4-40 Tap Drill Bit Size is a #43 Drill Bit

For a 4-40 UNC thread, the correct tap drill size is a #43 drill bit, which creates a 0.0890" (2.26 mm) hole size.

Correct Tap Drill Size for 4-40 Threads

For a #4-40 UNC tap, the best tap drill size is a #43 drill bit. This drill produces a 0.0890" hole size, which creates a thread engagement of approximately 71%.

Optional 4-40 Tap Drill Sizes

There is only one alternative drill size for a 4-40 tap drill that is in the good / preferred 70-77% thread engagement range.  That is the 2.25mm drill bit in the chart above.  The 2.2mm drill bit will cut with more torque and risk breakage or excessive wear.  The 2.3mm drill bit will produce a much shallower thread and the strength may not be sufficient to hold the screw.  Some sites recommend a 3/32" drill bit, but this is not a good choice.  The thread engagement for this size drill bit is 56% which is not sufficient for most metals.

Find Other Tap Drill Sizes

Use this tap drill calculator to compare additional drill sizes and understand the relationship of thread percentages to the drill and tap size.

Why Tap Drill Accuracy Matters

Accurate tap drill sizing helps maintain thread quality, reduce tool wear, and improve repeatability in CNC machining, manual tapping, and repair work. This is especially important when tapping stainless steel, alloy steel, aluminum, or other production materials where incorrect hole size can create excess torque or poor thread fit.

Reference Additional Drill and Tap Charts

For broader comparisons across thread sizes and drill systems, review the charts below to quickly find related tap drill sizes and equivalents.

• NPT Tap Drill Chart
• Standard Tap Drill Size Chart
• Drill Tap Size Chart MM
• Drill Bit and Screw Size Chart

Find #43 Drill Bits Below