Imperial Thread Pitch Chart (UNC/UNF)
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- **Title:** Imperial Thread Pitch Chart | UNC & UNF Sizes #0 to 4" | Tap & Clearance Drill Sizes- Meta Description: Complete imperial thread pitch chart covering UNC and UNF sizes from #0 to 4 inch. Includes major diameter, TPI, tap drill sizes, clearance holes, and Class 2A/2B fit specifications.
- Keywords: imperial thread pitch chart, UNC thread chart, UNF thread chart, tap drill size chart, clearance hole chart, threads per inch, unified national coarse, unified national fine
Introduction
Understanding imperial thread specifications is essential for proper fastener selection and installation. This comprehensive guide covers both Unified National Coarse (UNC) and Unified National Fine (UNF) thread standards, the two most common imperial thread series used in North America.
Whether you're selecting tap drill sizes, determining clearance holes, or verifying thread compatibility, this chart provides the technical data you need for sizes #0 through 4 inches.
UNC (Unified National Coarse) Thread Chart
UNC threads are the most commonly used imperial thread series. They're preferred for general-purpose fastening where quick assembly, debris tolerance, and resistance to cross-threading are important.
Machine Screw Sizes (#0 - #12)
| Size | Major Dia (in) | TPI | Tap Drill | Tap Drill (in) | Close Fit Clearance | Free Fit Clearance |
|---|---|---|---|---|---|---|
| #0 | 0.0600 | 80 | 3/64" | 0.0469 | #52 (0.0635) | #50 (0.0700) |
| #1 | 0.0730 | 64 | #53 | 0.0595 | #48 (0.0760) | #46 (0.0810) |
| #2 | 0.0860 | 56 | #50 | 0.0700 | #43 (0.0890) | #41 (0.0960) |
| #3 | 0.0990 | 48 | #47 | 0.0785 | #37 (0.1040) | #35 (0.1100) |
| #4 | 0.1120 | 40 | #43 | 0.0890 | #32 (0.1160) | #30 (0.1285) |
| #5 | 0.1250 | 40 | #38 | 0.1015 | #30 (0.1285) | #29 (0.1360) |
| #6 | 0.1380 | 32 | #36 | 0.1065 | #27 (0.1440) | #25 (0.1495) |
| #8 | 0.1640 | 32 | #29 | 0.1360 | #18 (0.1695) | #16 (0.1770) |
| #10 | 0.1900 | 24 | #25 | 0.1495 | #9 (0.1960) | #7 (0.2010) |
| #12 | 0.2160 | 24 | #16 | 0.1770 | #2 (0.2210) | #1 (0.2280) |
Fractional Sizes (1/4" - 1")
| Size | Major Dia (in) | TPI | Tap Drill | Tap Drill (in) | Close Fit Clearance | Free Fit Clearance |
|---|---|---|---|---|---|---|
| 1/4" | 0.2500 | 20 | #7 | 0.2010 | F (0.2570) | H (0.2660) |
| 5/16" | 0.3125 | 18 | F | 0.2570 | P (0.3230) | Q (0.3320) |
| 3/8" | 0.3750 | 16 | 5/16" | 0.3125 | W (0.3860) | X (0.3970) |
| 7/16" | 0.4375 | 14 | U | 0.3680 | 29/64" (0.4531) | 15/32" (0.4688) |
| 1/2" | 0.5000 | 13 | 27/64" | 0.4219 | 33/64" (0.5156) | 17/32" (0.5313) |
| 9/16" | 0.5625 | 12 | 31/64" | 0.4844 | 37/64" (0.5781) | 19/32" (0.5938) |
| 5/8" | 0.6250 | 11 | 17/32" | 0.5313 | 41/64" (0.6406) | 21/32" (0.6563) |
| 3/4" | 0.7500 | 10 | 21/32" | 0.6563 | 49/64" (0.7656) | 25/32" (0.7813) |
| 7/8" | 0.8750 | 9 | 49/64" | 0.7656 | 57/64" (0.8906) | 29/32" (0.9063) |
| 1" | 1.0000 | 8 | 7/8" | 0.8750 | 1-1/64" (1.0156) | 1-1/32" (1.0313) |
Large Fractional Sizes (1-1/8" - 4")
| Size | Major Dia (in) | TPI | Tap Drill | Close Fit Clearance | Free Fit Clearance |
|---|---|---|---|---|---|
| 1-1/8" | 1.1250 | 7 | 63/64" | 1-9/64" (1.1406) | 1-5/32" (1.1563) |
| 1-1/4" | 1.2500 | 7 | 1-7/64" | 1-17/64" (1.2656) | 1-9/32" (1.2813) |
| 1-3/8" | 1.3750 | 6 | 1-7/32" | 1-25/64" (1.3906) | 1-13/32" (1.4063) |
| 1-1/2" | 1.5000 | 6 | 1-11/32" | 1-33/64" (1.5156) | 1-17/32" (1.5313) |
| 1-3/4" | 1.7500 | 5 | 1-9/16" | 1-49/64" (1.7656) | 1-25/32" (1.7813) |
| 2" | 2.0000 | 4.5 | 1-25/32" | 2-1/32" (2.0313) | 2-1/16" (2.0625) |
| 2-1/4" | 2.2500 | 4.5 | 2-1/32" | 2-9/32" (2.2813) | 2-5/16" (2.3125) |
| 2-1/2" | 2.5000 | 4 | 2-1/4" | 2-17/32" (2.5313) | 2-9/16" (2.5625) |
| 2-3/4" | 2.7500 | 4 | 2-1/2" | 2-25/32" (2.7813) | 2-13/16" (2.8125) |
| 3" | 3.0000 | 4 | 2-3/4" | 3-1/32" (3.0313) | 3-1/16" (3.0625) |
| 3-1/4" | 3.2500 | 4 | 3" | 3-9/32" (3.2813) | 3-5/16" (3.3125) |
| 3-1/2" | 3.5000 | 4 | 3-1/4" | 3-17/32" (3.5313) | 3-9/16" (3.5625) |
| 3-3/4" | 3.7500 | 4 | 3-1/2" | 3-25/32" (3.7813) | 3-13/16" (3.8125) |
| 4" | 4.0000 | 4 | 3-3/4" | 4-1/32" (4.0313) | 4-1/16" (4.0625) |
UNF (Unified National Fine) Thread Chart
UNF threads have more threads per inch than their UNC counterparts. They're preferred for applications requiring fine adjustment, higher tensile strength in the same diameter, or better vibration resistance.
Machine Screw Sizes (#0 - #12)
| Size | Major Dia (in) | TPI | Tap Drill | Tap Drill (in) | Close Fit Clearance | Free Fit Clearance |
|---|---|---|---|---|---|---|
| #0 | 0.0600 | 80 | 3/64" | 0.0469 | #52 (0.0635) | #50 (0.0700) |
| #1 | 0.0730 | 72 | #53 | 0.0595 | #48 (0.0760) | #46 (0.0810) |
| #2 | 0.0860 | 64 | #50 | 0.0700 | #43 (0.0890) | #41 (0.0960) |
| #3 | 0.0990 | 56 | #45 | 0.0820 | #37 (0.1040) | #35 (0.1100) |
| #4 | 0.1120 | 48 | #42 | 0.0935 | #32 (0.1160) | #30 (0.1285) |
| #5 | 0.1250 | 44 | #37 | 0.1040 | #30 (0.1285) | #29 (0.1360) |
| #6 | 0.1380 | 40 | #33 | 0.1130 | #27 (0.1440) | #25 (0.1495) |
| #8 | 0.1640 | 36 | #29 | 0.1360 | #18 (0.1695) | #16 (0.1770) |
| #10 | 0.1900 | 32 | #21 | 0.1590 | #9 (0.1960) | #7 (0.2010) |
| #12 | 0.2160 | 28 | #14 | 0.1820 | #2 (0.2210) | #1 (0.2280) |
Fractional Sizes (1/4" - 1-1/2")
| Size | Major Dia (in) | TPI | Tap Drill | Tap Drill (in) | Close Fit Clearance | Free Fit Clearance |
|---|---|---|---|---|---|---|
| 1/4" | 0.2500 | 28 | #3 | 0.2130 | F (0.2570) | H (0.2660) |
| 5/16" | 0.3125 | 24 | I | 0.2720 | P (0.3230) | Q (0.3320) |
| 3/8" | 0.3750 | 24 | Q | 0.3320 | W (0.3860) | X (0.3970) |
| 7/16" | 0.4375 | 20 | 25/64" | 0.3906 | 29/64" (0.4531) | 15/32" (0.4688) |
| 1/2" | 0.5000 | 20 | 29/64" | 0.4531 | 33/64" (0.5156) | 17/32" (0.5313) |
| 9/16" | 0.5625 | 18 | 33/64" | 0.5156 | 37/64" (0.5781) | 19/32" (0.5938) |
| 5/8" | 0.6250 | 18 | 37/64" | 0.5781 | 41/64" (0.6406) | 21/32" (0.6563) |
| 3/4" | 0.7500 | 16 | 11/16" | 0.6875 | 49/64" (0.7656) | 25/32" (0.7813) |
| 7/8" | 0.8750 | 14 | 13/16" | 0.8125 | 57/64" (0.8906) | 29/32" (0.9063) |
| 1" | 1.0000 | 12 | 15/16" | 0.9375 | 1-1/64" (1.0156) | 1-1/32" (1.0313) |
| 1-1/8" | 1.1250 | 12 | 1-3/64" | 1.0469 | 1-9/64" (1.1406) | 1-5/32" (1.1563) |
| 1-1/4" | 1.2500 | 12 | 1-11/64" | 1.1719 | 1-17/64" (1.2656) | 1-9/32" (1.2813) |
| 1-3/8" | 1.3750 | 12 | 1-19/64" | 1.2969 | 1-25/64" (1.3906) | 1-13/32" (1.4063) |
| 1-1/2" | 1.5000 | 12 | 1-27/64" | 1.4219 | 1-33/64" (1.5156) | 1-17/32" (1.5313) |
Understanding Thread Class Fits
Thread class designations indicate the tolerance and allowance for imperial threads. The most common classes for general fastener applications are Class 2A (external) and Class 2B (internal).
Class Designations
| Class | Type | Fit Description | Applications |
|---|---|---|---|
| 1A/1B | Loose | Maximum allowance, easy assembly | Quick assembly, dirty environments |
| 2A/2B | Standard | Normal production fit | General fastening (most common) |
| 3A/3B | Tight | Close tolerance, minimal allowance | Precision equipment, aerospace |
Class 2A/2B Specifications
Class 2A (External Threads):
- Used on bolts, screws, and studs
- Allowance provides clearance for plating/coating
- Tolerances adequate for most commercial applications
Class 2B (Internal Threads):
- Used on nuts and tapped holes
- Zero allowance at maximum material condition
- Mates properly with Class 2A external threads
Thread Fit Recommendations
| Application | Recommended Class |
|---|---|
| General commercial | 2A/2B |
| Automotive | 2A/2B |
| Aerospace | 3A/3B |
| Quick assembly | 1A/1B |
| Plated fasteners | 2A/2B (before plating) |
Practical Tips for Using This Chart
Tap Drill Selection
1. 75% Thread Engagement: The tap drill sizes listed produce approximately 75% thread engagement, which provides optimal strength without excessive tap breakage risk.
2. For Harder Materials: Consider going one drill size larger to reduce tapping torque and tap breakage.
3. For Softer Materials: Can use the listed size or slightly smaller for better thread quality.
Clearance Hole Selection
- Close Fit: Use when alignment is critical and fasteners must locate parts precisely
- Free Fit: Use for general assembly where slight misalignment is acceptable
Common Conversions
| Number Size | Decimal Equivalent | Nearest Metric |
|---|---|---|
| #4 | 0.112" | 3mm |
| #6 | 0.138" | 3.5mm |
| #8 | 0.164" | 4mm |
| #10 | 0.190" | 5mm |
| 1/4" | 0.250" | 6mm |
Frequently Asked Questions
What's the difference between UNC and UNF threads?
UNC (Unified National Coarse) has fewer threads per inch than UNF (Unified National Fine). For example, a 1/4" UNC has 20 TPI while 1/4" UNF has 28 TPI. UNC is better for general use; UNF is better for fine adjustment and vibration resistance.
How do I know which tap drill to use?
Use the tap drill sizes in this chart for approximately 75% thread engagement. This is the industry standard that balances thread strength with ease of tapping. For softer materials like aluminum, you can use the exact size; for harder materials, consider one size larger.
What does "Class 2A/2B" mean?
Class 2A refers to external threads (bolts/screws) and 2B refers to internal threads (nuts/tapped holes). Class 2 is the standard fit for most commercial fasteners, providing proper clearance for assembly while maintaining adequate strength.
Can I use a UNC bolt in a UNF nut?
No. While they may have the same nominal diameter, the different thread pitches make them incompatible. Forcing them together will damage the threads. Always match UNC with UNC and UNF with UNF.
What clearance hole should I use?
Use "close fit" clearance when precise alignment is important, such as in fixtures or when dowel pins aren't used. Use "free fit" for general assembly where slight misalignment is acceptable and easier assembly is preferred.
Why are machine screw sizes given as numbers (#0, #4, etc.)?
This is a legacy sizing system predating standardization. The number roughly correlates to wire gauge sizes. To convert: Major Diameter = (Number × 0.013) + 0.060. For example, #10 = (10 × 0.013) + 0.060 = 0.190".
Related Resources
- [Metric Thread Pitch Chart](/reference/thread-pitch-chart-metric)
- [UNC vs UNF Selection Guide](/reference/unc-vs-unf-guide)
- [Thread Engagement Calculator](/reference/thread-engagement-chart)
- [Thread Identification Guide](/reference/thread-identification-guide)
Last Updated: January 2025
Technical data based on ASME B1.1-2019 Unified Inch Screw Threads standard.