Look up tap drill sizes at 50%, 65%, and 75% thread engagement. Search by thread size. Enter a custom drill to calculate the resulting thread percentage.
| Size | Major ∅ | 75% drill | 65% drill | 50% drill |
|---|---|---|---|---|
| #1-64 | 0.073" | #53 (0.0595") | #52 (0.0635") | #51 (0.0670") |
| #2-56 | 0.086" | #50 (0.0700") | #49 (0.0730") | #48 (0.0760") |
| #3-48 | 0.099" | #47 (0.0785") | #44 (0.0860") | #43 (0.0890") |
| #4-40 | 0.112" | #43 (0.0890") | #42 (0.0935") | #41 (0.0960") |
| #5-40 | 0.125" | #38 (0.1015") | #37 (0.1040") | #36 (0.1065") |
| #6-32 | 0.138" | #36 (0.1065") | #33 (0.1130") | #31 (0.1200") |
| #8-32 | 0.164" | #29 (0.1360") | #28 (0.1405") | #26 (0.1470") |
| #10-24 | 0.190" | #25 (0.1495") | #21 (0.1590") | #16 (0.1770") |
| #12-24 | 0.216" | #16 (0.1770") | #14 (0.1820") | #10 (0.1935") |
| 1/4-20 | 0.250" | #7 (0.2010") | #4 (0.2090") | 7/32 (0.2188") |
| 5/16-18 | 0.313" | F (0.2570") | G (0.2610") | H (0.2660") |
| 3/8-16 | 0.375" | 5/16 (0.3125") | Q (0.3320") | R (0.3390") |
| 7/16-14 | 0.438" | U (0.3680") | 25/64 (0.3906") | X (0.3970") |
| 1/2-13 | 0.500" | 27/64 (0.4219") | 29/64 (0.4531") | 15/32 (0.4688") |
| 9/16-12 | 0.563" | 31/64 (0.4844") | 33/64 (0.5156") | 17/32 (0.5313") |
| 5/8-11 | 0.625" | 17/32 (0.5313") | 9/16 (0.5625") | 37/64 (0.5781") |
| 3/4-10 | 0.750" | 21/32 (0.6563") | 11/16 (0.6875") | 45/64 (0.7031") |
| 7/8-9 | 0.875" | 49/64 (0.7656") | 51/64 (0.7969") | 53/64 (0.8281") |
| 1-8 | 1.000" | 7/8 (0.8750") | 57/64 (0.8906") | 59/64 (0.9219") |
A tap drill is the hole you drill before threading. The drill size determines how much material the tap has to cut, which is expressed as percent of thread engagement. Higher engagement means a stronger thread but requires more torque to tap and increases tap breakage risk.
100% engagement would mean the tap cuts a full-depth thread with no clearance — impossible in practice. 75% is the standard recommendation for most applications. It provides adequate thread strength with reasonable tapping torque. Above 83%, tap breakage risk increases sharply with minimal strength gain.
The formula for thread engagement percentage is:
% engagement = (1 − (drill size − minor dia) / (pitch dia − minor dia)) × 100
Hard materials (stainless, titanium, Inconel):Use 60–65% engagement. The thread is already strong because the material is strong. Lower engagement dramatically reduces tap breakage.
Blind holes:Use 60–65% engagement to reduce tapping torque and chip packing problems at the bottom of the hole.
Long thread engagement (>1.5× diameter): You can go as low as 50% because the additional thread length compensates for lower engagement per thread.
In soft materials like aluminum or brass where tap breakage is not a concern and the thread length is short (less than 1× diameter), 80–83% engagement provides additional pull-out resistance.
UNC (Unified National Coarse): Larger pitch, easier to tap, more forgiving of misalignment. Standard for general-purpose fastening.
UNF (Unified National Fine): Smaller pitch, stronger thread for a given diameter, better vibration resistance. Common in aerospace and automotive. Requires more precision and is less forgiving of cross-threading.
Metric (ISO):Standard outside the US and increasingly common in US shops. The coarse pitch series (e.g., M6×1.0) is analogous to UNC.
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