Tap Drill Size for M4x0.5

Thread Specifications for M4x0.5

Here are the complete specifications for the M4x0.5 thread:

• Major Diameter: 4.0mm (0.1575″)
• Pitch: 0.5mm
• Tap Drill Size: 3.5mm (0.1378″ / 3.5mm)
• Thread Engagement: ~75% (standard)

How Is the Tap Drill Size Calculated?

The tap drill size for metric threads is calculated using this simple formula:

Tap Drill Diameter = Major Diameter – Pitch

For M4x0.5:

Tap Drill = 4.0mm – 0.5mm = 3.5mm

The recommended tap drill is 3.5mm, which gives approximately 75% thread engagement — the industry standard for most applications.

Why 75% Thread Engagement?

Thread engagement percentage determines how deep the threads are cut. While more might seem better, there is a practical limit:

• 50% engagement provides about 90% of full thread strength — ideal for softer materials or when tap breakage is a concern.
• 75% engagement is the industry standard — excellent strength with reasonable tapping effort. This is what the 3.5mm drill provides for M4x0.5.
• Beyond 83% adds almost no additional strength but significantly increases tapping torque and tap breakage risk.

Pro tip: In aluminum, brass, or other soft materials, 50-60% thread engagement is often preferred to reduce tap breakage. Save 75%+ for steel and harder materials.

Related Thread Sizes

• Tap Drill for M3x0.35 — 2.65mm (0.1043″)
• Tap Drill for M5x0.5 — 4.5mm (0.1772″)
• Tap Drill for M6x0.75 — 5.25mm (0.2067″)