Tap Drill Size Calculator

The formula subtracts a fraction of the thread pitch from the major diameter. Specifically, tap drill diameter equals the major diameter minus the desired engagement percentage divided by 100, multiplied by the thread pitch, multiplied by 1.0825. For imperial threads the pitch is one divided by the threads-per-inch count; for metric threads it is the pitch value in millimeters listed in the thread designation. The 1.0825 constant comes from the geometry of a 60-degree thread form and accounts for the relationship between pitch diameter and minor diameter.

For most steels and aluminum alloys, 75 percent thread engagement is the standard starting point. It delivers close to the maximum pull-out strength a fastener can achieve while keeping tapping torque at a manageable level. When working with harder or more brittle materials such as hardened steel, stainless, or cast iron, backing off to 60 to 65 percent engagement significantly reduces the risk of snapping the tap without sacrificing much useful holding strength. In softer materials like plastics or magnesium alloys, going up toward 85 percent can be worthwhile to compensate for lower shear strength in the parent material.

Only marginally beyond about 75 percent. Research on thread strength shows that jumping from 75 to 100 percent engagement increases pull-out strength by less than 5 percent in most metallic materials, because the weakest link shifts from thread shear to bolt tensile strength well before full engagement is reached. The cost of that small gain is steep: tapping torque roughly doubles from 65 to 100 percent engagement, and the probability of snapping the tap in a blind hole rises sharply. In most production environments 75 percent is the practical optimum that engineers and machinists have settled on over decades of experience.

Use the calculator as a starting point for selecting your drill size, then cross-check the result against a printed tap drill chart from a reputable reference such as Machinery's Handbook or the drill manufacturer's published tables. For any load-bearing, structural, pressure-retaining, or safety-critical application, also verify that your chosen engagement percentage is appropriate for the specific material, thread standard, and fastener specification involved. When tolerances are tight or the consequence of failure is severe, have a qualified engineer review the selection before committing metal.