Tap Drill Size Calculator
Find the correct drill size for any thread. Supports imperial (UNC/UNF) and metric (ISO) threads with adjustable thread percentage.
Thread Selection
Thread % Guidelines
- 75% - Standard for most applications
- 65-70% - Easier tapping, less torque
- 80-85% - High strength requirements
- 50-60% - Soft materials, hand tapping
Recommended Tap Drill
Calculated Drill Size
209.4 thou (0.2094")
Closest Standard Drill
#4
Thread Dimensions
Tapping Tips
- • Use cutting oil or tapping fluid for best results
- • Back out frequently to clear chips (1/2 turn forward, 1/4 turn back)
- • Start tap perpendicular to surface
- • Use a bottoming tap for blind holes after starting tap
Understanding Tap Drills
A tap drill creates the hole that will be threaded. The drill size determines how much material remains for the tap to cut, expressed as thread percentage. Choosing the right size balances thread strength against tapping difficulty.
Thread Percentage Guidelines
- 50-60%: Easy tapping, soft materials, hand taps
- 65-70%: Reduced torque, hard materials, CNC tapping
- 75%: Standard - full strength with reasonable effort
- 80-85%: Maximum strength, requires care
Common Thread Sizes
Imperial (UNC)
- #6-32: #36 drill (0.1065")
- #8-32: #29 drill (0.136")
- #10-24: #25 drill (0.1495")
- 1/4-20: #7 drill (0.201")
- 5/16-18: F drill (0.257")
- 3/8-16: 5/16" drill (0.3125")
Metric (ISO)
- M3 × 0.5: 2.5mm drill
- M4 × 0.7: 3.3mm drill
- M5 × 0.8: 4.2mm drill
- M6 × 1.0: 5.0mm drill
- M8 × 1.25: 6.8mm drill
- M10 × 1.5: 8.5mm drill
Frequently Asked Questions
What is thread percentage and why does it matter?
Thread percentage (or thread engagement) is how much of the theoretical thread form is cut into the hole. 75% is standard for most applications - it provides full strength with reasonable tapping torque. Going above 75% adds minimal strength but significantly increases tap breakage risk.
What's the difference between UNC and UNF threads?
UNC (Unified National Coarse) has fewer threads per inch and is the default for most applications - faster to assemble and more tolerant of damage. UNF (Unified National Fine) has more threads per inch, providing finer adjustment, higher tensile strength, and better resistance to vibration loosening.
Why use a smaller drill for harder materials?
For hard materials like stainless steel, using a larger drill (lower thread percentage like 65%) reduces tapping torque and tap breakage. The threads are still plenty strong. For soft materials like aluminum, you can use a smaller drill (higher thread percentage) since tapping is easier.
What if I don't have the exact drill size?
Use the closest standard drill size. Going slightly larger is generally safer (reduces thread percentage slightly but prevents tap breakage). Going smaller increases tap breakage risk. Letter and number drills often provide closer sizes than fractional drills.
How do I convert between numbered/letter drills and decimal?
Numbered drills (#1-#80) and letter drills (A-Z) are specified by their decimal inch size. Our calculator shows the exact decimal equivalent. #1 is 0.228", #80 is 0.0135". Letter A is 0.234", Z is 0.413".
What about pipe threads (NPT)?
NPT (National Pipe Thread) uses tapered threads and requires specific tap drill sizes based on the pipe size. NPT tap drills are standardized and don't follow the same percentage calculations as straight threads.
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