Imperial to Metric Fastener Matcher for UNC and UNF ThreadsApproximate M-size matches for sourcing and engineering review
This fastener matcher helps engineers and buyers compare imperial UNC/UNF fasteners with approximate metric equivalents. It is useful for sourcing, BOM cleanup, and supplier discussions, but metric and imperial threads are not directly interchangeable unless the joint, pitch, clearance, strength grade, and mating parts are verified.
How to match an imperial fastener to a metric size
Start with the imperial diameter, shortlist the nearest M size, then verify thread pitch, clearance, strength grade, wrench size, and whether the joint uses a tapped hole or a nut.
UNC and UNF thread equivalents are approximate
Fastener safety note: metric equivalents are approximate planning references. Do not install metric fasteners into imperial tapped holes, safety-critical assemblies, pressure-retaining parts, or load-bearing joints without engineering approval and supplier verification.
Quick Examples
Common Fastener Conversions
| Imperial Thread | Diameter (in) | Metric Equivalent | Diameter (mm) | Pitch (mm) | Compatibility |
|---|---|---|---|---|---|
| #0-80 | 0.0600 | M1.6 | 1.6 | 0.35 | close |
| #4-40 | 0.1120 | M2.5 | 2.5 | 0.45 | close |
| #8-32 | 0.1640 | M4 | 4.0 | 0.7 | close |
| 1/4"-20 | 0.2500 | M6 | 6.0 | 1 | close |
| 3/8"-16 | 0.3750 | M10 | 10.0 | 1.5 | close |
| 1/2"-13 | 0.5000 | M12 | 12.0 | 1.75 | close |
| 5/8"-11 | 0.6250 | M16 | 16.0 | 2 | close |
| 7/8"-9 | 0.8750 | M22 | 22.0 | 2.5 | close |
Strength Grade Equivalents
Tensile Strength: 64,000 psi (440 MPa)
Low carbon steel, general purpose
Tensile Strength: 120,000 psi (830 MPa)
Medium carbon steel, most common
Tensile Strength: 150,000 psi (1040 MPa)
Alloy steel, high strength
Why Match Imperial to Metric Fasteners?
International Sourcing
Sourcing fasteners from overseas suppliers? Most international manufacturers use metric threads. Find the right metric equivalent to avoid ordering incorrect parts.
Replacement Parts
Need to replace discontinued imperial fasteners? Find compatible metric substitutes with confidence using our compatibility ratings and diameter difference calculations.
Mixed-Unit Assemblies
Working on projects that mix imperial and metric components? Understand thread compatibility, required tap drill sizes, and clearance hole specifications.
Strength Verification
Compare imperial Grade 5/8 with metric Class 8.8/10.9. Ensure your replacement fasteners meet or exceed the original strength requirements for safety-critical applications.
Important Considerations
⚠️ Thread Form Differences
- • Imperial: 60° thread angle, different pitch
- • Metric: 60° thread angle, metric pitch
- • Threads are NOT directly interchangeable
- • Never force metric bolt into imperial threaded hole
⚠️ Diameter Tolerances
- • Most conversions have 1-10% diameter difference
- • Check clearance holes in precision assemblies
- • Verify thread engagement length
- • Consider washer size compatibility
⚠️ Strength Ratings
- • Grade 5 ≈ Class 8.8 (similar strength)
- • Grade 8 ≈ Class 10.9 (high strength)
- • Always verify for structural applications
- • Consider material certifications
⚠️ Application Verification
- • Consult engineer for safety-critical parts
- • Verify torque specifications
- • Check regulatory compliance (aerospace, medical)
- • Test fit before production quantities
Fastener Terminology
Imperial Thread Notation
- • 1/4" = Major diameter (0.250 inches)
- • 20 = Threads per inch (TPI)
- • UNC = Unified National Coarse (standard)
- • UNF = Unified National Fine (finer pitch)
Metric Thread Notation
- • M = Metric thread
- • 6 = Major diameter (6mm)
- • 1.0 = Thread pitch (distance between threads)
- • Coarse pitch often omitted (M6 = M6×1.0)
Imperial to Metric Fastener FAQ
What is the metric equivalent of a 1/4-20 bolt?
The common approximate metric equivalent for 1/4-20 is M6, but it is not a direct thread replacement. Check diameter, pitch, clearance, and the mating threaded part.
Can I screw a metric bolt into an imperial tapped hole?
No. Metric and imperial thread forms and pitches are different. Do not force a metric bolt into an imperial tapped hole; retap, replace the mating part, or use the correct fastener.
Is UNC the same as metric coarse thread?
No. UNC and metric coarse are different thread systems. They may have similar diameters, but the thread pitch and profile are not the same.
Should I match by diameter or thread pitch first?
Start with diameter for approximate sizing, then check pitch, thread engagement, strength grade, and application safety before substitution.
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View Wrench Chart →Start Matching Fasteners
Join thousands of engineers and procurement specialists who use our fastener matcher for international sourcing. Find compatible metric equivalents with confidence using our comprehensive compatibility checks and specifications.