- D
- Thread size: the major diameter (M6 = 6 mm, 1/4-20 = 1/4 inch)
- d
- Tap drill: the hole you drill before tapping
- P
- Pitch: the distance from one thread to the next (1 ÷ TPI for inch threads)
- h
- Thread depth: 75% of the full thread height. At 100% the hole would be the minor diameter
Metric coarse threads
| Thread | Tap drill | Thread % | Pitch (mm) | Minor Ø (mm) | Stress area (mm²) |
|---|---|---|---|---|---|
| M1.6 | 1.3 mm | 66% | 0.35 | 1.221 | 1.3 |
| M2 | 1.6 mm | 77% | 0.4 | 1.567 | 2.1 |
| M2.5 | 2.1 mm | 68% | 0.45 | 2.013 | 3.4 |
| M3 | 2.5 mm | 77% | 0.5 | 2.459 | 5 |
| M3.5 | 2.9 mm | 77% | 0.6 | 2.850 | 6.8 |
| M4 | 3.3 mm | 77% | 0.7 | 3.242 | 8.8 |
| M5 | 4.2 mm | 77% | 0.8 | 4.134 | 14.2 |
| M6 | 5.0 mm | 77% | 1 | 4.917 | 20.1 |
| M8 | 6.8 mm | 74% | 1.25 | 6.647 | 36.6 |
| M10 | 8.5 mm | 77% | 1.5 | 8.376 | 58 |
| M12 | 10.3 mm | 75% | 1.75 | 10.106 | 84.3 |
| M14 | 12.1 mm | 73% | 2 | 11.835 | 115.4 |
| M16 | 14.0 mm | 77% | 2 | 13.835 | 156.7 |
| M18 | 15.5 mm | 77% | 2.5 | 15.294 | 192.5 |
| M20 | 17.5 mm | 77% | 2.5 | 17.294 | 244.8 |
| M22 | 19.5 mm | 77% | 2.5 | 19.294 | 303.4 |
| M24 | 21.0 mm | 77% | 3 | 20.752 | 352.5 |
| M27 | 24.0 mm | 77% | 3 | 23.752 | 459.4 |
| M30 | 26.5 mm | 77% | 3.5 | 26.211 | 560.6 |
| M33 | 29.5 mm | 77% | 3.5 | 29.211 | 693.6 |
| M36 | 32.0 mm | 77% | 4 | 31.670 | 816.7 |
Coarse is the everyday metric thread (ISO 261 / ISO 262); an M6 bolt is 6 mm across with a 1 mm pitch. Drills are 0.1 mm steps to 13 mm, then 0.5 mm. Some charts round a few sizes differently, such as 1.25 mm for M1.6, 2.05 mm for M2.5, 10.2 mm for M12 and 12 mm for M14: a slightly deeper thread, and just as good if you have that drill.
Metric fine threads
| Thread | Tap drill | Thread % | Pitch (mm) | Minor Ø (mm) | Stress area (mm²) |
|---|---|---|---|---|---|
| M8 × 1 | 7.0 mm | 77% | 1 | 6.917 | 39.2 |
| M10 × 1.25 | 8.8 mm | 74% | 1.25 | 8.647 | 61.2 |
| M10 × 1 | 9.0 mm | 77% | 1 | 8.917 | 64.5 |
| M12 × 1.5 | 10.5 mm | 77% | 1.5 | 10.376 | 88.1 |
| M12 × 1.25 | 10.8 mm | 74% | 1.25 | 10.647 | 92.1 |
| M14 × 1.5 | 12.5 mm | 77% | 1.5 | 12.376 | 124.5 |
| M16 × 1.5 | 14.5 mm | 77% | 1.5 | 14.376 | 167.2 |
| M18 × 1.5 | 16.5 mm | 77% | 1.5 | 16.376 | 216.2 |
| M20 × 1.5 | 18.5 mm | 77% | 1.5 | 18.376 | 271.5 |
| M22 × 1.5 | 20.5 mm | 77% | 1.5 | 20.376 | 333.1 |
| M24 × 2 | 22.0 mm | 77% | 2 | 21.835 | 384.4 |
| M24 × 1.5 | 22.5 mm | 77% | 1.5 | 22.376 | 400.9 |
Common fine pitches. A fine thread has a smaller pitch, so its tap drill is bigger than the coarse one for the same size.
UNC (unified coarse) threads
| Thread | Tap drill | Thread % | TPI | Size | Minor Ø (in) | Stress area (in²) |
|---|---|---|---|---|---|---|
| #2-56 | #50 (0.07 in · 1.78 mm) | 69% | 56 | 0.086 in (2.18 mm) | 0.0667 | 0.0037 |
| #4-40 | #43 (0.089 in · 2.26 mm) | 71% | 40 | 0.112 in (2.84 mm) | 0.0849 | 0.0060 |
| #5-40 | #38 (0.1015 in · 2.58 mm) | 72% | 40 | 0.125 in (3.18 mm) | 0.0979 | 0.0080 |
| #6-32 | #36 (0.1065 in · 2.71 mm) | 78% | 32 | 0.138 in (3.51 mm) | 0.1042 | 0.0091 |
| #8-32 | #29 (0.136 in · 3.45 mm) | 69% | 32 | 0.164 in (4.17 mm) | 0.1302 | 0.0140 |
| #10-24 | #25 (0.1495 in · 3.8 mm) | 75% | 24 | 0.190 in (4.83 mm) | 0.1449 | 0.0175 |
| #12-24 | #16 (0.177 in · 4.5 mm) | 72% | 24 | 0.216 in (5.49 mm) | 0.1709 | 0.0242 |
| 1/4-20 | #7 (0.201 in · 5.11 mm) | 75% | 20 | 0.250 in (6.35 mm) | 0.1959 | 0.0318 |
| 5/16-18 | F (0.257 in · 6.53 mm) | 77% | 18 | 0.3125 in (7.94 mm) | 0.2524 | 0.0524 |
| 3/8-16 | 5/16 in (7.94 mm) | 77% | 16 | 0.375 in (9.53 mm) | 0.3073 | 0.0775 |
| 7/16-14 | U (0.368 in · 9.35 mm) | 75% | 14 | 0.4375 in (11.11 mm) | 0.3602 | 0.1063 |
| 1/2-13 | 27/64 in (10.72 mm) | 78% | 13 | 0.500 in (12.7 mm) | 0.4167 | 0.1419 |
| 9/16-12 | 31/64 in (12.3 mm) | 72% | 12 | 0.5625 in (14.29 mm) | 0.4723 | 0.1819 |
| 5/8-11 | 17/32 in (13.49 mm) | 79% | 11 | 0.625 in (15.88 mm) | 0.5266 | 0.2260 |
| 3/4-10 | 21/32 in (16.67 mm) | 72% | 10 | 0.750 in (19.05 mm) | 0.6417 | 0.3345 |
| 7/8-9 | 49/64 in (19.45 mm) | 76% | 9 | 0.875 in (22.23 mm) | 0.7547 | 0.4617 |
| 1-8 | 7/8 in (22.23 mm) | 77% | 8 | 1.000 in (25.4 mm) | 0.8647 | 0.6057 |
TPI is threads per inch: a 1/4-20 bolt is 1/4 inch across with 20 threads per inch (ASME B1.1). Drills are fractional (1/64 inch steps), number (#1 to #56) and letter (A to Z).
UNF (unified fine) threads
| Thread | Tap drill | Thread % | TPI | Size | Minor Ø (in) | Stress area (in²) |
|---|---|---|---|---|---|---|
| #4-48 | #42 (0.0935 in · 2.37 mm) | 68% | 48 | 0.112 in (2.84 mm) | 0.0894 | 0.0066 |
| #6-40 | #33 (0.113 in · 2.87 mm) | 77% | 40 | 0.138 in (3.51 mm) | 0.1109 | 0.0101 |
| #8-36 | #29 (0.136 in · 3.45 mm) | 78% | 36 | 0.164 in (4.17 mm) | 0.1339 | 0.0147 |
| #10-32 | #21 (0.159 in · 4.04 mm) | 76% | 32 | 0.190 in (4.83 mm) | 0.1562 | 0.0200 |
| 1/4-28 | #3 (0.213 in · 5.41 mm) | 80% | 28 | 0.250 in (6.35 mm) | 0.2113 | 0.0364 |
| 5/16-24 | I (0.272 in · 6.91 mm) | 75% | 24 | 0.3125 in (7.94 mm) | 0.2674 | 0.0581 |
| 3/8-24 | Q (0.332 in · 8.43 mm) | 79% | 24 | 0.375 in (9.53 mm) | 0.3299 | 0.0878 |
| 7/16-20 | 25/64 in (9.92 mm) | 72% | 20 | 0.4375 in (11.11 mm) | 0.3834 | 0.1187 |
| 1/2-20 | 29/64 in (11.51 mm) | 72% | 20 | 0.500 in (12.7 mm) | 0.4459 | 0.1600 |
| 9/16-18 | 33/64 in (13.1 mm) | 65% | 18 | 0.5625 in (14.29 mm) | 0.5024 | 0.2030 |
| 5/8-18 | 37/64 in (14.68 mm) | 65% | 18 | 0.625 in (15.88 mm) | 0.5649 | 0.2560 |
| 3/4-16 | 11/16 in (17.46 mm) | 77% | 16 | 0.750 in (19.05 mm) | 0.6823 | 0.3730 |
| 7/8-14 | 13/16 in (20.64 mm) | 67% | 14 | 0.875 in (22.23 mm) | 0.7977 | 0.5095 |
| 1-12 | 59/64 in (23.42 mm) | 72% | 12 | 1.000 in (25.4 mm) | 0.9098 | 0.6630 |
Fine threads have more threads per inch, so their tap drills are bigger than the coarse ones for the same size.
About this chart
The sizes of each standard thread and the drill to use before tapping it. Each tap drill is the nearest standard size to the hole that cuts a 75% thread, the usual choice for steel and most metals, and the chart shows the thread percentage that drill actually gives. Choose the thread form above the chart.
For a different thread percentage, a size that isn't listed, or inch drills for a metric thread, use the tap drill calculator.
How it's worked out (for inch threads the pitch is 1 ÷ TPI):
- Minor Ø (the basic minor diameter of the hole) = D − 1.0825 × pitch (ISO 724, ASME B1.1)
- Stress area (the area that carries a bolt's load, for strength and torque) = π/4 × (D − 0.9382 × pitch)² for metric, π/4 × (D − 0.9743 × pitch)² for inch threads
- Tap drill = D − 1.299 × pitch × 75%, then the nearest standard drill. A 75% thread is nearly as strong as a full one and much easier to tap. Go one drill size bigger for hard or tough materials, or smaller for soft metals and plastics.
Last checked 30 October 2026 · Last rev. 31 October 2026 · a718