How Tap Drill Sizing Works
A tap cuts threads into a hole. Before you tap, you drill a smaller hole called the tap drill. If the hole is too small, the tap has to remove too much metal and can break. If the hole is too big, the threads are shallow and weak. The goal is to pick a drill that leaves just enough material for the tap to cut strong threads without too much force.
The Math Behind the Drill Size
For UN inch threads the standard formula from Machinery's Handbook is: Drill = Major − 1.299 × Pitch × (%/100). For ISO metric threads it is: Drill = Major − Pitch × (%/100). The factors come from the 60 degree thread geometry and match every industry tap drill chart. Most shops target 75 percent engagement, which gives close to the tightest strong thread without overloading the tap.
Why 75 Percent Is the Default
Research shows that going from 75 percent to 100 percent thread engagement only adds about 5 percent to thread strength. But the tap torque more than doubles. That means at 75 percent you get almost all the strength with way less risk of breaking the tap. For production shops running hundreds of holes, 75 percent is the sweet spot.
Pro tip: For blind holes in tough materials like Ti-6Al-4V or 4140 heat-treated steel, drop to 50 or 55 percent engagement. You lose a little strength but the tap lasts 5 to 10 times longer.
Picking the Right Standard Drill
Your calculated drill size will almost never match a stocked drill exactly. Round up to the next number, letter, fractional, or metric drill (see our drill bit size chart for a complete list). Rounding up cuts engagement by 1 to 3 percent, which is safe. Rounding down makes the hole undersized and loads the tap past its design limit.
Common Tap Drill Sizes
The most-used taps in US machine shops are 4-40, 6-32, 8-32, 10-24, 10-32, 1/4-20, 3/8-16, and 1/2-13. In metric the common sizes are M3, M4, M5, M6, M8, and M10. The reference table below gives the 75 percent tap drill for each of these.