Imperial to Metric Fastener Chart for UNC and UNF ThreadsApproximate M-size equivalents for purchasing and engineering checks
Use this conversion chart to compare imperial UNC and UNF fasteners with approximate metric M-size equivalents. It is designed for procurement specialists, mechanical engineers, and maintenance teams who need thread, diameter, wrench, and strength context before approving a substitution.
Complete Coverage
#0-80 to 1"-8 UNC/UNF threads with common sizes highlighted for production use.
Detailed Specs
Diameter, pitch, wrench size, and clearance information for quick engineering checks.
Strength Grades
SAE to ISO strength grade comparison to ensure safe substitutions in the field.
How to Use This Chart
π For Procurement
- β’Find the imperial thread size in column 1
- β’Check the metric equivalent and size difference
- β’Verify compatibility based on your application
- β’Match strength grades (Grade 5 β 8.8)
β οΈ Important Notes
- β’Metric equivalents are closest standard sizes
- β’Direct substitution may not work for threaded holes
- β’Always verify for safety-critical applications
- β’Consider diameter difference for precision assemblies
Imperial Thread | Imperial Diameter | Imperial Pitch | Metric Equiv. | Metric Diameter | Metric Pitch | Size Diff | Wrench Size | Compat. |
|---|---|---|---|---|---|---|---|---|
| #0-80 | 0.0600" | 80 TPI | M1.6 | 1.60 mm | 0.35 mm | +4.2% | 1/16" 3.2 mm | Low |
| #1-64 | 0.0730" | 64 TPI | M2 | 2.00 mm | 0.40 mm | +7.9% | 5/64" 4.0 mm | Medium |
| #2-56 | 0.0860" | 56 TPI | M2.5 | 2.50 mm | 0.45 mm | +14.0% | 3/32" 5.0 mm | Medium |
| #3-48 | 0.0990" | 48 TPI | M2.5 | 2.50 mm | 0.45 mm | -0.7% | 1/8" 5.5 mm | Medium |
| #4-40 | 0.1120" | 40 TPI | M3 | 3.00 mm | 0.50 mm | +5.5% | 1/8" 5.5 mm | Medium |
| #5-40 | 0.1250" | 40 TPI | M3 | 3.00 mm | 0.50 mm | -5.5% | 9/64" 5.5 mm | Medium |
| #6-32 | 0.1380" | 32 TPI | M3.5 | 3.50 mm | 0.60 mm | -0.1% | 5/32" 6.0 mm | High |
| #8-32 | 0.1640" | 32 TPI | M4 | 4.00 mm | 0.70 mm | -3.8% | 11/64" 7.0 mm | High |
| #10-24 | 0.1900" | 24 TPI | M5 | 5.00 mm | 0.80 mm | +3.6% | 3/16" 8.0 mm | High |
| #10-32 | 0.1900" | 32 TPI | M5 | 5.00 mm | 0.80 mm | +3.6% | 3/16" 8.0 mm | High |
| 1/4"-20 | 0.2500" | 20 TPI | M6 | 6.00 mm | 1.00 mm | -5.5% | 7/16" 10 mm | Medium |
| 1/4"-28 | 0.2500" | 28 TPI | M6 | 6.00 mm | 1.00 mm | -5.5% | 7/16" 10 mm | Medium |
| 5/16"-18 | 0.3125" | 18 TPI | M8 | 8.00 mm | 1.25 mm | +0.8% | 1/2" 13 mm | High |
| 5/16"-24 | 0.3125" | 24 TPI | M8 | 8.00 mm | 1.00 mm | +0.8% | 1/2" 13 mm | High |
| 3/8"-16 | 0.3750" | 16 TPI | M10 | 10.00 mm | 1.50 mm | +5.0% | 9/16" 17 mm | High |
| 3/8"-24 | 0.3750" | 24 TPI | M10 | 10.00 mm | 1.25 mm | +5.0% | 9/16" 17 mm | High |
| 7/16"-14 | 0.4375" | 14 TPI | M12 | 12.00 mm | 1.75 mm | +7.9% | 5/8" 19 mm | Medium |
| 7/16"-20 | 0.4375" | 20 TPI | M12 | 12.00 mm | 1.25 mm | +7.9% | 5/8" 19 mm | Medium |
| 1/2"-13 | 0.5000" | 13 TPI | M12 | 12.00 mm | 1.75 mm | -5.5% | 3/4" 19 mm | Medium |
| 1/2"-20 | 0.5000" | 20 TPI | M12 | 12.00 mm | 1.25 mm | -5.5% | 3/4" 19 mm | Medium |
| 9/16"-12 | 0.5625" | 12 TPI | M14 | 14.00 mm | 2.00 mm | -1.9% | 13/16" 22 mm | High |
| 9/16"-18 | 0.5625" | 18 TPI | M14 | 14.00 mm | 1.50 mm | -1.9% | 13/16" 22 mm | High |
| 5/8"-11 | 0.6250" | 11 TPI | M16 | 16.00 mm | 2.00 mm | +0.8% | 15/16" 24 mm | High |
| 5/8"-18 | 0.6250" | 18 TPI | M16 | 16.00 mm | 1.50 mm | +0.8% | 15/16" 24 mm | High |
| 3/4"-10 | 0.7500" | 10 TPI | M20 | 20.00 mm | 2.50 mm | +5.0% | 1-1/8" 30 mm | Medium |
| 3/4"-16 | 0.7500" | 16 TPI | M20 | 20.00 mm | 1.50 mm | +5.0% | 1-1/8" 30 mm | Medium |
| 7/8"-9 | 0.8750" | 9 TPI | M22 | 22.00 mm | 2.50 mm | -0.9% | 1-5/16" 34 mm | High |
| 7/8"-14 | 0.8750" | 14 TPI | M22 | 22.00 mm | 1.50 mm | -0.9% | 1-5/16" 34 mm | High |
| 1"-8 | 1.0000" | 8 TPI | M24 | 24.00 mm | 3.00 mm | -5.5% | 1-1/2" 36 mm | Medium |
| 1"-12 | 1.0000" | 12 TPI | M24 | 24.00 mm | 2.00 mm | -5.5% | 1-1/2" 36 mm | Medium |
Compatibility Legend:
Size difference <3%. Generally suitable for clearance hole applications.
Size difference 3-7%. Verify application requirements before substitution.
Size difference >7%. Not recommended for direct substitution.
Understanding Size Differences
Why Aren't They Exact Matches?
Imperial and metric fastener standards were developed independently. The metric equivalents shown are the closest standard metric sizes available, not exact conversions. This means:
- β’A 1/4"-20 bolt (Γ6.35mm) is replaced by M6 (Γ6.00mm) - 5.5% smaller
- β’Thread pitch and form are different (60Β° for both, but different root/crest profiles)
- β’Tap drill sizes are not interchangeable
β When Substitution Works
- βClearance holes: Holes drilled oversized (e.g., Γ7mm+ for M6/1/4")
- βNon-critical assemblies: Where 5-10% size difference is acceptable
- βLow-stress applications: Furniture, enclosures, light brackets
β When NOT to Substitute
- βThreaded holes: Metric tap won't fit imperial thread, and vice versa
- βPrecision assemblies: Bearing mounts, alignment pins, close-tolerance fits
- βSafety-critical: Aerospace, automotive suspension, pressure vessels
- βHigh-stress: Clamping forces, vibration, cyclic loading
Material Strength Grade Comparison
| SAE/ASTM (Imperial) | ISO 898-1 (Metric) | Min. Tensile Strength | Common Use |
|---|---|---|---|
| Grade 2 | Class 4.6 | 400 MPa (58,000 psi) | Low-strength, general purpose |
| Grade 5 | Class 8.8 | 800 MPa (116,000 psi) | Medium-strength, automotive |
| Grade 8 | Class 10.9 | 1040 MPa (150,000 psi) | High-strength, critical applications |
| A307 Grade A | Class 4.6 | 414 MPa (60,000 psi) | Structural, non-critical |
| A325 | Class 8.8 | 827 MPa (120,000 psi) | Structural steel construction |
Note: Always consult with a structural engineer for safety-critical applications. Grade equivalence is approximate and may vary by manufacturer and certification.
Imperial to Metric Fastener Chart FAQ
Can I use M6 bolts instead of 1/4"-20 UNC?
M6 is close in diameter to 1/4 inch, but it is not thread-compatible with a 1/4-20 tapped hole. Use it only when the joint design, clearance hole, nut, strength grade, and supplier specification are verified.
What is the metric equivalent of Grade 5 bolts?
SAE Grade 5 is often compared with ISO property class 8.8, but certification, material, proof load, and application requirements still need engineering or supplier confirmation.
Why do imperial and metric thread pitches not interchange?
Imperial threads use threads per inch while metric threads use pitch in millimeters. Similar diameters can still have different thread forms and lead, so the mating parts are not interchangeable.
When should I use the chart versus the matcher tool?
Use the chart for scanning standard sizes and procurement references. Use the matcher tool when you want compatibility warnings, tap drill hints, and a faster shortlist for a specific imperial callout.
Related Tools & Resources
Fastener Matcher
Interactive tool to find metric equivalents with compatibility analysis and warnings
Use Tool βWrench Size Chart
Compare SAE wrench and socket sizes with closest common metric references
View Chart βDrill Bit Chart
Tap drill sizes for imperial and metric threads
View Chart βTolerance Grades
ANSI to ISO tolerance grade conversion reference
View Chart βSurface Finish Chart
Ra ΞΌin to ΞΌm reference for machined parts and inspection notes
View Chart βMaster Fastener Selection for Your Projects
Use this fastener conversion chart together with our Fastener Matcher to choose compatible metric substitutes for imperial threads. Quickly verify diameter differences, thread pitch, wrench sizes, and strength grades before ordering or assembling.