A practical reference for Unified National Coarse (UNC) and Unified National Fine (UNF) threads, including major diameter, TPI, pitch, and common tap drill sizes.
Technical Diagram
60° unified thread geometry
Nominal Diameter
Inch
Unified thread sizes are specified by nominal inch diameter.
TPI
Threads / Inch
Thread density is expressed as the number of threads per inch.
Tap Drill
Drill → Tap
Common starting drill sizes for producing internal threads.
Coverage
UNC + UNF
Common machine screw and fractional inch thread sizes.
Common unified inch thread dimensions and tap drill references.
| Size | Designation | Series | Major Ø | TPI | Pitch | Tap Drill | Drill Ø |
|---|---|---|---|---|---|---|---|
| #0 | #0-80 | UNF | 0.0600 in 1.52 mm | 80 | 0.318 mm | 3/64 | 1.20 mm |
| #1 | #1-64 | UNC | 0.0730 in 1.85 mm | 64 | 0.397 mm | 53 | 1.50 mm |
| #1 | #1-72 | UNF | 0.0730 in 1.85 mm | 72 | 0.353 mm | 53 | 1.50 mm |
| #2 | #2-56 | UNC | 0.0860 in 2.18 mm | 56 | 0.454 mm | 50 | 1.75 mm |
| #2 | #2-64 | UNF | 0.0860 in 2.18 mm | 64 | 0.397 mm | 50 | 1.75 mm |
| #4 | #4-40 | UNC | 0.1120 in 2.84 mm | 40 | 0.635 mm | 43 | 2.25 mm |
| #4 | #4-48 | UNF | 0.1120 in 2.84 mm | 48 | 0.529 mm | 42 | 2.30 mm |
| #6 | #6-32 | UNC | 0.1380 in 3.51 mm | 32 | 0.794 mm | 36 | 2.85 mm |
| #6 | #6-40 | UNF | 0.1380 in 3.51 mm | 40 | 0.635 mm | 33 | 2.85 mm |
| #8 | #8-32 | UNC | 0.1640 in 4.17 mm | 32 | 0.794 mm | 29 | 3.40 mm |
| #8 | #8-36 | UNF | 0.1640 in 4.17 mm | 36 | 0.706 mm | 29 | 3.40 mm |
| #10 | #10-24 | UNC | 0.1900 in 4.83 mm | 24 | 1.058 mm | 25 | 4.50 mm |
| #10 | #10-32 | UNF | 0.1900 in 4.83 mm | 32 | 0.794 mm | 21 | 4.00 mm |
| 1/4 | 1/4-20 | UNC | 0.2500 in 6.35 mm | 20 | 1.270 mm | #7 | 5.10 mm |
| 5/16 | 5/16-18 | UNC | 0.3125 in 7.94 mm | 18 | 1.411 mm | F | 6.60 mm |
| 3/8 | 3/8-16 | UNC | 0.3750 in 9.53 mm | 16 | 1.588 mm | 5/16 | 7.94 mm |
| 7/16 | 7/16-14 | UNC | 0.4375 in 11.11 mm | 14 | 1.814 mm | 25/64 | 9.92 mm |
| 1/2 | 1/2-13 | UNC | 0.5000 in 12.70 mm | 13 | 1.954 mm | 27/64 | 10.72 mm |
| 9/16 | 9/16-12 | UNC | 0.5625 in 14.29 mm | 12 | 2.117 mm | 31/64 | 12.30 mm |
| 5/8 | 5/8-11 | UNC | 0.6250 in 15.88 mm | 11 | 2.309 mm | 17/32 | 13.49 mm |
| 3/4 | 3/4-10 | UNC | 0.7500 in 19.05 mm | 10 | 2.540 mm | 21/32 | 16.67 mm |
| 7/8 | 7/8-9 | UNC | 0.8750 in 22.23 mm | 9 | 2.822 mm | 49/64 | 19.45 mm |
| 1 | 1-8 | UNC | 1.0000 in 25.40 mm | 8 | 3.175 mm | 7/8 | 22.23 mm |
| 1/4 | 1/4-28 | UNF | 0.2500 in 6.35 mm | 28 | 0.907 mm | #3 | 5.40 mm |
| 5/16 | 5/16-24 | UNF | 0.3125 in 7.94 mm | 24 | 1.058 mm | I | 7.10 mm |
| 3/8 | 3/8-24 | UNF | 0.3750 in 9.53 mm | 24 | 1.058 mm | Q | 8.43 mm |
| 7/16 | 7/16-20 | UNF | 0.4375 in 11.11 mm | 20 | 1.270 mm | 25/64 | 9.92 mm |
| 1/2 | 1/2-20 | UNF | 0.5000 in 12.70 mm | 20 | 1.270 mm | 29/64 | 11.51 mm |
| 9/16 | 9/16-18 | UNF | 0.5625 in 14.29 mm | 18 | 1.411 mm | 33/64 | 13.10 mm |
| 5/8 | 5/8-18 | UNF | 0.6250 in 15.88 mm | 18 | 1.411 mm | 37/64 | 14.68 mm |
| 3/4 | 3/4-16 | UNF | 0.7500 in 19.05 mm | 16 | 1.588 mm | 11/16 | 17.46 mm |
| 7/8 | 7/8-14 | UNF | 0.8750 in 22.23 mm | 14 | 1.814 mm | 13/16 | 20.64 mm |
| 1 | 1-12 | UNF | 1.0000 in 25.40 mm | 12 | 2.117 mm | 59/64 | 23.42 mm |
Coarse Series
UNC threads use fewer threads per inch and therefore a larger pitch than UNF threads of the same nominal diameter. They are widely used for general-purpose fastening because they are generally easier to assemble, more tolerant of contamination and damage, and provide a robust thread form for machinery.
Typical Use
General machinery
Key Characteristic
Larger pitch
Fine Series
UNF threads use more threads per inch and therefore a smaller pitch than UNC threads of the same nominal diameter. They can be advantageous where finer adjustment, greater tensile stress area, or increased thread engagement is beneficial.
Typical Use
Precision assemblies
Key Characteristic
Smaller pitch
Manufacturing Reference
The tap drill is the hole diameter drilled before cutting an internal thread. The final drill size depends on the desired thread percentage, workpiece material, tap geometry, tolerance, and manufacturing process.
Select the Thread
Example: 1/2-13 UNC.
Drill the Hole
Select the appropriate nominal tap drill diameter.
Cut the Thread
Use the appropriate tap to form the internal thread.
Important
Tap drill values are provided for reference only. The final hole diameter should be verified based on the workpiece material, tap type, target thread percentage, tolerances, lubrication, and manufacturing process. For critical fasteners and production applications, always verify the required dimensions against the applicable standard and the cutting-tool manufacturer's recommendations.