Calculate the correct drill bit size for tapping internal threads based on tap size, thread percentage, and pitch.
Last updated: March 2026
standard UNC/UNF
typical: 75%
A tap is a cutting tool used to create internal threads inside a pre-drilled hole. The correct drill bit size (called the tap drill size) is critical: too large and threads will be weak or stripped; too small and the tap may break or create excessive friction. Drill size is calculated based on the tap's major diameter, pitch, and desired thread engagement depth (thread percentage).
Thread percentage refers to the depth of thread cut into the hole, typically 50-90%. A 75% thread engagement balances strength with ease of tapping. This calculator uses the standard formula: Drill Size = Major Diameter - (1.299038 × %Thread / (2 × TPI)). Accurate calculations prevent broken taps, stripped threads, and wasted materials.
Calculate drill size for a 1/4-20 UNC tap at 75% thread engagement:
Drilling too large reduces thread engagement and creates weak, possibly stripped threads that won't hold fasteners securely. The threads won't develop their rated strength.
Drilling too small causes excessive friction, making tapping very difficult. The tap may break or bind, damaging both the tool and the workpiece. Always use correct tap drill size.
Slight variations (±0.005") are often acceptable, especially for non-critical applications. However, always aim for the calculated size for maximum thread strength and tapping ease.
75% thread is easier to tap and provides good strength (standard choice). 90% thread is harder to tap but creates stronger, more load-bearing fasteners. Use 90% for critical applications.
Yes. UNC (National Coarse) and UNF (National Fine) have different pitch (TPI), so they require different tap drill sizes and tap tools. Verify which standard you're using.
Currently, this calculator focuses on imperial (UNC/UNF) threads. Metric threads use the formula: Drill = Major - (1.0825 × %Thread / (2 × TPI equivalent)).
Drilling too small, using wrong thread %, applying too much pressure, or not using adequate coolant. Always use the correct drill size, light pressure, and proper coolant.
Coarse (UNC) is faster to tap and more forgiving of misalignment. Fine (UNF) provides better load distribution and is used where vibration resistance is critical.
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