304.8 mm | 30.48 cm | 1 foot |
Engineering and construction drawings often use fractional inches. Our inches to millimetres calculator converts both decimal and fractional measurements with precision required for technical specifications.
| Fractional Inches | Decimal Inches | Millimetres (mm) | Tolerance (±mm) |
|---|---|---|---|
| 1/64" | 0.0156" | 0.3969 mm | ±0.05 |
| 1/32" | 0.0313" | 0.7938 mm | ±0.05 |
| 1/16" | 0.0625" | 1.5875 mm | ±0.08 |
| 3/32" | 0.0938" | 2.3813 mm | ±0.08 |
| 1/8" | 0.1250" | 3.1750 mm | ±0.10 |
| 5/32" | 0.1563" | 3.9688 mm | ±0.10 |
| 3/16" | 0.1875" | 4.7625 mm | ±0.13 |
| 7/32" | 0.2188" | 5.5563 mm | ±0.13 |
| 1/4" | 0.2500" | 6.3500 mm | ±0.15 |
| 5/16" | 0.3125" | 7.9375 mm | ±0.15 |
| 3/8" | 0.3750" | 9.5250 mm | ±0.18 |
| 7/16" | 0.4375" | 11.1125 mm | ±0.18 |
| 1/2" | 0.5000" | 12.7000 mm | ±0.20 |
| 9/16" | 0.5625" | 14.2875 mm | ±0.20 |
| 5/8" | 0.6250" | 15.8750 mm | ±0.23 |
| 11/16" | 0.6875" | 17.4625 mm | ±0.23 |
| 3/4" | 0.7500" | 19.0500 mm | ±0.25 |
| 13/16" | 0.8125" | 20.6375 mm | ±0.25 |
| 7/8" | 0.8750" | 22.2250 mm | ±0.28 |
| 15/16" | 0.9375" | 23.8125 mm | ±0.28 |
Converting inches to millimetres is critical in various industries. Understanding precise conversions ensures compatibility between imperial and metric specifications in international projects and manufacturing processes.
Applications: Architectural drawings, material specifications, foundation dimensions
Common conversions: 2×4 lumber (38×89mm), 8" blocks (200mm), rebar spacing
Tolerance: Typically ±3mm for structural work
Standards: Building codes accept both systems with proper conversion
Applications: Machine parts, bearing sizes, shaft diameters, thread specifications
Common conversions: 1/2" bolts (12.7mm), 3/4" shafts (19.05mm)
Tolerance: ±0.05mm to ±0.5mm depending on fit class
Standards: ISO metric preferred for new designs
Applications: Pipe sizing, fitting dimensions, duct measurements
Common conversions: 1/2" pipe (15mm nominal), 3/4" pipe (20mm nominal)
Note: Nominal sizes don't match exact conversions
Standards: Different standards for pipes vs actual diameter
Applications: CNC programming, tool sizes, component dimensions
Common conversions: Drill bits, milling cutters, sheet metal gauges
Tolerance: High precision: ±0.01mm to ±0.02mm
Standards: ISO 2768 for general tolerances
Applications: Circuit board dimensions, component spacing, enclosure sizing
Common conversions: 0.1" pitch (2.54mm), IC pin spacing
Tolerance: Very tight: ±0.05mm for PCB manufacturing
Standards: IPC standards specify metric dimensions
Applications: Engine specifications, wheel sizes, bolt patterns
Common conversions: 16" wheels (406mm), 1/2" wheel studs
Mixed use: Many specs still imperial despite metric trend
Standards: SAE (imperial) and ISO (metric) both used
Understanding the mathematical relationship between inches and millimetres helps verify calculations and perform manual conversions when digital tools aren't available.
✅ Conversion Formulas:
Convert: 5 inches to millimetres
Calculation:
5 inches × 25.4 = 127 mm
Result: 5" = 127 mm exactly
Verification: 127 ÷ 25.4 = 5 inches ✓
Convert: 3/8 inch to millimetres
Calculation:
3 ÷ 8 = 0.375 inches
0.375 × 25.4 = 9.525 mm
Result: 3/8" = 9.525 mm
Rounded: 9.5 mm (practical use)
Convert: 2 feet 6 inches to mm
Calculation:
2 ft = 24 inches
Total = 24 + 6 = 30 inches
30 × 25.4 = 762 mm
Result: 2'6" = 762 mm = 0.762 m
Convert: 50 mm to inches
Calculation:
50 ÷ 25.4 = 1.9685 inches
Result: 50 mm ≈ 1.969"
Fraction: Approximately 2" (1-31/32")
Different applications require different levels of precision. Our inches to millimetres calculator provides full precision, but practical use often requires appropriate rounding.
| Application | Recommended Precision | Rounding | Example |
|---|---|---|---|
| General Construction | 1 decimal place | ±1mm acceptable | 5.5" = 139.7 mm → 140 mm |
| Carpentry & Joinery | 1-2 decimal places | ±0.5mm acceptable | 3/4" = 19.05 mm → 19.1 mm |
| Engineering Drawings | 2 decimal places | ±0.1mm typical | 1.250" = 31.75 mm (exact) |
| CNC Machining | 3-4 decimal places | ±0.01mm required | 0.375" = 9.5250 mm (exact) |
| Precision Engineering | 4+ decimal places | ±0.001mm critical | 0.0625" = 1.5875 mm (exact) |
| PCB Manufacturing | 3 decimal places | ±0.05mm standard | 0.100" = 2.540 mm (exact) |
Errors in conversion between imperial and metric systems have caused costly mistakes in manufacturing and construction. Understanding common pitfalls prevents measurement errors.
⚠️ Critical Conversion Errors:
Error: Software used imperial units while NASA expected metric
Impact: $327 million spacecraft lost
Lesson: Always verify units in all calculations and communications
Error: Fuel calculated in pounds instead of kilograms
Impact: Aircraft ran out of fuel mid-flight (landed safely)
Lesson: Double-check unit conversions in critical systems
Common error: Beam lengths ordered in wrong units
Impact: Material waste, project delays, cost overruns
Prevention: Always mark drawings clearly with units
Memorizing common conversions speeds up everyday calculations. These quick reference values cover most construction and engineering scenarios.
1 inch = 25.4 mm = 2.54 cm
1 foot = 304.8 mm = 30.48 cm
1 yard = 914.4 mm = 91.44 cm
1 mile = 1.609 km
1/8" ≈ 3.2 mm
1/4" ≈ 6.4 mm
1/2" ≈ 12.7 mm
3/4" ≈ 19.1 mm
2×4 lumber = 38×89 mm
2×6 lumber = 38×140 mm
4×4 post = 89×89 mm
8" block ≈ 200 mm
1/4" drive ≈ 6mm
3/8" drive ≈ 10mm
1/2" drive ≈ 13mm
3/4" drive ≈ 19mm