What drill size should I use for a thread forming tap?

1. Metric Thread Forming Taps (ISO Standards)

Metric taps are labeled with their major diameter (e.g., M10) and thread pitch (e.g., 1.5 mm for M10×1.5). The basic formula for metric thread forming taps is:

This formula yields a hole size that creates approximately 75% thread— the sweet

spot for balance between thread strength and tap life for most ductile materials (aluminum, mild steel, copper). For example:

• M8×1.25 tap: 8 mm (major diameter) – 1.25 mm (pitch) = 6.75 mm drill size. However, standard drill bits may not come in 6.75 mm—so you’d use the closest standard size (6.8 mm or 6.9 mm) based on material toughness.
• M10×1.5 tap: 10 mm – 1.5 mm = 8.5 mm drill size (a common standard size for this tap).

For precise thread percentage control (critical for high-volume production), use this advanced formula:

Example: For an M6×1.0 tap with 65% thread (ideal for softer materials):

2. Imperial Thread Forming Taps (UNC/UNF Standards)

Imperial taps are labeled with their major diameter (e.g., 1/4”) and threads per inch (TPI, e.g., 20 for 1/4–20 UNC). First, calculate the pitch (1/TPI), then use the formula:

For thread percentage control, use this formula:

Examples:

• 1/4–20 UNC tap: Major diameter = 0.250 inch, TPI = 20, pitch = 0.050 inch. Basic formula: 0.250 – (0.050/2) = 0.225 inch (use a #4 drill bit, which is 0.221 inch—close enough for most applications).
• 3/8–16 UNC tap: Major diameter = 0.375 inch, TPI = 16, pitch = 0.0625 inch. Basic formula: 0.375 – (0.0625/2) = 0.34375 inch (use a 5/16 inch drill bit, which is 0.3125 inch? No—adjust for thread percentage: 0.375 – (0.0068×75)/16 ≈ 0.340 inch, so use a #3 drill bit (0.3437 inch)).

Quick Reference Chart: Common Thread Forming Tap Drill Sizes

To save time, use this chart for the most popular metric and imperial thread forming taps. All sizes are based on 75% thread (ideal for ductile materials like aluminum, mild steel, and brass).