Machining

Drill & Tap Chart

Quick reference for drill sizes, tap sizes, and clearance holes for standard threads.

Input Parameters

Units:

Thread Size

Thread Engagement

Results

Select a thread size and click Look Up

What is a Drill & Tap Chart?

A drill and tap chart provides the correct drill bit size to use before tapping threads. The tap drill creates a hole smaller than the major diameter, leaving material for the tap to cut threads into.

Choosing the right tap drill is critical: too small and you risk breaking taps; too large and you get weak threads. The optimal size depends on desired thread engagement percentage.

How to Use

Select your thread system (Imperial or Metric)
Use search to filter or select thread size directly
Choose thread engagement (75% standard, 50% for soft materials)
Click Look Up to get drill sizes
Use clearance drill for through-holes, tap drill for threaded holes

FAQs

Thread engagement is the percentage of full thread depth achieved. 75% engagement is the standard recommendation for steel - it provides strong threads without excessive tapping torque. 50% engagement is used for softer materials like aluminum, brass, and plastics where full engagement isn't needed and would increase tap breakage risk.

The imperial drill system uses three overlapping size ranges: numbered drills (#1-#80, with #80 being smallest), lettered drills (A-Z, with Z being largest), and fractional drills. This historical system fills in the gaps between fractional sizes, giving machinists more precise hole sizing options.

Yes, but you need to match the decimal equivalent. For example, a 5mm drill (0.1968") is close to a #9 drill (0.196") and can substitute. However, slight size differences affect thread engagement percentage, so verify the decimal sizes match closely for critical applications.

UNC (Unified National Coarse) has fewer threads per inch and is more common for general fastening. UNF (Unified National Fine) has more TPI, providing finer adjustment, better vibration resistance, and higher tensile strength in the same diameter, but is more prone to cross-threading and stripping.

Drill at least 1.5x the tap depth plus the tap's chamfer length (typically 2-3 threads). For example, if you need 1/2" of threads with a 2-thread chamfer on a 1/4-20 tap, drill at least 0.75" + 0.10" = 0.85" deep. This ensures full threads and chip clearance.

Limitations

• Covers standard UNC/UNF and metric coarse/fine only
• Does not include pipe threads (NPT, BSPT) or specialty threads
• Engagement percentages are calculated values, not measured
• Actual results vary with drill wear and material
• Always verify with tap manufacturer recommendations