Thread / Tap Drill Calculator

Calculate the recommended tap drill size for threading operations instantly. This tool helps engineers and machinists determine the correct drill diameter before tapping, ensuring proper thread engagement and avoiding tool breakage.

Thread / Tap Drill
Calculator

Type
M
Tap Drill: 0 mm
Metric formula: Drill ≈ Major Diameter - Pitch
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Quick Answer

👉 Metric thread rule:

Tap Drill ≈ Major Diameter − Pitch

✔ Example:
M10 × 1.5
→ Drill ≈ 10 − 1.5 = 8.5 mm

Formula (Tap Drill Calculation)

Metric Threads:

Drill Size (mm) ≈ Major Diameter − Pitch

Inch Threads (Reference)

Drill Size (inch) ≈ Major Diameter − (1 / TPI)

  • TPI = Threads per inch

✔ Example:
1/4″-20 UNC
→ Drill ≈ 0.250 − (1/20) = 0.200″ (~#7 drill)

Common Metric Tap Drill Sizes

Thread Size Pitch (mm) Tap Drill (mm)
M3 0.5 2.5
M4 0.7 3.3
M5 0.8 4.2
M6 1.0 5.0
M8 1.25 6.8
M10 1.5 8.5
M12 1.75 10.2
M16 2.0 14.0

How to Use This Calculator

  1. Select thread type (metric or inch)
  2. Choose thread size (e.g., M6, M10)
  3. The calculator outputs the recommended tap drill size

✔ Instant results
✔ Reduces trial-and-error in machining

Common Engineering Uses

1. CNC Tapping Operations

  • Ensure correct hole size before threading

2. Manual Tapping

  • Prevent tap breakage due to undersized holes

3. Thread Quality Control

  • Achieve proper thread engagement

4. Design for Manufacturability (DFM)

  • Specify correct drill sizes in drawings

Important Notes

  • This formula gives an approximate drill size
  • Actual selection depends on:
    • Material type (hard vs soft)
    • Thread tolerance class
    • % thread engagement (typically 60–75%)

👉 Softer materials → slightly smaller drill
👉 Hard materials → slightly larger drill

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FAQ

What is a tap drill?

It is the hole diameter drilled before tapping threads.

Why subtract pitch from diameter?

Because the thread depth is approximately equal to the pitch.

Is this formula exact?

No, it is a widely used engineering approximation.

What happens if the drill is too small?

  • Tap breakage
  • High torque
  • Poor thread quality

What happens if the drill is too large?

  • Weak threads
  • Reduced holding strength
CNC Machining Shops

CNC Machining Shops

Need Precision Threaded CNC Parts?

Correct tap drill size ensures strong and accurate threads.

👉 We offer:

  • CNC machining with precision threading
  • Metric and imperial thread standards
  • Tapping, thread milling, and inspection
  • Fast quoting and global delivery

Upload your drawing today and get a professional machining quote.