Tap Drill Size Calculator
What drill size for M6 tapping? Pick a spec, get the answer.
Thread System
Calculation Results
Breakdown
Thread Specification
Thread Size Comparison
🟢 Green = pilot hole (what you drill) 🔵 Blue = major dia. (bolt OD) 🟠 Orange = minor dia. (thread root)
Calculation Formula
About Tap Drill Size Calculator
The Tap Drill Size Calculator is a professional tool designed for CNC operators, machinists, and DIY enthusiasts. Simply enter your thread specification to instantly get the recommended drill size - no more guessing "what drill do I need for an M6 tap?"
How to Use
- Select thread system: Metric (M), UNC (Coarse), or UNF (Fine)
- Choose target thread size, such as M6×1.0 or 1/4-20
- Select appropriate thread engagement based on material hardness
- Choose tap type (cutting or forming)
- View recommended drill size and start machining
Formula Explanation
This calculator uses the Machinery's Handbook standard formula: D = D_major - (1.08253 × P × Pft/100), where D_major is major diameter, P is pitch, and Pft is thread engagement percentage. The constant 1.08253 represents the theoretical height coefficient (5H/8) for 60° V-threads.
Thread Engagement Guide
- 75% Standard: Suitable for general low carbon steel and aluminum, the most common setting
- 65% Hard Materials: For stainless steel and tool steel, reduces tap breakage risk
- 50% Loose Fit: For deep hole tapping and softer materials, reduces tapping resistance
Forming vs Cutting Taps
Forming taps (roll taps) create threads through plastic deformation without producing chips. They're ideal for aluminum, copper, and plastics. Since no material is removed, a larger tap drill hole is required (approximately D_major - 0.5P).
FAQ
Why is the recommended drill different from theoretical?
The theoretical value is an exact calculation, but commercial drills come in standard sizes. The calculator matches the closest available standard drill.
What does confidence level mean?
Standard sizes (like M6) have well-established tap drill sizes with high confidence; manual input for special sizes should be verified with reference handbooks.
What if my tap keeps breaking?
Consider using a larger tap drill (lower thread engagement), appropriate cutting fluid, slower tapping speed, or switching to forming taps.
