Select the thread
Choose a thread designation from either the inch (UNC/UNF) or metric (ISO) series.
Interactive dimensional reference
Select a UNC, UNF, or metric thread size to instantly see the recommended tap drill for standard ~75% thread engagement. The complete reference tables for both inch (UNC/UNF) and metric (ISO coarse and fine) threads are available below.
Choose a thread designation from either the inch (UNC/UNF) or metric (ISO) series.
Values update instantly from the selected thread designation.
Thread engagement describes how much of the full theoretical thread depth is actually cut into the hole. A drill sized for 100% engagement removes the least material and creates the deepest, theoretically strongest thread — but in practice it also creates dramatically more friction and cutting torque while tapping, which sharply increases the risk of tap breakage, especially in blind holes and harder alloys.
75% engagement is the long-standing general-purpose default used across machine shops and published in standard reference tables (Machinery's Handbook, ASME/ANSI B1.1 practice) because it captures roughly 95% of the holding strength of a 100% thread while requiring meaningfully less tapping torque and reducing tap breakage risk. Beyond ~75%, the strength gained per additional percent of engagement drops off quickly, so the extra torque and tap wear are rarely worth it for general-purpose fastening.
Softer materials (aluminum, brass, plastics) can often tolerate engagement toward the higher end of this range since they cut more easily, while tougher alloys and stainless steels are sometimes tapped closer to 50–65% specifically to reduce breakage risk in production. The tables below use the standard ~75% engagement figures published for general-purpose tapping — always confirm against your specific tap manufacturer's recommendation for unusual materials or blind-hole depths.
We stock the jobber-length and drill-mill bits used to drill the tap-drill-sized pilot hole before tapping. We don't sell taps — just the precision drills that get the hole ready.
Complete reference
Tap drill sizes for ≈75% thread engagement, per ASME/ANSI B1.1.
| Thread designation | Series | Major diameter | Threads per inch | Tap drill (≈75%) | Decimal equivalent | Minor diameter |
|---|
Complete reference
Tap drill sizes for ≈75% thread engagement, per ISO 965.
| Thread designation | Series | Pitch | Tap drill (≈75%) | Decimal / fractional equivalent |
|---|
Reference: Tap drill sizes are based on ≈75% thread engagement per ASME/ANSI B1.1 (Unified inch screw threads — UN and UNR thread form) for UNC/UNF sizes, and ISO 965 (ISO general-purpose metric screw threads — tolerances) for metric coarse and fine sizes. Always verify critical or blind-hole applications against your tap manufacturer's specification.