Tap Drill Size Calculator
Look up the correct tap drill size for any Unified (UNC/UNF) or Metric thread. Complete drill size table with inch and mm dimensions.
Introduction
The Tap Drill Size Calculator is a critical reference tool for ensuring the structural integrity of threaded assemblies. Selecting the correct drill size before tapping is one of the most common yet vital tasks in any machine shop. A hole that is too small will cause the tap to break due to excessive torque, while a hole that is too large will result in 'shallow' threads that lack the necessary shear strength to hold the fastener under load. This tool provides the precise diameter needed for approximately 75% thread engagement, the industry standard for most applications.
How it Works
The calculator determines the recommended drill size based on the nominal diameter and the pitch of the thread. For imperial Unified threads (UNC/UNF), the formula is: Drill Size = Major Diameter - (1 / TPI). For metric threads, it is: Drill Size = Major Diameter - Pitch. The tool also provides decimal and metric equivalents to help you choose the closest standard drill bit available in your cabinet, whether it is a fractional, letter, or wire gauge size.
Usage Scenarios
- Selecting the correct drill for a 1/2-13 UNC thread in 6061 aluminum to ensure clean, strong threads without risk of tap breakage.
- Looking up the metric equivalent for an M10 x 1.5 tap when only metric drill bits are available in the shop.
- Verifying the correct starting hole size for a thread forming (roll) tap, which requires a larger hole than a traditional cutting tap.
FAQ
Why 75% thread engagement?
75% engagement provides a balance between thread strength and ease of tapping. Increasing engagement beyond 80% significantly increases tap breakage risk without adding meaningful strength.
What if I don't have the exact drill size?
Use the closest standard size that is slightly larger rather than smaller to reduce the risk of breaking the tap, especially in harder materials.