Imperial to Metric Fastener MatcherFind Compatible Thread Sizes
Find metric equivalents for imperial fasteners (UNC, UNF threads). Essential for international sourcing, replacing discontinued parts, and working with mixed-unit assemblies. Includes compatibility checks, strength grade comparison, and machining specifications.
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)
Frequently Asked Questions
Can I use M6 bolts instead of 1/4"-20?
M6 (6.0mm) is slightly smaller than 1/4"-20 (6.35mm) - about 5.5% difference. They work fine for clearance holes, but cannot be used interchangeably for threaded holes. The thread forms (pitch) are completely different and not compatible.
What does "close" compatibility mean?
"Close" means the diameter difference is small (typically under 6%) and the metric fastener makes a good substitute for most applications. However, you still cannot thread a metric bolt into an imperial threaded hole - you need matching threads. Use for new designs or clearance holes.
Is Grade 5 the same as Class 8.8?
Very close. Grade 5 (SAE J429) has ~120,000 psi tensile strength, while Class 8.8 (ISO 898) has ~116,000 psi (800 MPa). They are considered equivalent for most applications, but always verify for structural or safety-critical uses.
How do I tap for metric threads?
Use the tap drill size shown in our results (75% thread engagement). For M6×1.0, drill Ø5.0mm, then tap with M6 tap. Never use imperial tap drill sizes for metric threads - the pitch is different and won't work correctly.
Can I mix imperial and metric fasteners in the same assembly?
Yes, but only in clearance holes, not threaded holes. Ensure clearance holes are sized correctly for the larger fastener. Keep track of which holes use which thread type. Consider using thread inserts (Helicoil) to convert thread types if needed.
Why is the diameter difference a percentage?
Percentage shows relative size difference regardless of absolute dimensions. A 5% difference means the metric fastener is 5% larger or smaller than imperial. Under 5% is generally acceptable for non-precision applications; over 10% requires careful verification.
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Convert BOM →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.