Thread Tap Drill Size Calculator
Search any metric or imperial thread size to instantly get the recommended tap drill, clearance drill, and thread engagement percentage. Works offline - no data sent anywhere.
Learn more: tap drills, engagement, and thread standards
Why 75% thread engagement is the engineering sweet spot
A tap drill hole that is too small leaves too much material for the tap to cut, increasing the risk of tap breakage - especially in stainless steel and titanium where taps snap at nuisance rates. A hole that is too large produces weak threads. 75% engagement gives approximately 95% of full thread strength while being easy to tap. In plastics and soft aluminium, 65% engagement is often preferred for faster cutting and less tool wear.
Metric ISO, UNC, and UNF thread standards
Metric ISO (M1 to M42) threads are specified by nominal diameter and pitch in millimeters - for example, M6x1.0 is a 6mm diameter thread with 1mm pitch. UNC (Unified National Coarse) and UNF (Unified National Fine) are imperial standards, specified in inches and threads-per-inch (TPI). UNC is more common for fasteners; UNF is used where vibration resistance or finer adjustment is needed. The calculator handles all three standards and converts between mm and fractional inches.
How tap drill size affects thread strength and ease of cutting
The tap drill leaves a hole sized so the tap cuts threads at the desired engagement percentage. Too small, and the tap hits maximum resistance and breaks; too large, and the thread is shallow and weak. Different materials need different approaches: soft materials like aluminium and plastics tolerate larger drills (60-65% engagement), while hard materials like stainless steel and cast iron demand careful sizing to minimize tap breakage. The calculator shows the standard 75% recommendation and flags when material-specific adjustments are wise.
FAQ
What drill bit do I use for an M6 tap?
An M6 thread has a 1.0mm pitch. The standard tap drill size is 5.0mm (leaving 75% thread engagement, the engineering sweet spot). If you are tapping into a harder material like steel you can use 5.2mm for 65% engagement and reduced tap breakage risk.
What is thread engagement percentage?
Thread engagement is the percentage of the theoretical maximum thread depth that your drilled hole allows. 75% engagement is the standard recommendation - it provides approximately 95% of the strength of a 100% engagement thread, while being much easier to tap and far less likely to break the tap.
What is the tap drill for 1/4-20 UNC?
The standard tap drill for 1/4-20 UNC is #7 drill (5.1mm). This gives approximately 75% thread engagement in most materials. The clearance drill (for a through-hole that a 1/4 inch bolt passes freely) is 17/64 inch (6.75mm).