Thread Engagement Calculator —
How Deep to Tap, and the Tap Drill for Any Percent of Thread

Tap Drill for Any Percent of Thread

A tap does not have to cut the full thread height. The tap drill sets how much of the theoretical sharp-V height the tap engages, and the standard shop practice is about 75%, which gives nearly the full strength of a 100% thread at roughly half the tapping torque. Going below 60% is common in tough materials like Inconel and titanium to save taps; going above 80% rarely adds strength and breaks taps.[1]

The calculator then picks the closest standard drill from the drill size chart and reports the percent of thread that drill actually gives, so you can see whether it landed slightly above or below your target.

How Deep to Tap: Length of Engagement

A bolt in a tapped hole can fail three ways: the bolt breaks in tension, the bolt's external threads shear off, or the internal threads in the hole strip out. Good design makes the bolt break first, because a broken bolt is obvious and replaceable while stripped threads in a housing are neither. The engagement length that guarantees it comes from comparing the thread shear areas with the bolt's tensile stress area.[2]

Shear strength is taken as 0.6 times tensile strength for every material in the tables, a common engineering approximation. The calculator uses basic thread dimensions; class 2A/2B allowances make the real shear areas a few percent smaller, so treat the result as a design minimum and round up to the next practical depth. It also assumes uniform load sharing along the engaged threads and does not cover thread inserts, fatigue, or fine-pitch threads in soft materials, which strip more easily than the areas suggest.

Worked Example: 1/4-20 Grade 5 Bolt in 6061-T6

The result lands on the rule of thumb machinists already use, one diameter of engagement for a steel bolt in aluminum, and shows where it comes from. The same bolt in a 4140 pre-hard housing needs only the 0.14 in equal-material value, and in Delrin the multiplier climbs past 8, which is why plastic parts get threaded inserts.

Threads in the Calculator

Basic major diameters and pitches per ASME B1.1 (unified inch) and ISO 261 / ASME B1.13M (metric). The calculator reads this table when the page loads.[3]

Material Strengths Used

Fastener strengths are the specification minimum tensile strengths; tapped-material strengths are the typical values stated in this library's material guides, so the calculator agrees with the articles. Shear strength is taken as 0.6 × tensile throughout.[4]