Imperial to Metric Fastener Size ChartUNC/UNF Threads to Metric Equivalents
Complete conversion reference for imperial threads (UNC/UNF) to metric equivalents. Designed for procurement specialists, mechanical engineers, and manufacturing professionals sourcing metric fasteners to replace imperial hardware.
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
📋 For Procurement
- •Find the imperial thread size in column 1
- •Check the metric equivalent and size difference
- •Verify compatibility based on your application
- •Match strength grades (Grade 5 ≈ 8.8)
⚠️ Important Notes
- •Metric equivalents are closest standard sizes
- •Direct substitution may not work for threaded holes
- •Always verify for safety-critical applications
- •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:
- •A 1/4"-20 bolt (Ø6.35mm) is replaced by M6 (Ø6.00mm) - 5.5% smaller
- •Thread pitch and form are different (60° for both, but different root/crest profiles)
- •Tap drill sizes are not interchangeable
✅ When Substitution Works
- ✓Clearance holes: Holes drilled oversized (e.g., Ø7mm+ for M6/1/4")
- ✓Non-critical assemblies: Where 5-10% size difference is acceptable
- ✓Low-stress applications: Furniture, enclosures, light brackets
❌ When NOT to Substitute
- ✗Threaded holes: Metric tap won't fit imperial thread, and vice versa
- ✗Precision assemblies: Bearing mounts, alignment pins, close-tolerance fits
- ✗Safety-critical: Aerospace, automotive suspension, pressure vessels
- ✗High-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.
Frequently Asked Questions
Can I use M6 bolts instead of 1/4"-20 UNC?
M6 is 5.5% smaller than 1/4" (6.00mm vs 6.35mm). It works for clearance holes (Ø7mm+) but NOT for threaded holes. Thread form is incompatible - you cannot thread an M6 bolt into a 1/4"-20 tapped hole. For safety-critical or precision applications, do not substitute.
What's the metric equivalent of Grade 5 bolts?
Grade 5 (SAE J429) is approximately equivalent to Class 8.8 (ISO 898-1). Both have minimum tensile strength around 800 MPa (116,000 psi). Always verify with your structural engineer for critical applications.
Why are thread pitches different between imperial and metric?
Imperial threads use TPI (threads per inch) while metric uses pitch in millimeters. For example, 1/4"-20 has 20 threads per inch (1.27mm pitch), while M6 has 1.0mm pitch (25.4 TPI). This makes the threads completely incompatible despite similar diameters.
How do I tap a metric hole for imperial threads?
You cannot tap a metric hole for imperial threads or vice versa. The thread profiles and pitches are incompatible. You must either: (1) use the correct thread standard for your hole, (2) drill out and re-tap with the desired thread, or (3) use a helicoil/threaded insert adapter.
Master 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.