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Thread Tap Calculator

Tap drill size for UNC, UNF, and metric threads by thread percentage

Tap Drill Size

0.2013"

Thread

1/4-20

% Thread

75%

75% is standard. 50–60% for hard materials.

Tap Drill Size

0.2013"

(5.11 mm)

Thread

1/4-20

% Thread

75%

TPI

20

Thread Dimensions

Major Diameter0.25"
Pitch Diameter0.2175"
Minor Diameter0.1959"
Nearest 64th13/64"

Example Calculations

11/4-20 UNC at 75% thread

Inputs

Thread1/4-20 UNC
Thread %75%

Result

Tap Drill0.2013"
Metric5.11 mm

0.250 - (0.75 * 1.299 * 0.05) = 0.2013". Use #7 drill.

2M8x1.25 at 75% thread

Inputs

ThreadM8x1.25
Thread %75%

Result

Tap Drill6.78 mm
Imperial0.267"

8 - (0.75 * 1.299 * 1.25) = 6.78mm. Use 6.8mm drill.

Frequently Asked Questions

Q

How do I calculate tap drill size?

Tap drill = major diameter - (thread% * 2 * 0.6495 * pitch). For 1/4-20 UNC at 75%: 0.250 - (0.75 * 1.299 * 0.05) = 0.2013" (use #7 drill).

  • D_drill = D_major - (%T * 1.299 * pitch)
  • Standard thread % is 75%
  • 1/4-20 at 75%: 0.201" (#7 drill)
  • M6x1.0 at 75%: 5.03mm (5.0mm drill)
ThreadPitch75% Drill60% Drill
1/4-20 UNC0.050"#7 (0.201")13/64" (0.203")
3/8-16 UNC0.0625"5/16" (0.313")Q (0.332")
M6x1.01.0mm5.0mm5.1mm
M8x1.251.25mm6.8mm6.9mm
Q

What thread percentage should I use?

75% is standard for most applications. Use 50-60% for hard materials (stainless, titanium) to reduce tap breakage. 60-65% is good for production tapping.

  • 75%: standard, maximum strength
  • 60-65%: production, easier tapping
  • 50-55%: hard materials, CNC tapping
  • Below 50%: threads too shallow
Q

What is the difference between UNC and UNF?

UNC (coarse) has fewer threads per inch, easier assembly. UNF (fine) has more TPI, giving finer adjustment and more thread engagement per inch of depth.

  • UNC: coarse pitch, easier assembly
  • UNF: fine pitch, stronger holding
  • 1/4-20 UNC vs 1/4-28 UNF
  • UNF preferred for vibration resistance
Q

How do I convert between imperial and metric tap drills?

Multiply inches by 25.4 to get mm. Choose the nearest standard drill size in your preferred system, always rounding to the nearest available size without going smaller.

  • 1 inch = 25.4 mm
  • 0.201" = 5.11mm
  • 5.0mm = 0.197"
  • Use nearest standard size, never smaller

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This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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