Why thread percentage matters more than it seems
A tap drill chart gives a single recommended size, but that size is really targeting a specific percentage of full thread engagement, not the absolute maximum possible. Drilling closer to the theoretical minor diameter increases thread strength but also increases tapping torque and the risk of tap breakage, especially in tougher materials, so the actual drill size chosen is always a tradeoff between strength and tappability.
How the number is calculated
Thread percentage equals the major diameter minus the actual tap drill size, divided by the major diameter minus the theoretical minor diameter, expressed as a percentage. A drill that produces the full theoretical minor diameter gives 100 percent thread engagement, while a larger drill leaves less material removed and a correspondingly lower percentage.
Practical tips
75 percent thread engagement is the commonly cited sweet spot for most general-purpose tapping, offering nearly the full strength of a 100 percent thread while noticeably reducing tapping torque and tap breakage risk. In harder materials, dropping to 65 or even 60 percent thread is common practice specifically to keep taps from snapping, since the strength lost is often small compared to the reliability gained in production.