Quick Overview
Quick Overview
| Feature | Metric (ISO) | Imperial (Inch/US) |
|---|---|---|
| Size designation | Millimeters (M8) | Fractions/decimals (5/16") |
| Pitch measurement | mm between threads | Threads Per Inch (TPI) |
| Thread angle | 60° | 60° |
| Grades | Class 8.8, 10.9, 12.9 | Grade 2, 5, 8 |
| Used primarily in | Worldwide (outside US) | USA, some UK legacy |
Thread Designation Systems
Metric System
- M = Metric
- 10 = 10mm nominal diameter
- 1.5 = 1.5mm thread pitch
- Coarse pitch often omitted (M10 = M10×1.5)
Imperial System (Unified)
- 3/8 = 3/8" (0.375") nominal diameter
- 16 = 16 threads per inch
- UNC = Unified National Coarse
- UNF = Unified National Fine
Numbered Screws (Imperial)
| Number | Decimal Diameter | Nearest Metric |
|---|---|---|
| #2 | 0.086" | M2.2 |
| #4 | 0.112" | M2.9 |
| #6 | 0.138" | M3.5 |
| #8 | 0.164" | M4.2 |
| #10 | 0.190" | M4.8 |
| #12 | 0.216" | M5.5 |
Size Comparison Chart
Common Size Cross-Reference
| Metric | Diameter (mm) | Closest Imperial | Diameter (inch) | Compatible? |
|---|---|---|---|---|
| M3 | 3.00mm | #4-40 | 2.84mm | NO |
| M4 | 4.00mm | #8-32 | 4.17mm | NO |
| M5 | 5.00mm | #10-24 | 4.83mm | NO |
| M6 | 6.00mm | 1/4-20 | 6.35mm | NO |
| M8 | 8.00mm | 5/16-18 | 7.94mm | NO |
| M10 | 10.00mm | 3/8-16 | 9.53mm | NO |
| M12 | 12.00mm | 1/2-13 | 12.70mm | NO |
| M16 | 16.00mm | 5/8-11 | 15.88mm | NO |
| M20 | 20.00mm | 3/4-10 | 19.05mm | NO |
Critical: None of these are interchangeable! Even similar sizes have different thread pitches.
Grade/Property Class Comparison
Strength Equivalents
| Metric Class | Tensile Strength | US Grade | Tensile Strength |
|---|---|---|---|
| Class 4.6 | 400 MPa | Grade 2 | 510 MPa (74 ksi) |
| Class 8.8 | 800 MPa | Grade 5 | 827 MPa (120 ksi) |
| Class 10.9 | 1040 MPa | Grade 8 | 1034 MPa (150 ksi) |
| Class 12.9 | 1220 MPa | (none std) | - |
Grade Marking Comparison
| System | Low Strength | Medium Strength | High Strength |
|---|---|---|---|
| Metric | "4.6" or unmarked | "8.8" | "10.9" or "12.9" |
| US | Unmarked | 3 radial lines | 6 radial lines |
Understanding Metric Class Numbers
Thread Pitch Conversion
Converting Between Systems
Example Conversions
| Thread | Pitch/TPI | Converted | Difference |
|---|---|---|---|
| 1/4-20 UNC | 20 TPI | 1.27mm pitch | - |
| M6×1.0 | 1.0mm | 25.4 TPI | Different! |
| 3/8-16 UNC | 16 TPI | 1.59mm pitch | - |
| M10×1.5 | 1.5mm | 16.9 TPI | Different! |
Even when pitch seems close, they are NOT compatible.
Wrench Size Comparison
| Metric Bolt | Metric Wrench | US Bolt | US Wrench | Same Wrench? |
|---|---|---|---|---|
| M5 | 8mm | #10 | 3/8" (9.5mm) | NO |
| M6 | 10mm | 1/4" | 7/16" (11.1mm) | NO |
| M8 | 13mm | 5/16" | 1/2" (12.7mm) | Close |
| M10 | 16mm | 3/8" | 9/16" (14.3mm) | NO |
| M12 | 18mm | 1/2" | 3/4" (19.1mm) | Close |
| M16 | 24mm | 5/8" | 15/16" (23.8mm) | Close |
Tip: 13mm ≈ 1/2", 19mm ≈ 3/4" — but exact matches are rare.
Advantages of Each System
Metric Advantages
- Simpler math (base 10)
- International standard (ISO)
- Growing global adoption
- Logical size progression
- Required for export products
Imperial Advantages
- Established US infrastructure
- Legacy equipment compatibility
- Widely available in US market
- US construction/plumbing standard
- Fractional sizes intuitive for many
Industry Usage
| Industry | Primary System | Notes |
|---|---|---|
| Automotive (US domestic) | Mixed | Trending metric |
| Automotive (import) | Metric | Japanese/European |
| Aerospace (US) | Mostly imperial | AN/MS/NAS standards |
| Aerospace (EU) | Metric | EN/ISO standards |
| Construction (US) | Imperial | Structural steel |
| Construction (EU) | Metric | EN standards |
| Electronics | Metric | Global supply chain |
| Plumbing (US) | Imperial | NPT threads |
| Plumbing (EU) | Metric | BSP/ISO threads |
| Medical devices | Metric | Global standard |
| Consumer products | Metric | Global manufacturing |
Conversion Challenges
Why You Can't Just "Convert"
1. Thread pitch differs — Even similar diameters have incompatible threads
2. No exact matches — Sizes don't align precisely
3. Strength grades differ — Class 8.8 ≠ exactly Grade 5
4. Standards differ — Tolerances, head dimensions vary
5. Legal/specification requirements — Many specs mandate one system
What "Close Enough" Causes
- Cross-threading
- Stripped threads
- Loose connections
- Joint failure
- Liability issues
Identification Tips
How to Identify Metric Fasteners
- Size marked with "M" (M8, M10)
- Property class with decimal (8.8, 10.9)
- Head dimensions match metric wrenches
- Pitch measured in mm
How to Identify Imperial Fasteners
- Size in fractions (1/4", 3/8") or numbers (#8, #10)
- Grade marks (radial lines for Grade 5/8)
- Head dimensions match inch wrenches
- TPI measured with thread gauge
When Unmarked
1. Measure diameter precisely (mm or inch)
2. Count threads per inch OR measure pitch in mm
3. Check head dimensions against standards
4. Use thread gauge for confirmation
Mixing Metric and Imperial
NEVER Do This:
- Metric bolt in imperial nut (or vice versa)
- Assume similar sizes are compatible
- Force threads that don't engage smoothly
- Use imperial torque on metric without conversion
What Happens When Mixed:
- Threads may start but won't fully engage
- Apparent fit but no clamping force
- Threads strip under load
- Joint fails unexpectedly
Practical Recommendations
For New Designs
| Market | Recommendation |
|---|---|
| US domestic only | Either (imperial common) |
| Export/global | Metric (ISO standard) |
| Automotive | Metric (industry trend) |
| Aerospace | Per specification |
| General industrial | Metric (future-proof) |
For Maintenance/Repair
- Identify existing fasteners before ordering
- Keep separate metric and imperial stock
- Never assume compatibility
- Label storage clearly
Tool Kits
- Maintain both metric and imperial sets
- Don't rely on "close enough" substitutions
- Quality thread gauges for identification
FAQ
Q: Can I use a metric bolt in a slightly larger imperial hole?
A: The hole might fit, but if you're threading into something, the threads won't match. For through-bolts with nuts, use the correct nut for the bolt.
Q: Is the world going fully metric?
A: Mostly yes for new manufacturing. US is a holdout, but even US automotive is largely metric now. Legacy equipment will need imperial support indefinitely.
Q: How do I know if my car uses metric or imperial?
A: Most vehicles since 1980s use metric, including US domestics. Pre-1980 US vehicles are typically imperial. Japanese/European always metric.
Q: Are metric fasteners stronger?
A: Not inherently. Strength depends on grade (Class 10.9 ≈ Grade 8). Both systems have equivalent strength options.
Q: Why do both systems use 60° thread angle?
A: The 60° angle was adopted by both ISO and Unified standards, making thread form similar. But pitch and diameter are still different!
Q: Can I retap a metric hole to imperial?
A: Sometimes, if diameters allow. But you're creating a non-standard thread. Generally better to use the correct fastener.
Need help identifying fasteners? Our technical team can assist with thread identification and proper fastener selection.