UNC Thread Chart (#1 to 4 in)
UNC thread chart from #1 to 4 in: TPI, major diameter, pitch, pitch diameter, minor diameter and approximate tap drill. mm and inch, labelled drawing, ASME B1.1.
For preliminary reference and dimensional comparison. Thread class tolerances, stress area and joint safety are not included. Confirm against the ASME B1.1 edition and the tap manufacturer before manufacture or final selection.
Dimension drawing and symbols
- d
- Major diameter mm
- P
- Pitch, 1/TPI mm
- d2
- Basic pitch diameter mm
- D1
- Basic minor diameter (internal) mm
On the basic profile d = D and d2 = D2 for external and internal threads. D1 is the basic minor diameter of the internal thread, not a manufacturing limit. The profile angle is 60°.
Columns
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Visible rows: 33
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| Profile diameters, mm | ||||||
|---|---|---|---|---|---|---|
| Size | TPI | dmajor dia. | Ppitch | d2pitch dia. | D1minor dia. | tap drillmm |
| #1 | 64 | 1.854 | 0.397 | 1.596 | 1.425 | 1.46 |
| #2 | 56 | 2.184 | 0.454 | 1.890 | 1.693 | 1.73 |
| #3 | 48 | 2.515 | 0.529 | 2.171 | 1.942 | 1.99 |
| #4 | 40 | 2.845 | 0.635 | 2.432 | 2.157 | 2.21 |
| #5 | 40 | 3.175 | 0.635 | 2.763 | 2.488 | 2.54 |
| #6 | 32 | 3.505 | 0.794 | 2.990 | 2.646 | 2.71 |
| #8 | 32 | 4.166 | 0.794 | 3.650 | 3.306 | 3.37 |
| #10 | 24 | 4.826 | 1.058 | 4.139 | 3.680 | 3.77 |
| #12 | 24 | 5.486 | 1.058 | 4.799 | 4.341 | 4.43 |
| 1/4 | 20 | 6.350 | 1.270 | 5.525 | 4.975 | 5.08 |
| 5/16 | 18 | 7.938 | 1.411 | 7.021 | 6.410 | 6.53 |
| 3/8 | 16 | 9.525 | 1.588 | 8.494 | 7.806 | 7.94 |
| 7/16 | 14 | 11.113 | 1.814 | 9.934 | 9.148 | 9.30 |
| 1/2 | 13 | 12.700 | 1.954 | 11.431 | 10.585 | 10.75 |
| 9/16 | 12 | 14.288 | 2.117 | 12.913 | 11.996 | 12.17 |
| 5/8 | 11 | 15.875 | 2.309 | 14.375 | 13.375 | 13.57 |
| 3/4 | 10 | 19.050 | 2.540 | 17.400 | 16.300 | 16.51 |
| 7/8 | 9 | 22.225 | 2.822 | 20.392 | 19.170 | 19.40 |
| 1 | 8 | 25.400 | 3.175 | 23.338 | 21.963 | 22.23 |
| 1 1/8 | 7 | 28.575 | 3.629 | 26.218 | 24.647 | 24.95 |
| 1 1/4 | 7 | 31.750 | 3.629 | 29.393 | 27.822 | 28.12 |
| 1 3/8 | 6 | 34.925 | 4.233 | 32.175 | 30.342 | 30.69 |
| 1 1/2 | 6 | 38.100 | 4.233 | 35.350 | 33.517 | 33.87 |
| 1 3/4 | 5 | 44.450 | 5.080 | 41.150 | 38.951 | 39.37 |
| 2 | 4.5 | 50.800 | 5.644 | 47.134 | 44.690 | 45.16 |
| 2 1/4 | 4.5 | 57.150 | 5.644 | 53.484 | 51.040 | 51.51 |
| 2 1/2 | 4 | 63.500 | 6.350 | 59.376 | 56.626 | 57.15 |
| 2 3/4 | 4 | 69.850 | 6.350 | 65.726 | 62.976 | 63.50 |
| 3 | 4 | 76.200 | 6.350 | 72.076 | 69.326 | 69.85 |
| 3 1/4 | 4 | 82.550 | 6.350 | 78.426 | 75.676 | 76.20 |
| 3 1/2 | 4 | 88.900 | 6.350 | 84.776 | 82.026 | 82.55 |
| 3 3/4 | 4 | 95.250 | 6.350 | 91.126 | 88.376 | 88.90 |
| 4 | 4 | 101.600 | 6.350 | 97.476 | 94.726 | 95.25 |
Quick answers
- #10 UNC: 24 TPI, major diameter 4.826 mm, pitch diameter 4.139 mm, approximate tap drill 3.77 mm. View in table
- 1/4 UNC: 20 TPI, major diameter 6.35 mm, pitch diameter 5.525 mm, approximate tap drill 5.08 mm. View in table
- 5/16 UNC: 18 TPI, major diameter 7.938 mm, pitch diameter 7.021 mm, approximate tap drill 6.53 mm. View in table
- 3/8 UNC: 16 TPI, major diameter 9.525 mm, pitch diameter 8.494 mm, approximate tap drill 7.94 mm. View in table
- 1/2 UNC: 13 TPI, major diameter 12.7 mm, pitch diameter 11.431 mm, approximate tap drill 10.75 mm. View in table
- 3/4 UNC: 10 TPI, major diameter 19.05 mm, pitch diameter 17.4 mm, approximate tap drill 16.51 mm. View in table
- 1 UNC: 8 TPI, major diameter 25.4 mm, pitch diameter 23.338 mm, approximate tap drill 22.23 mm. View in table
Conditions and notes
ASME B1.1 Unified inch screw thread, UNC series, basic profile. Class tolerances (1A, 2A, 3A, 1B, 2B, 3B) are not applied.
- TPI and major diameters were compared with three separate publishers. Pitch and minor diameters are calculated from the relations and checked to four decimals against two separate numeric tables.
- The approximate tap drill is d − P, not rounded to a standard drill size. Check the tap manufacturer's chart for the actual thread percentage and drill.
- The metric and inch switch changes only the display. Calculations use the exact inch definition (1 inch = 25.4 mm).
- CSV download is unavailable because the sources do not establish permission for reuse.
UNC is the coarse series of the Unified inch screw thread. For numbered sizes the major diameter is d = 0.060 + 0.013 × N inch, for fractional sizes it is the nominal size itself. Pitch is P = 1/TPI.
Pitch and minor diameters follow the ASME B1.1 basic profile with exact coefficients: H = √3/2 × P, d2 = d − 3H/4 = d − 0.649519P, D1 = d − 5H/4 = d − 1.082532P. Class tolerances are not included. The approximate tap drill d − P is a preliminary choice.
Sources
This table is for preliminary reference. Check the applicable standard and product conditions before use.
- Source 1. Version accessed. Accessed: 2026-10-16
- Source 2. Version accessed. Accessed: 2026-10-16
- Source 3. Version accessed. Accessed: 2026-10-16
- Source 4. Version accessed. Accessed: 2026-10-16
- Source 5. Version accessed. Accessed: 2026-10-16
Data version: 2026-10-16