1/4-20 UNC tap-drill size
The cutting-tap recommendation and thread dimensions for 1/4-20 UNC.
- Basic major diameter
- 6.35 mm
- Threads per inch
- 20 TPI
- Thread family
- UNC
- Thread form
- 60° · parallel
What this measurement means
The recommended 5.1 mm hole is a drilling dimension. The thread’s 6.35 mm basic major diameter describes a different part of the thread geometry. The interactive chart can centre either one while retaining the full designation.
This recommendation is attached to 1/4-20 UNC in the source data. It is not inferred from the closest drill size or calculated from a diameter-minus-pitch estimate.
Explore the thread diameterRecommendations vary with material, engagement and tool. These values are transcribed from the Alpha / Sterling chart, not calculated by subtracting pitch. They are not intended for forming taps. A nearby catalogue drill is a dimensional comparison, not a substitute recommendation.
Check the thread family
UNC and UNF are separate thread series. A matching major diameter is not enough to identify the thread. Confirm the threads per inch and the complete designation before choosing a tapping drill.
See the UNF tableSources & scope
Data edition 1.0. Values come from the reference sources below. SizeAxis is a selected reference catalogue; it does not certify manufactured dimensions or thread compatibility.
Read the measurement definitions and full coverage